Forming an Ion Changes the Electrons and Never the Protons
Learn why a third particle was needed inside the nucleus, the charges and masses of the three subatomic particles, where element symbols come from, and how to count protons, neutrons and electrons.
When an atom becomes an ion, what actually changes?
Only the electrons. The protons and neutrons are untouched.
A sodium atom has protons, neutrons and electrons. Lose one electron and it becomes the sodium ion, , with protons, neutrons and ** electrons.
The atomic number stays , because that counts protons. The mass number stays , because electrons are far too light to matter. All that has happened is that one electron has left, leaving an excess of one positive charge.
So a question that gives you a charge is telling you about electrons**, and nothing else. Students who adjust the proton count to explain a positive charge change the element itself — sodium with protons would be neon, which is not what forming an ion does.
This page covers the second part of the CBSE Class 9 Science chapter on the journey inside the atom.
A sodium atom has protons, neutrons and electrons. Lose one electron and it becomes the sodium ion, , with protons, neutrons and ** electrons.
The atomic number stays , because that counts protons. The mass number stays , because electrons are far too light to matter. All that has happened is that one electron has left, leaving an excess of one positive charge.
So a question that gives you a charge is telling you about electrons**, and nothing else. Students who adjust the proton count to explain a positive charge change the element itself — sodium with protons would be neon, which is not what forming an ion does.
This page covers the second part of the CBSE Class 9 Science chapter on the journey inside the atom.
Why was a third particle needed inside the nucleus?
Because protons and electrons together could not account for the mass of an atom.
Take helium. It carries a charge that requires two protons, so its nucleus should have a mass of about u — and electrons are far too light to add anything noticeable. Yet a helium atom has a mass of about ** u.
Half the mass was unaccounted for. What was needed was a particle with no charge — so that it would not upset the charge balance — and a mass about the same as a proton's**, so that two of them would supply the missing u. That particle is the neutron.
The three subatomic particles.
- Electron — charge , and a mass of roughly one two-thousandth of a proton's. So small that it is taken as negligible in mass calculations.
- Proton — charge , relative mass u. Found in the nucleus.
- Neutron — charge ****, relative mass u. Found in the nucleus.
Protons and neutrons together are called nucleons, because both live in the nucleus. Electrons occupy the shells outside it.
Where the atom's mass is. Since electrons are negligible and both nucleons weigh about u, nearly all the mass of an atom is in its nucleus — which is exactly what the gold foil experiment showed in the previous part of this chapter. The two conclusions arrive from completely different directions and agree.
An atom is neutral because protons and electrons balance. The proton's charge and the electron's are equal in size and opposite in sign, so equal numbers of each give a neutral atom. The neutron plays no part in that balance, which is precisely why it went unnoticed while the charges were being accounted for and only the mass gave it away.
Take helium. It carries a charge that requires two protons, so its nucleus should have a mass of about u — and electrons are far too light to add anything noticeable. Yet a helium atom has a mass of about ** u.
Half the mass was unaccounted for. What was needed was a particle with no charge — so that it would not upset the charge balance — and a mass about the same as a proton's**, so that two of them would supply the missing u. That particle is the neutron.
The three subatomic particles.
- Electron — charge , and a mass of roughly one two-thousandth of a proton's. So small that it is taken as negligible in mass calculations.
- Proton — charge , relative mass u. Found in the nucleus.
- Neutron — charge ****, relative mass u. Found in the nucleus.
Protons and neutrons together are called nucleons, because both live in the nucleus. Electrons occupy the shells outside it.
Where the atom's mass is. Since electrons are negligible and both nucleons weigh about u, nearly all the mass of an atom is in its nucleus — which is exactly what the gold foil experiment showed in the previous part of this chapter. The two conclusions arrive from completely different directions and agree.
An atom is neutral because protons and electrons balance. The proton's charge and the electron's are equal in size and opposite in sign, so equal numbers of each give a neutral atom. The neutron plays no part in that balance, which is precisely why it went unnoticed while the charges were being accounted for and only the mass gave it away.
Why is the symbol for sodium Na and not So?
Because several symbols come from the element's Latin name rather than its English one.
A symbol is a one- or two-letter abbreviation for an element. The rule of writing is strict: the first letter is a capital, and a second letter, where there is one, is small.
Symbols from the English name — the majority:
- H hydrogen, C carbon, N nitrogen, O oxygen
- Ca calcium, Mg magnesium, Al aluminium, Zn zinc, Si silicon
Symbols from the Latin name — the ones worth memorising, because they cannot be guessed:
- Sodium, from natrium — Na
- Potassium, from kalium — K
- Iron, from ferrum — Fe
- Copper, from cuprum — Cu
- Silver, from argentum — Ag
- Gold, from aurum — Au
- Lead, from plumbum — Pb
- Tin, from stannum — Sn
- Mercury, from hydrargyrum — Hg
- Antimony, from stibium — Sb
Everyday traces of those names. A plumber is named for working with lead pipes, from plumbum. The word cuprous in chemistry comes from cuprum, and ferrous from ferrum — which is why an iron compound is called ferrous sulphate rather than ironous anything.
The capitalisation carries meaning, and is not a matter of neatness. Co is the element cobalt. CO is the compound carbon monoxide, made of one carbon atom and one oxygen atom. Writing one when you mean the other changes an element into a poisonous gas, so NA, na and nA are all genuinely wrong for sodium, and only Na is right.
A symbol is not simply shorthand. It stands for one atom of the element, which is why a formula such as can state a composition exactly — two atoms of hydrogen to one of oxygen. That is the whole basis of writing chemical formulas, and it depends on the symbols being unambiguous.
A symbol is a one- or two-letter abbreviation for an element. The rule of writing is strict: the first letter is a capital, and a second letter, where there is one, is small.
Symbols from the English name — the majority:
- H hydrogen, C carbon, N nitrogen, O oxygen
- Ca calcium, Mg magnesium, Al aluminium, Zn zinc, Si silicon
Symbols from the Latin name — the ones worth memorising, because they cannot be guessed:
- Sodium, from natrium — Na
- Potassium, from kalium — K
- Iron, from ferrum — Fe
- Copper, from cuprum — Cu
- Silver, from argentum — Ag
- Gold, from aurum — Au
- Lead, from plumbum — Pb
- Tin, from stannum — Sn
- Mercury, from hydrargyrum — Hg
- Antimony, from stibium — Sb
Everyday traces of those names. A plumber is named for working with lead pipes, from plumbum. The word cuprous in chemistry comes from cuprum, and ferrous from ferrum — which is why an iron compound is called ferrous sulphate rather than ironous anything.
The capitalisation carries meaning, and is not a matter of neatness. Co is the element cobalt. CO is the compound carbon monoxide, made of one carbon atom and one oxygen atom. Writing one when you mean the other changes an element into a poisonous gas, so NA, na and nA are all genuinely wrong for sodium, and only Na is right.
A symbol is not simply shorthand. It stands for one atom of the element, which is why a formula such as can state a composition exactly — two atoms of hydrogen to one of oxygen. That is the whole basis of writing chemical formulas, and it depends on the symbols being unambiguous.
Formula
How do you count the protons, neutrons and electrons in an atom or ion?
so that
For a neutral atom, the number of electrons equals the number of protons, so electrons .
For an ion, adjust only the electrons:
- A positive ion (cation) has lost electrons, so electrons
- A negative ion (anion) has gained electrons, so electrons
Worked example 1 — sodium atom. , .
Worked example 2 — chlorine atom. , .
Worked example 3 — aluminium atom. , : protons, neutrons, electrons.
**Worked example 4 — the sodium ion .** The charge is , so one electron has been lost.
**Worked example 5 — the chloride ion .** The charge is , so one electron has been gained.
**Worked example 6 — the oxide ion .** With and :
**Worked example 7 — the calcium ion .** With and :
Notice something about examples 5, 6 and 7. The chloride ion, the oxide ion and the calcium ion have , and electrons — so and have the same number of electrons despite being different elements. Electron count does not identify an element; only the proton count does.
Worked example 8 — the one atom with no neutrons. Hydrogen has and .
The commonest form of hydrogen is the only common atom with no neutrons at all, and it is worth remembering as the boundary case that the formula still handles correctly.
Add electrons for a negative charge and subtract for a positive one. The direction feels backwards, and that is exactly why it is the commonest error. A negative ion is negative because it gained electrons — so the count goes up. Say it once in words before substituting and the sign looks after itself.
How do you write an element with its mass number and atomic number?
Mass number above, atomic number below, both to the left of the symbol:
Worked examples.
- Sodium:
- Chlorine:
- Oxygen:
- Carbon:
- Nitrogen:
- Helium:
- Hydrogen:
Reading one off. Given , the atomic number is and the mass number is , so phosphorus has protons, electrons and neutrons.
**Given **: protons, electrons and neutrons.
The check that catches a swap. The mass number is always the larger of the two, since it counts the protons and the neutrons. The only case where they are equal is hydrogen, at and , because it has no neutrons.
So a notation with the bigger number at the bottom has the two swapped, and that is a one-second check worth making every time. A written is wrong on its face.
The charge goes to the top right, if there is one. An ion is written or , with the charge on the right and the two numbers still on the left. The three positions carry three different pieces of information, and keeping them apart is what makes the notation readable.
The atomic number is often omitted in ordinary writing, because the symbol already tells you which element it is and therefore what must be. So and are complete descriptions. But when a question asks you to represent an element using the notation, both numbers are expected — and leaving one out loses the mark even though the meaning survives.
Worked examples.
- Sodium:
- Chlorine:
- Oxygen:
- Carbon:
- Nitrogen:
- Helium:
- Hydrogen:
Reading one off. Given , the atomic number is and the mass number is , so phosphorus has protons, electrons and neutrons.
**Given **: protons, electrons and neutrons.
The check that catches a swap. The mass number is always the larger of the two, since it counts the protons and the neutrons. The only case where they are equal is hydrogen, at and , because it has no neutrons.
So a notation with the bigger number at the bottom has the two swapped, and that is a one-second check worth making every time. A written is wrong on its face.
The charge goes to the top right, if there is one. An ion is written or , with the charge on the right and the two numbers still on the left. The three positions carry three different pieces of information, and keeping them apart is what makes the notation readable.
The atomic number is often omitted in ordinary writing, because the symbol already tells you which element it is and therefore what must be. So and are complete descriptions. But when a question asks you to represent an element using the notation, both numbers are expected — and leaving one out loses the mark even though the meaning survives.
Exam tip
Exam tip: adjust only the electrons for an ion
** counts protons, counts protons plus neutrons**, so neutrons . For a neutral atom, electrons .
For an ion, change only the electrons. A negative ion has gained electrons, so add; a positive ion has lost them, so subtract. has electrons; has .
Protons and the mass number never change when an ion forms — changing the protons would change the element.
The mass number is always the larger of the two numbers, except for hydrogen where both are and there are no neutrons.
Write the notation as , with any charge on the top right: .
Capitalise correctly — Co is cobalt, CO is carbon monoxide. Only the first letter is a capital.
Learn the Latin-derived symbols by heart: Na, K, Fe, Cu, Ag, Au, Pb, Sn, Hg, Sb. They cannot be worked out.
Remember the particle facts: proton and u, neutron and u, electron and negligible mass.
Nearly all the mass is in the nucleus, and protons and neutrons together are nucleons.
And set out a counting answer as three labelled lines — protons, neutrons, electrons — rather than three bare numbers.
For an ion, change only the electrons. A negative ion has gained electrons, so add; a positive ion has lost them, so subtract. has electrons; has .
Protons and the mass number never change when an ion forms — changing the protons would change the element.
The mass number is always the larger of the two numbers, except for hydrogen where both are and there are no neutrons.
Write the notation as , with any charge on the top right: .
Capitalise correctly — Co is cobalt, CO is carbon monoxide. Only the first letter is a capital.
Learn the Latin-derived symbols by heart: Na, K, Fe, Cu, Ag, Au, Pb, Sn, Hg, Sb. They cannot be worked out.
Remember the particle facts: proton and u, neutron and u, electron and negligible mass.
Nearly all the mass is in the nucleus, and protons and neutrons together are nucleons.
And set out a counting answer as three labelled lines — protons, neutrons, electrons — rather than three bare numbers.
Did you know
Why the neutron stayed hidden while the charges were counted
Of the three particles in an atom, the neutron is the one that announces itself least — and the reason is that it has no charge.
Almost everything known about the early atom came from electrical evidence. Electrons were identified because they are deflected by electric and magnetic fields. The nucleus was located because positively charged alpha particles were repelled by it. The atom's neutrality was explained by balancing positive against negative.
A neutral particle takes no part in any of that. Put a neutron in an electric field and nothing happens. Fire it past a nucleus and it is not repelled. It leaves no trace in any experiment that works by watching charges being pushed around, which is why the charge accounting of the atom could be completed without it.
What gave it away was mass. Helium's charge needed two protons and its mass needed four units, and no amount of rearranging charges could close that gap. The missing mass had to belong to something that weighed as much as a proton and carried no charge at all.
The same property makes neutrons unusually useful. Because they are uncharged, they are not repelled by a nucleus and can reach it directly, where a proton would be pushed away long before arriving. That is why neutrons are the particles used to start reactions in a nuclear reactor — a charged particle would need enormous energy to get close enough.
So the neutron's lack of charge explains both facts about it at once: why it was the last of the three to be accounted for, and why it turned out to be the one that can be aimed at a nucleus.
Almost everything known about the early atom came from electrical evidence. Electrons were identified because they are deflected by electric and magnetic fields. The nucleus was located because positively charged alpha particles were repelled by it. The atom's neutrality was explained by balancing positive against negative.
A neutral particle takes no part in any of that. Put a neutron in an electric field and nothing happens. Fire it past a nucleus and it is not repelled. It leaves no trace in any experiment that works by watching charges being pushed around, which is why the charge accounting of the atom could be completed without it.
What gave it away was mass. Helium's charge needed two protons and its mass needed four units, and no amount of rearranging charges could close that gap. The missing mass had to belong to something that weighed as much as a proton and carried no charge at all.
The same property makes neutrons unusually useful. Because they are uncharged, they are not repelled by a nucleus and can reach it directly, where a proton would be pushed away long before arriving. That is why neutrons are the particles used to start reactions in a nuclear reactor — a charged particle would need enormous energy to get close enough.
So the neutron's lack of charge explains both facts about it at once: why it was the last of the three to be accounted for, and why it turned out to be the one that can be aimed at a nucleus.
Exam relevance
Why does counting electrons in ions matter for NEET and JEE?
Because every one of the three Class 11 chapters that follow this one begins by counting electrons, and an error made here propagates through all of them.
This is the foundation for the Class 11 Chemistry chapter Structure of Atom, examined in both JEE Main and NEET. The atomic number and mass number keep exactly the definitions given here, and the chapter adds the actual charges and masses in coulombs and kilograms, along with the isotopes and isobars that the next part of this chapter introduces.
Where the electron count is used. Class 11 Classification of Elements and Periodicity places every element by its atomic number, and explains periodic trends by how many electrons are in the outermost shell. Class 11 Chemical Bonding is entirely about electrons being lost, gained or shared — so the ion counting done above is the arithmetic that whole chapter rests on. And in Class 12 Coordination Compounds and The d-Block Elements, the electron count of a metal ion determines its behaviour, with and behaving quite differently for exactly this reason.
The notation is used unchanged in Class 12 Physics Nuclei and Chemistry Nuclear Chemistry, where nuclear equations are balanced by requiring the mass numbers on each side to agree and the atomic numbers on each side to agree — a check that only works if the two are never confused.
Latin-derived symbols matter more than they appear to, because formulas and equations in every later chapter use them without comment.
What the questions look like. Numericals giving , and a charge and asking for the three particle counts are common and quick. Assertion-reason items favour the statement that forming an ion does not change the mass number. Match-the-column questions pair an ion with its electron count, and the pairs that catch students out are ones like and , which have the same electron count and different elements. Statement-based questions ask which of several species are isoelectronic — meaning they have equal electron counts — which is precisely the observation made after worked example 7 above.
How board and competitive emphasis differ. A board paper asks you to define atomic number and mass number, state the charge and mass of the three particles, and count the particles in one or two atoms. A competitive paper gives an ion rather than an atom, or asks which two species are isoelectronic, so the direction of the electron adjustment is what is being tested rather than the definitions.
The single trap that costs the most marks. Adjusting the protons to account for a charge. A positive ion has lost electrons, and changing its protons would change the element entirely. Write charge means electrons at the top of the working and the error becomes impossible.
A second trap worth naming. Adding electrons for a positive charge. A negative ion is negative because it gained electrons, so its count goes up — has , not . Saying the reason in words before substituting is the reliable fix, and it is worth doing every time because the direction genuinely feels reversed.
This is the foundation for the Class 11 Chemistry chapter Structure of Atom, examined in both JEE Main and NEET. The atomic number and mass number keep exactly the definitions given here, and the chapter adds the actual charges and masses in coulombs and kilograms, along with the isotopes and isobars that the next part of this chapter introduces.
Where the electron count is used. Class 11 Classification of Elements and Periodicity places every element by its atomic number, and explains periodic trends by how many electrons are in the outermost shell. Class 11 Chemical Bonding is entirely about electrons being lost, gained or shared — so the ion counting done above is the arithmetic that whole chapter rests on. And in Class 12 Coordination Compounds and The d-Block Elements, the electron count of a metal ion determines its behaviour, with and behaving quite differently for exactly this reason.
The notation is used unchanged in Class 12 Physics Nuclei and Chemistry Nuclear Chemistry, where nuclear equations are balanced by requiring the mass numbers on each side to agree and the atomic numbers on each side to agree — a check that only works if the two are never confused.
Latin-derived symbols matter more than they appear to, because formulas and equations in every later chapter use them without comment.
What the questions look like. Numericals giving , and a charge and asking for the three particle counts are common and quick. Assertion-reason items favour the statement that forming an ion does not change the mass number. Match-the-column questions pair an ion with its electron count, and the pairs that catch students out are ones like and , which have the same electron count and different elements. Statement-based questions ask which of several species are isoelectronic — meaning they have equal electron counts — which is precisely the observation made after worked example 7 above.
How board and competitive emphasis differ. A board paper asks you to define atomic number and mass number, state the charge and mass of the three particles, and count the particles in one or two atoms. A competitive paper gives an ion rather than an atom, or asks which two species are isoelectronic, so the direction of the electron adjustment is what is being tested rather than the definitions.
The single trap that costs the most marks. Adjusting the protons to account for a charge. A positive ion has lost electrons, and changing its protons would change the element entirely. Write charge means electrons at the top of the working and the error becomes impossible.
A second trap worth naming. Adding electrons for a positive charge. A negative ion is negative because it gained electrons, so its count goes up — has , not . Saying the reason in words before substituting is the reliable fix, and it is worth doing every time because the direction genuinely feels reversed.
Key takeaways
Subatomic particles, symbols and counting: quick revision
- A third particle was needed because protons and electrons could not account for an atom's mass — helium's charge needs protons but its mass is about u.
- Electron: charge , mass roughly one two-thousandth of a proton's, taken as negligible.
- Proton: charge , relative mass u, in the nucleus.
- Neutron: charge ****, relative mass u, in the nucleus.
- Protons and neutrons are nucleons, and nearly all the atom's mass is in the nucleus — matching the gold foil conclusion.
- An atom is neutral because protons and electrons balance; the neutron takes no part in that balance, which is why its mass gave it away.
- A symbol has a capital first letter and a small second letter. Co is cobalt; CO is carbon monoxide.
- Latin-derived symbols: Na (natrium), K (kalium), Fe (ferrum), Cu (cuprum), Ag (argentum), Au (aurum), Pb (plumbum), Sn (stannum), Hg (hydrargyrum), Sb (stibium).
- ** = protons, = protons + neutrons, so neutrons **. For a neutral atom, electrons .
- For an ion, change only the electrons: subtract for a positive charge, add for a negative one.
- Sodium (, ): p, n, e. Chlorine (, ): p, n, e. Aluminium (, ): p, n, e.
- : p, n, **** e. : p, n, **** e. : p, n, **** e. : p, n, **** e.
- and have the same electron count — only the proton count identifies an element.
- Hydrogen (, ) has no neutrons — the boundary case the formula still handles.
- Protons and the mass number never change when an ion forms.
- Notation is , with any charge at the top right: , .
- Examples: , , , , (with neutrons), (with ).
- The mass number is always the larger, except for hydrogen where both are .
Take five ions with their atomic and mass numbers and write out the three counts for each, then find which two of them turn out to have the same number of electrons.
- Electron: charge , mass roughly one two-thousandth of a proton's, taken as negligible.
- Proton: charge , relative mass u, in the nucleus.
- Neutron: charge ****, relative mass u, in the nucleus.
- Protons and neutrons are nucleons, and nearly all the atom's mass is in the nucleus — matching the gold foil conclusion.
- An atom is neutral because protons and electrons balance; the neutron takes no part in that balance, which is why its mass gave it away.
- A symbol has a capital first letter and a small second letter. Co is cobalt; CO is carbon monoxide.
- Latin-derived symbols: Na (natrium), K (kalium), Fe (ferrum), Cu (cuprum), Ag (argentum), Au (aurum), Pb (plumbum), Sn (stannum), Hg (hydrargyrum), Sb (stibium).
- ** = protons, = protons + neutrons, so neutrons **. For a neutral atom, electrons .
- For an ion, change only the electrons: subtract for a positive charge, add for a negative one.
- Sodium (, ): p, n, e. Chlorine (, ): p, n, e. Aluminium (, ): p, n, e.
- : p, n, **** e. : p, n, **** e. : p, n, **** e. : p, n, **** e.
- and have the same electron count — only the proton count identifies an element.
- Hydrogen (, ) has no neutrons — the boundary case the formula still handles.
- Protons and the mass number never change when an ion forms.
- Notation is , with any charge at the top right: , .
- Examples: , , , , (with neutrons), (with ).
- The mass number is always the larger, except for hydrogen where both are .
Take five ions with their atomic and mass numbers and write out the three counts for each, then find which two of them turn out to have the same number of electrons.