Most of the Alpha Particles Went Straight Through the Foil
Count protons, neutrons and electrons from the atomic number and mass number, read the alpha-scattering experiment as observations and conclusions, fill shells by the 2n squared rule, and separate isotopes from isobars.
What did firing alpha particles at a gold foil actually reveal?
Suppose you are handed a sealed box and told to work out what is inside it without opening it. One reasonable approach is to fire something small and fast at it and watch what comes out the other side.
If everything passes straight through, the box is mostly empty. If a few things bounce back hard, there is something small, heavy and solid in there.
That is the entire logic of the alpha particle scattering experiment. Fast, positively charged alpha particles were directed at a very thin sheet of gold, and a fluorescent screen recorded where each one ended up.
Most went straight through. A few were deflected slightly. A very small number bounced almost straight back.
Those three observations are enough to build the nuclear model of the atom: an atom is mostly empty space, with all of its positive charge and nearly all of its mass packed into a very small centre.
This page covers the first part of the ICSE Class 9 Chemistry chapter on atomic structure — counting the three particles from the atomic number and mass number, the scattering experiment and what it did and did not explain, how electrons are distributed among shells, the octet rule, and isotopes.
If everything passes straight through, the box is mostly empty. If a few things bounce back hard, there is something small, heavy and solid in there.
That is the entire logic of the alpha particle scattering experiment. Fast, positively charged alpha particles were directed at a very thin sheet of gold, and a fluorescent screen recorded where each one ended up.
Most went straight through. A few were deflected slightly. A very small number bounced almost straight back.
Those three observations are enough to build the nuclear model of the atom: an atom is mostly empty space, with all of its positive charge and nearly all of its mass packed into a very small centre.
This page covers the first part of the ICSE Class 9 Chemistry chapter on atomic structure — counting the three particles from the atomic number and mass number, the scattering experiment and what it did and did not explain, how electrons are distributed among shells, the octet rule, and isotopes.
Formula
How do you find protons, neutrons and electrons from Z and A?
Three relations settle every question of this kind.
Here is the atomic number — the number of protons, which is what fixes the identity of the element — and is the mass number, the total number of protons and neutrons, since the electrons weigh almost nothing.
**Worked example — sodium, , .**
- protons
- electrons (it is a neutral atom)
- neutrons
Three more, done the same way.
- Chlorine, , : 17 protons, 17 electrons, neutrons
- Aluminium, , : 13 protons, 13 electrons, neutrons
- Calcium, , : 20 protons, 20 electrons, neutrons
Only the electron count changes when an ion forms. The nucleus is untouched, so the proton and neutron counts stay exactly as they were:
- — 11 protons, 12 neutrons, but electrons
- — 17 protons, 18 neutrons, and electrons
- — 8 protons, 8 neutrons, and electrons
- — 13 protons, 14 neutrons, and electrons
A positive charge means electrons were lost and a negative charge means electrons were gained. A common slip is to subtract for a negative ion because the sign looks like subtraction — but the charge is negative because extra negative electrons were added. Read the sign as a count of electrons rather than as an instruction.
**Notice that , and all have 10 electrons. They are different elements with different nuclei, and they have arrived at the same electron count — the count belonging to neon. That is the whole point of ion formation, and the octet rule below explains why.
The three particles and their properties.
- Proton** — charge , mass 1 unit, in the nucleus
- Neutron — no charge, mass 1 unit, in the nucleus
- Electron — charge , mass about of a proton, in the shells outside
So the mass sits in the nucleus and the chemistry happens outside it. Nearly all the mass is protons and neutrons; every reaction in the rest of this course involves only the outermost electrons.
Here is the atomic number — the number of protons, which is what fixes the identity of the element — and is the mass number, the total number of protons and neutrons, since the electrons weigh almost nothing.
**Worked example — sodium, , .**
- protons
- electrons (it is a neutral atom)
- neutrons
Three more, done the same way.
- Chlorine, , : 17 protons, 17 electrons, neutrons
- Aluminium, , : 13 protons, 13 electrons, neutrons
- Calcium, , : 20 protons, 20 electrons, neutrons
Only the electron count changes when an ion forms. The nucleus is untouched, so the proton and neutron counts stay exactly as they were:
- — 11 protons, 12 neutrons, but electrons
- — 17 protons, 18 neutrons, and electrons
- — 8 protons, 8 neutrons, and electrons
- — 13 protons, 14 neutrons, and electrons
A positive charge means electrons were lost and a negative charge means electrons were gained. A common slip is to subtract for a negative ion because the sign looks like subtraction — but the charge is negative because extra negative electrons were added. Read the sign as a count of electrons rather than as an instruction.
**Notice that , and all have 10 electrons. They are different elements with different nuclei, and they have arrived at the same electron count — the count belonging to neon. That is the whole point of ion formation, and the octet rule below explains why.
The three particles and their properties.
- Proton** — charge , mass 1 unit, in the nucleus
- Neutron — no charge, mass 1 unit, in the nucleus
- Electron — charge , mass about of a proton, in the shells outside
So the mass sits in the nucleus and the chemistry happens outside it. Nearly all the mass is protons and neutrons; every reaction in the rest of this course involves only the outermost electrons.
What were the observations and conclusions of the scattering experiment?
Apparatus. A source of fast-moving alpha particles, which are positively charged and comparatively heavy; a sheet of gold beaten so thin that it is only a few atoms thick; and a fluorescent screen placed around it, which gives a tiny flash of light wherever a particle strikes.
By counting flashes in different directions you learn how the particles were deflected.
Observations.
- Most of the alpha particles passed straight through the foil with no deflection at all
- A small fraction were deflected through small angles
- A very few — a tiny number — were deflected through large angles, some bouncing almost straight back the way they came
Conclusions drawn from each observation.
- Most passing straight through the atom is mostly empty space. A solid atom would have stopped them
- Small deflections these particles passed near a concentration of positive charge, and like charges repel
- Large deflections and rebounds these few scored a direct hit on something small, positively charged and heavy — a positive particle would not be turned back by anything lighter than itself
- The rebounds being so rare that heavy centre is very small compared with the whole atom
That small heavy positive centre is the nucleus.
The model that follows. An atom has a tiny, positively charged nucleus carrying nearly all of its mass, with the electrons occupying the comparatively enormous volume outside it and moving around the nucleus.
Merits of the model.
- It established the existence of the nucleus and located the positive charge and the mass in it
- It showed that the atom is largely empty space, which no earlier picture had suggested
- It explained all three scattering observations from one arrangement
Demerits of the model.
- It could not explain why the electrons do not spiral into the nucleus. A charge moving in a circle should radiate energy continuously, lose energy and fall inwards — so on this model the atom should collapse, and atoms plainly do not
- It predicted a continuous spectrum from an atom, because a steadily spiralling electron would emit every wavelength. Real atoms give a line spectrum, only certain sharp wavelengths
- It said nothing at all about how the electrons are arranged outside the nucleus
Those demerits are all the same gap seen three ways. The model placed the electrons outside the nucleus without saying anything about their energies, and every failure follows from that. The repair was to allow electrons only certain fixed energy levels — the shells of the next section — so that an electron in a shell does not radiate at all and light is emitted only when it jumps between levels.
A scattering question is asking you to link an observation to a conclusion. Write them as pairs — observation, then what it proves — and the marks follow the pairing. A bare description of the apparatus earns very little, because the reasoning is the answer.
By counting flashes in different directions you learn how the particles were deflected.
Observations.
- Most of the alpha particles passed straight through the foil with no deflection at all
- A small fraction were deflected through small angles
- A very few — a tiny number — were deflected through large angles, some bouncing almost straight back the way they came
Conclusions drawn from each observation.
- Most passing straight through the atom is mostly empty space. A solid atom would have stopped them
- Small deflections these particles passed near a concentration of positive charge, and like charges repel
- Large deflections and rebounds these few scored a direct hit on something small, positively charged and heavy — a positive particle would not be turned back by anything lighter than itself
- The rebounds being so rare that heavy centre is very small compared with the whole atom
That small heavy positive centre is the nucleus.
The model that follows. An atom has a tiny, positively charged nucleus carrying nearly all of its mass, with the electrons occupying the comparatively enormous volume outside it and moving around the nucleus.
Merits of the model.
- It established the existence of the nucleus and located the positive charge and the mass in it
- It showed that the atom is largely empty space, which no earlier picture had suggested
- It explained all three scattering observations from one arrangement
Demerits of the model.
- It could not explain why the electrons do not spiral into the nucleus. A charge moving in a circle should radiate energy continuously, lose energy and fall inwards — so on this model the atom should collapse, and atoms plainly do not
- It predicted a continuous spectrum from an atom, because a steadily spiralling electron would emit every wavelength. Real atoms give a line spectrum, only certain sharp wavelengths
- It said nothing at all about how the electrons are arranged outside the nucleus
Those demerits are all the same gap seen three ways. The model placed the electrons outside the nucleus without saying anything about their energies, and every failure follows from that. The repair was to allow electrons only certain fixed energy levels — the shells of the next section — so that an electron in a shell does not radiate at all and light is emitted only when it jumps between levels.
A scattering question is asking you to link an observation to a conclusion. Write them as pairs — observation, then what it proves — and the marks follow the pairing. A bare description of the apparatus earns very little, because the reasoning is the answer.
How are electrons distributed among the shells?
**The maximum number of electrons a shell can hold is **, where is the shell number counting outwards from the nucleus.
- K shell, : electrons
- L shell, : electrons
- M shell, : electrons
- N shell, : electrons
Two extra conditions, known as the Bohr–Bury scheme, decide the order of filling.
- The outermost shell of an atom can never hold more than 8 electrons
- A shell starts filling only after the shell before it has taken as many as it is allowed
The two rules together explain potassium. Potassium has 19 electrons. The M shell can hold 18 in principle, so you might write 2, 8, 9. But 9 in the outermost shell is not allowed, so the M shell stops at 8 and the nineteenth electron starts the N shell: 2, 8, 8, 1.
Calcium, with 20 electrons, does the same thing: 2, 8, 8, 2.
**So gives a capacity and the outermost-shell rule decides when that capacity is actually used. The M shell does fill to 18 in heavier atoms, once it is no longer the outermost shell. Writing 2, 8, 9 for potassium is the standard error — and the giveaway is that it puts 9 electrons on the outside.
Electron distribution of the first twenty elements.**
- Hydrogen — 1
- Helium — 2
- Lithium — 2, 1
- Beryllium — 2, 2
- Boron — 2, 3
- Carbon — 2, 4
- Nitrogen — 2, 5
- Oxygen — 2, 6
- Fluorine — 2, 7
- Neon — 2, 8
- Sodium — 2, 8, 1
- Magnesium — 2, 8, 2
- Aluminium — 2, 8, 3
- Silicon — 2, 8, 4
- Phosphorus — 2, 8, 5
- Sulphur — 2, 8, 6
- Chlorine — 2, 8, 7
- Argon — 2, 8, 8
- Potassium — 2, 8, 8, 1
- Calcium — 2, 8, 8, 2
**Check the total against every time. The distribution 2, 8, 7 adds to 17, which is chlorine — if your digits do not sum to the atomic number you have made an arithmetic slip. That one addition catches almost every mistake in this topic.
The outermost electrons are called the valence electrons**, and they are the ones that take part in bonding. Sodium has one, chlorine has seven, and argon has eight — which is exactly why the first two react and the third does not.
- K shell, : electrons
- L shell, : electrons
- M shell, : electrons
- N shell, : electrons
Two extra conditions, known as the Bohr–Bury scheme, decide the order of filling.
- The outermost shell of an atom can never hold more than 8 electrons
- A shell starts filling only after the shell before it has taken as many as it is allowed
The two rules together explain potassium. Potassium has 19 electrons. The M shell can hold 18 in principle, so you might write 2, 8, 9. But 9 in the outermost shell is not allowed, so the M shell stops at 8 and the nineteenth electron starts the N shell: 2, 8, 8, 1.
Calcium, with 20 electrons, does the same thing: 2, 8, 8, 2.
**So gives a capacity and the outermost-shell rule decides when that capacity is actually used. The M shell does fill to 18 in heavier atoms, once it is no longer the outermost shell. Writing 2, 8, 9 for potassium is the standard error — and the giveaway is that it puts 9 electrons on the outside.
Electron distribution of the first twenty elements.**
- Hydrogen — 1
- Helium — 2
- Lithium — 2, 1
- Beryllium — 2, 2
- Boron — 2, 3
- Carbon — 2, 4
- Nitrogen — 2, 5
- Oxygen — 2, 6
- Fluorine — 2, 7
- Neon — 2, 8
- Sodium — 2, 8, 1
- Magnesium — 2, 8, 2
- Aluminium — 2, 8, 3
- Silicon — 2, 8, 4
- Phosphorus — 2, 8, 5
- Sulphur — 2, 8, 6
- Chlorine — 2, 8, 7
- Argon — 2, 8, 8
- Potassium — 2, 8, 8, 1
- Calcium — 2, 8, 8, 2
**Check the total against every time. The distribution 2, 8, 7 adds to 17, which is chlorine — if your digits do not sum to the atomic number you have made an arithmetic slip. That one addition catches almost every mistake in this topic.
The outermost electrons are called the valence electrons**, and they are the ones that take part in bonding. Sodium has one, chlorine has seven, and argon has eight — which is exactly why the first two react and the third does not.
Why do atoms bother to react at all, and what are isotopes?
The octet rule: atoms combine so as to acquire eight electrons in their outermost shell. For the first shell, which holds only two, the stable arrangement is a pair — the duplet.
An arrangement of eight in the outermost shell is the arrangement the noble gases already have — helium has a duplet, and neon and argon have octets. Those elements are chemically inert, and the inference is that the arrangement itself is what confers stability.
Every other element reaches it by one of three routes.
- 1, 2 or 3 valence electrons — easier to lose them. Sodium loses one and becomes (2, 8); magnesium loses two; aluminium loses three
- 5, 6 or 7 valence electrons — easier to gain the few that are missing. Chlorine gains one and becomes (2, 8, 8); oxygen gains two; nitrogen gains three
- 4 valence electrons — losing or gaining four is too costly, so the atom shares instead. Carbon does this in every compound it forms
So the 10-electron ions from the earlier section make sense now. , and all reached the neon arrangement 2, 8 — from three different starting points, by losing one, gaining two and losing three. The octet is the destination, and the charge is the receipt for how far the atom had to travel to get there.
Isotopes: atoms of the same element with the same atomic number but different mass numbers. Same protons, same electrons, different neutrons — so the same element chemically, with a different mass.
Isotopes of hydrogen.
- Protium, — 1 proton, 0 neutrons, 1 electron
- Deuterium, — 1 proton, 1 neutron, 1 electron
- Tritium, — 1 proton, 2 neutrons, 1 electron
Protium is the only atom with no neutrons at all. Its nucleus is a single proton, which is why hydrogen is the simplest atom there is.
Isotopes of carbon and chlorine.
- Carbon: (6 protons, 6 neutrons), (6, 7), (6, 8)
- Chlorine: (17 protons, 18 neutrons) and (17, 20)
Isotopes have identical chemical properties and different physical properties. Chemistry is decided by the electrons, and isotopes have the same electron count and the same distribution — so they react identically. Physical properties that depend on mass, such as density and rate of diffusion, do differ.
Worked example — why chlorine's relative atomic mass is not a whole number. A natural sample of chlorine is about 75% and 25% . The relative atomic mass is the weighted average:
So the fractional value is an average over a mixture, not a fractional atom. No single chlorine atom has a mass number of 35.5 — every atom is either 35 or 37. That is the answer to "why is the relative atomic mass of chlorine 35.5?", and it is the reason the earlier part of this chapter warned that relative atomic masses are not all whole numbers.
Isobars are different — same mass number, different atomic number. Calcium (, ) and argon (, ) are isobars: they are different elements that happen to weigh the same. Isotopes share the atomic number; isobars share the mass number. So isotopes are one element and isobars are two, and that single sentence keeps the pair straight.
An arrangement of eight in the outermost shell is the arrangement the noble gases already have — helium has a duplet, and neon and argon have octets. Those elements are chemically inert, and the inference is that the arrangement itself is what confers stability.
Every other element reaches it by one of three routes.
- 1, 2 or 3 valence electrons — easier to lose them. Sodium loses one and becomes (2, 8); magnesium loses two; aluminium loses three
- 5, 6 or 7 valence electrons — easier to gain the few that are missing. Chlorine gains one and becomes (2, 8, 8); oxygen gains two; nitrogen gains three
- 4 valence electrons — losing or gaining four is too costly, so the atom shares instead. Carbon does this in every compound it forms
So the 10-electron ions from the earlier section make sense now. , and all reached the neon arrangement 2, 8 — from three different starting points, by losing one, gaining two and losing three. The octet is the destination, and the charge is the receipt for how far the atom had to travel to get there.
Isotopes: atoms of the same element with the same atomic number but different mass numbers. Same protons, same electrons, different neutrons — so the same element chemically, with a different mass.
Isotopes of hydrogen.
- Protium, — 1 proton, 0 neutrons, 1 electron
- Deuterium, — 1 proton, 1 neutron, 1 electron
- Tritium, — 1 proton, 2 neutrons, 1 electron
Protium is the only atom with no neutrons at all. Its nucleus is a single proton, which is why hydrogen is the simplest atom there is.
Isotopes of carbon and chlorine.
- Carbon: (6 protons, 6 neutrons), (6, 7), (6, 8)
- Chlorine: (17 protons, 18 neutrons) and (17, 20)
Isotopes have identical chemical properties and different physical properties. Chemistry is decided by the electrons, and isotopes have the same electron count and the same distribution — so they react identically. Physical properties that depend on mass, such as density and rate of diffusion, do differ.
Worked example — why chlorine's relative atomic mass is not a whole number. A natural sample of chlorine is about 75% and 25% . The relative atomic mass is the weighted average:
So the fractional value is an average over a mixture, not a fractional atom. No single chlorine atom has a mass number of 35.5 — every atom is either 35 or 37. That is the answer to "why is the relative atomic mass of chlorine 35.5?", and it is the reason the earlier part of this chapter warned that relative atomic masses are not all whole numbers.
Isobars are different — same mass number, different atomic number. Calcium (, ) and argon (, ) are isobars: they are different elements that happen to weigh the same. Isotopes share the atomic number; isobars share the mass number. So isotopes are one element and isobars are two, and that single sentence keeps the pair straight.
Exam tip
Exam tip: pair every observation with its conclusion
**Protons , electrons for a neutral atom, neutrons . Write the subtraction out; do not do it in your head.
For an ion, only the electron count changes. A positive charge means electrons lost, a negative charge means electrons gained — and the nucleus is untouched.
Check that your electron distribution adds up to . 2, 8, 7 sums to 17, so it is chlorine. This catches nearly every slip.
Never put more than 8 in the outermost shell. Potassium is 2, 8, 8, 1, not 2, 8, 9. Calcium is 2, 8, 8, 2.
For scattering, answer in pairs — observation, then the conclusion it supports. Most pass through, so the atom is mostly empty; a few rebound, so there is a small heavy positive nucleus.
Learn the demerits as one idea: the model said nothing about electron energies, so it could not stop the electron spiralling in, predicted a continuous instead of a line spectrum, and gave no electron arrangement.
Octet rule: atoms combine to reach 8 in the outermost shell, or a duplet of 2 in the first shell — the noble-gas arrangement.
1, 2, 3 valence electrons lose; 5, 6, 7 gain; 4 shares.
Isotopes** — same , different , different neutrons; identical chemical properties, different physical ones. Isobars — same , different , different elements.
And for chlorine's 35.5, show the weighted average and say that it is an average over a mixture.
For an ion, only the electron count changes. A positive charge means electrons lost, a negative charge means electrons gained — and the nucleus is untouched.
Check that your electron distribution adds up to . 2, 8, 7 sums to 17, so it is chlorine. This catches nearly every slip.
Never put more than 8 in the outermost shell. Potassium is 2, 8, 8, 1, not 2, 8, 9. Calcium is 2, 8, 8, 2.
For scattering, answer in pairs — observation, then the conclusion it supports. Most pass through, so the atom is mostly empty; a few rebound, so there is a small heavy positive nucleus.
Learn the demerits as one idea: the model said nothing about electron energies, so it could not stop the electron spiralling in, predicted a continuous instead of a line spectrum, and gave no electron arrangement.
Octet rule: atoms combine to reach 8 in the outermost shell, or a duplet of 2 in the first shell — the noble-gas arrangement.
1, 2, 3 valence electrons lose; 5, 6, 7 gain; 4 shares.
Isotopes** — same , different , different neutrons; identical chemical properties, different physical ones. Isobars — same , different , different elements.
And for chlorine's 35.5, show the weighted average and say that it is an average over a mixture.
Did you know
How empty is an atom, really?
"The atom is mostly empty space" is the standard phrase, and it is easy to read past it without noticing what it actually claims.
Put a number on it. If the nucleus of an atom were scaled up to the size of a pea placed at the centre of a large sports stadium, the electrons would be somewhere out in the stands. Everything between the pea and the stands — which is almost the entire stadium — is empty.
That is the ratio the scattering experiment measured. An alpha particle aimed at random at an atom almost always finds nothing in its way, which is why most of them went straight through the gold foil. Only the rare particle heading almost exactly at the nucleus was turned back.
And that same ratio has a consequence you can feel. Nearly all the mass of the gold foil was in those tiny nuclei, and the foil was still a solid sheet of metal you could pick up. Matter is dense in a very small fraction of its volume and empty in the rest.
Which raises a fair question: if atoms are that empty, why can you not push your hand through a table?
The answer is that the emptiness is not unoccupied. The volume outside the nucleus belongs to the electrons, and an electron cloud will not let another electron cloud into the same space. When you press on a table, the electrons in your hand and the electrons in the wood repel one another, and that repulsion is what you feel as solidity.
So the thing that makes a table solid is not the part of the atom that has the mass. The nucleus supplies the weight and the electrons supply the resistance — and the scattering experiment, which measured only the nucleus, was blind to the very property that makes matter feel like matter.
Put a number on it. If the nucleus of an atom were scaled up to the size of a pea placed at the centre of a large sports stadium, the electrons would be somewhere out in the stands. Everything between the pea and the stands — which is almost the entire stadium — is empty.
That is the ratio the scattering experiment measured. An alpha particle aimed at random at an atom almost always finds nothing in its way, which is why most of them went straight through the gold foil. Only the rare particle heading almost exactly at the nucleus was turned back.
And that same ratio has a consequence you can feel. Nearly all the mass of the gold foil was in those tiny nuclei, and the foil was still a solid sheet of metal you could pick up. Matter is dense in a very small fraction of its volume and empty in the rest.
Which raises a fair question: if atoms are that empty, why can you not push your hand through a table?
The answer is that the emptiness is not unoccupied. The volume outside the nucleus belongs to the electrons, and an electron cloud will not let another electron cloud into the same space. When you press on a table, the electrons in your hand and the electrons in the wood repel one another, and that repulsion is what you feel as solidity.
So the thing that makes a table solid is not the part of the atom that has the mass. The nucleus supplies the weight and the electrons supply the resistance — and the scattering experiment, which measured only the nucleus, was blind to the very property that makes matter feel like matter.
Exam relevance
How does atomic structure carry into JEE Main and NEET?
Because every one of these ideas is repeated at greater depth in Class 11 and then assumed by every chapter after it.
This is the foundation for Class 11 Chemistry Structure of Atom, examined in both JEE Main and NEET. That chapter starts from exactly this material and then supplies what the scattering model could not. Bohr's model repairs the demerits listed on this page — electrons are restricted to fixed energy levels, so an electron in a shell does not radiate and the atom does not collapse, and light is emitted only in jumps between levels, giving the line spectrum that a continuous model could not explain.
The hydrogen spectrum becomes a calculation. Class 11 gives the energy of each level and the wavelength of each transition, and the Lyman, Balmer and Paschen series are named. Questions asking for the wavelength of a particular jump are a standard type, and the qualitative reason for a line spectrum given here is what those calculations make quantitative.
The shells become subshells. The rule is replaced by , , and subshells filled according to the Aufbau principle, Hund's rule and the Pauli exclusion principle — and that machinery is what the Bohr–Bury outermost-shell rule was standing in for. Once you know that fills before , potassium's 2, 8, 8, 1 stops being a special case and becomes the obvious answer.
Isotopes and average atomic mass reappear directly. The weighted-average calculation done here for chlorine is examined in Class 11 Some Basic Concepts of Chemistry, sometimes run backwards — given the average mass, find the percentage abundance. That reverse form is a common JEE Main question, and it is the same arithmetic rearranged.
The nucleus becomes its own subject. Class 12 Physics covers Nuclei — binding energy, radioactivity, and the alpha particle as a helium nucleus. The alpha particles fired at the foil on this page are the alpha radiation of that chapter, and the same experiment appears in the Physics syllabus as the basis of the nuclear model of the atom.
For NEET, Structure of Atom is examined as recall and short calculation: electronic configurations, quantum numbers, the number of electrons in a given subshell, and isotope questions. The first-twenty distributions on this page must be automatic, because a NEET question will not give you time to derive them.
What the questions look like. For board work, expect find the number of protons, neutrons and electrons in a given atom or ion, write the electron distribution, state the observations and conclusions of the scattering experiment, give its merits and demerits, define isotopes and isobars with examples, and explain why chlorine's relative atomic mass is 35.5. Numerical answers need the subtraction shown. For JEE Main and NEET, expect quantum numbers, subshell configurations, spectral-line calculations and abundance problems.
How board and competitive emphasis differ. A board paper rewards the observation-and-conclusion pairing and a correctly written distribution. A competitive paper assumes both and asks for a transition wavelength or an abundance percentage.
The single trap that costs the most marks. Changing the neutron count when an ion forms. Ionisation adds or removes electrons only — the nucleus is not touched, so still has 17 protons and 18 neutrons and differs from in nothing but its 18 electrons. **The defence is to write the proton and neutron counts first, from and alone, and only then apply the charge to the electrons.**
This is the foundation for Class 11 Chemistry Structure of Atom, examined in both JEE Main and NEET. That chapter starts from exactly this material and then supplies what the scattering model could not. Bohr's model repairs the demerits listed on this page — electrons are restricted to fixed energy levels, so an electron in a shell does not radiate and the atom does not collapse, and light is emitted only in jumps between levels, giving the line spectrum that a continuous model could not explain.
The hydrogen spectrum becomes a calculation. Class 11 gives the energy of each level and the wavelength of each transition, and the Lyman, Balmer and Paschen series are named. Questions asking for the wavelength of a particular jump are a standard type, and the qualitative reason for a line spectrum given here is what those calculations make quantitative.
The shells become subshells. The rule is replaced by , , and subshells filled according to the Aufbau principle, Hund's rule and the Pauli exclusion principle — and that machinery is what the Bohr–Bury outermost-shell rule was standing in for. Once you know that fills before , potassium's 2, 8, 8, 1 stops being a special case and becomes the obvious answer.
Isotopes and average atomic mass reappear directly. The weighted-average calculation done here for chlorine is examined in Class 11 Some Basic Concepts of Chemistry, sometimes run backwards — given the average mass, find the percentage abundance. That reverse form is a common JEE Main question, and it is the same arithmetic rearranged.
The nucleus becomes its own subject. Class 12 Physics covers Nuclei — binding energy, radioactivity, and the alpha particle as a helium nucleus. The alpha particles fired at the foil on this page are the alpha radiation of that chapter, and the same experiment appears in the Physics syllabus as the basis of the nuclear model of the atom.
For NEET, Structure of Atom is examined as recall and short calculation: electronic configurations, quantum numbers, the number of electrons in a given subshell, and isotope questions. The first-twenty distributions on this page must be automatic, because a NEET question will not give you time to derive them.
What the questions look like. For board work, expect find the number of protons, neutrons and electrons in a given atom or ion, write the electron distribution, state the observations and conclusions of the scattering experiment, give its merits and demerits, define isotopes and isobars with examples, and explain why chlorine's relative atomic mass is 35.5. Numerical answers need the subtraction shown. For JEE Main and NEET, expect quantum numbers, subshell configurations, spectral-line calculations and abundance problems.
How board and competitive emphasis differ. A board paper rewards the observation-and-conclusion pairing and a correctly written distribution. A competitive paper assumes both and asks for a transition wavelength or an abundance percentage.
The single trap that costs the most marks. Changing the neutron count when an ion forms. Ionisation adds or removes electrons only — the nucleus is not touched, so still has 17 protons and 18 neutrons and differs from in nothing but its 18 electrons. **The defence is to write the proton and neutron counts first, from and alone, and only then apply the charge to the electrons.**
Key takeaways
Atomic structure, scattering and isotopes: quick revision
- **protons ; electrons for a neutral atom; neutrons **.
- Sodium (, ): 11 protons, 11 electrons, 12 neutrons. Chlorine (17, 35): 17, 17, 18. Aluminium (13, 27): 13, 13, 14. Calcium (20, 40): 20, 20, 20.
- For an ion only the electron count changes. — 11p, 12n, 10e; — 17p, 18n, 18e; — 8p, 8n, 10e; — 13p, 14n, 10e.
- Proton , mass 1, in the nucleus. Neutron neutral, mass 1, in the nucleus. Electron , negligible mass, in the shells.
- Scattering apparatus: fast alpha particles, a very thin gold foil, a fluorescent screen.
- Observations: most pass straight through; a small fraction deflect through small angles; a very few rebound through large angles.
- Conclusions: the atom is mostly empty space; the positive charge and nearly all the mass are in a very small central nucleus; the nucleus is positively charged and tiny compared with the atom.
- Merits: established the nucleus and the atom's emptiness, and explained all three observations.
- Demerits: could not explain why the electron does not spiral in; predicted a continuous instead of a line spectrum; said nothing about electron arrangement. All three follow from saying nothing about electron energies.
- **Shell capacity is : K 2, L 8, M 18, N 32.
- Bohr–Bury: the outermost shell never holds more than 8, and shells fill in order. So potassium is 2, 8, 8, 1 and calcium 2, 8, 8, 2 — never 2, 8, 9.
- Check your distribution sums to .
- Octet rule: atoms combine to get 8 in the outermost shell, or a duplet of 2 in the first shell — the noble gas arrangement, which is why noble gases are inert.
- 1, 2, 3 valence electrons lose; 5, 6, 7 gain; 4 shares.** , and all reach 2, 8.
- Isotopes: same element, same , **different — different neutrons**. Hydrogen: protium (1p, 0n), deuterium (1p, 1n), tritium (1p, 2n). Carbon: , , . Chlorine: , .
- Identical chemical properties (same electrons), different physical properties (different mass).
- Chlorine's 35.5 is a weighted average: — an average over a mixture, not a fractional atom.
- Isobars: same , different — different elements. Calcium-40 and argon-40. Isotopes share the atomic number; isobars share the mass number.
Pick any element from the table of twenty, cover the distribution, and see whether you can rebuild it from the atomic number alone — that is the one skill every later question in this chapter assumes.
- Sodium (, ): 11 protons, 11 electrons, 12 neutrons. Chlorine (17, 35): 17, 17, 18. Aluminium (13, 27): 13, 13, 14. Calcium (20, 40): 20, 20, 20.
- For an ion only the electron count changes. — 11p, 12n, 10e; — 17p, 18n, 18e; — 8p, 8n, 10e; — 13p, 14n, 10e.
- Proton , mass 1, in the nucleus. Neutron neutral, mass 1, in the nucleus. Electron , negligible mass, in the shells.
- Scattering apparatus: fast alpha particles, a very thin gold foil, a fluorescent screen.
- Observations: most pass straight through; a small fraction deflect through small angles; a very few rebound through large angles.
- Conclusions: the atom is mostly empty space; the positive charge and nearly all the mass are in a very small central nucleus; the nucleus is positively charged and tiny compared with the atom.
- Merits: established the nucleus and the atom's emptiness, and explained all three observations.
- Demerits: could not explain why the electron does not spiral in; predicted a continuous instead of a line spectrum; said nothing about electron arrangement. All three follow from saying nothing about electron energies.
- **Shell capacity is : K 2, L 8, M 18, N 32.
- Bohr–Bury: the outermost shell never holds more than 8, and shells fill in order. So potassium is 2, 8, 8, 1 and calcium 2, 8, 8, 2 — never 2, 8, 9.
- Check your distribution sums to .
- Octet rule: atoms combine to get 8 in the outermost shell, or a duplet of 2 in the first shell — the noble gas arrangement, which is why noble gases are inert.
- 1, 2, 3 valence electrons lose; 5, 6, 7 gain; 4 shares.** , and all reach 2, 8.
- Isotopes: same element, same , **different — different neutrons**. Hydrogen: protium (1p, 0n), deuterium (1p, 1n), tritium (1p, 2n). Carbon: , , . Chlorine: , .
- Identical chemical properties (same electrons), different physical properties (different mass).
- Chlorine's 35.5 is a weighted average: — an average over a mixture, not a fractional atom.
- Isobars: same , different — different elements. Calcium-40 and argon-40. Isotopes share the atomic number; isobars share the mass number.
Pick any element from the table of twenty, cover the distribution, and see whether you can rebuild it from the atomic number alone — that is the one skill every later question in this chapter assumes.