Each Model of the Atom Was Replaced by One Awkward Observation
Learn how the idea of indivisible particles became a testable theory, what Thomson's model explained and failed to, what the gold foil experiment showed, and how Bohr fixed the stability problem.
Why does the atom have so many different models?
Because each one was replaced by a single observation it could not explain.
That is the pattern worth carrying through this whole page. A model of the atom was not abandoned because someone decided it looked wrong. It was abandoned when a particular measurement came out in a way the model forbade, and the next model was built to accommodate exactly that measurement.
Dalton's atom was indivisible, until particles smaller than an atom turned up inside it. Thomson's atom had its positive charge spread thinly, until fast particles were found bouncing back off something very concentrated. Rutherford's atom had electrons circling freely, until that arrangement was shown to be unstable.
So the right question to ask of each model is not is it correct? but which observation defeated it? — and this page answers that question four times. It covers the first part of the CBSE Class 9 Science chapter on the journey inside the atom.
That is the pattern worth carrying through this whole page. A model of the atom was not abandoned because someone decided it looked wrong. It was abandoned when a particular measurement came out in a way the model forbade, and the next model was built to accommodate exactly that measurement.
Dalton's atom was indivisible, until particles smaller than an atom turned up inside it. Thomson's atom had its positive charge spread thinly, until fast particles were found bouncing back off something very concentrated. Rutherford's atom had electrons circling freely, until that arrangement was shown to be unstable.
So the right question to ask of each model is not is it correct? but which observation defeated it? — and this page answers that question four times. It covers the first part of the CBSE Class 9 Science chapter on the journey inside the atom.
How did the idea of indivisible particles become a scientific theory?
By being turned into statements that explained measurements, rather than remaining an argument from reasoning alone.
The idea that matter is built from tiny indivisible particles is very old. In early Indian philosophy such a particle was called anu, and the smallest conceivable unit parmanu. In Greek thought the word atomos meant that which cannot be cut, and the English word atom comes from it.
Both were philosophical ideas, argued from thinking about what happens if you keep dividing matter. Neither made a prediction that could be measured, so neither could be tested.
Dalton's atomic theory changed that by stating postulates precise enough to check against the laws of chemical combination:
- All matter is made of very small particles called atoms
- Atoms are indivisible and cannot be created or destroyed in a chemical reaction
- Atoms of the same element are identical in mass and chemical properties
- Atoms of different elements have different masses and different properties
- Atoms combine in small whole-number ratios to form compounds
What it explained. The second postulate accounts for the law of conservation of mass — if atoms are merely rearranged in a reaction, the total mass cannot change. The fifth accounts for compounds having a fixed composition, since a fixed ratio of atoms means a fixed ratio of masses.
Everyday consequence. Burning g of carbon in g of oxygen gives exactly g of carbon dioxide, because one carbon atom joins two oxygen atoms and none of them is created or destroyed. That arithmetic is Dalton's theory in use.
Two of its postulates turned out to be wrong, and each failure is a later section of this chapter:
- The atom is not indivisible — it contains electrons, protons and neutrons
- Atoms of the same element are not always identical in mass — isotopes differ, as the third part of this chapter shows
A theory can be enormously useful and still be incomplete. Dalton's postulates explain the laws of chemical combination perfectly well, and chemists still use that arithmetic daily. Being superseded on two points does not make a theory a mistake — it makes it a stage, which is exactly the law and theory distinction from the opening chapter of this course.
The idea that matter is built from tiny indivisible particles is very old. In early Indian philosophy such a particle was called anu, and the smallest conceivable unit parmanu. In Greek thought the word atomos meant that which cannot be cut, and the English word atom comes from it.
Both were philosophical ideas, argued from thinking about what happens if you keep dividing matter. Neither made a prediction that could be measured, so neither could be tested.
Dalton's atomic theory changed that by stating postulates precise enough to check against the laws of chemical combination:
- All matter is made of very small particles called atoms
- Atoms are indivisible and cannot be created or destroyed in a chemical reaction
- Atoms of the same element are identical in mass and chemical properties
- Atoms of different elements have different masses and different properties
- Atoms combine in small whole-number ratios to form compounds
What it explained. The second postulate accounts for the law of conservation of mass — if atoms are merely rearranged in a reaction, the total mass cannot change. The fifth accounts for compounds having a fixed composition, since a fixed ratio of atoms means a fixed ratio of masses.
Everyday consequence. Burning g of carbon in g of oxygen gives exactly g of carbon dioxide, because one carbon atom joins two oxygen atoms and none of them is created or destroyed. That arithmetic is Dalton's theory in use.
Two of its postulates turned out to be wrong, and each failure is a later section of this chapter:
- The atom is not indivisible — it contains electrons, protons and neutrons
- Atoms of the same element are not always identical in mass — isotopes differ, as the third part of this chapter shows
A theory can be enormously useful and still be incomplete. Dalton's postulates explain the laws of chemical combination perfectly well, and chemists still use that arithmetic daily. Being superseded on two points does not make a theory a mistake — it makes it a stage, which is exactly the law and theory distinction from the opening chapter of this course.
What did Thomson's model of the atom say, and where did it fail?
It described the atom as a sphere of positive charge with electrons embedded in it.
Once electrons were known to exist — negatively charged particles far lighter than an atom — a model had to place them somewhere and still leave the atom electrically neutral.
Thomson's model does that:
- The atom is a sphere of positive charge
- Electrons are embedded in that sphere, spread through it
- The total negative charge equals the total positive charge, so the atom as a whole is neutral
The standard picture is a watermelon: the red flesh is the positive charge filling the whole volume, and the black seeds scattered through it are the electrons. A plum pudding with plums set into it is the same image.
What it explained. The atom's electrical neutrality, and the existence of electrons as a part of every atom. Both were real achievements, and neither had a place in Dalton's indivisible atom.
Where it failed. The model spreads the positive charge thinly through the whole atom. Fast, heavy, positively charged particles fired at a thin sheet of metal should therefore meet only a weak, diffuse repulsion everywhere and pass through with slight deflections at most. Nothing in the model could turn such a particle back the way it came.
When exactly that was observed — and it was, in the experiment described in the next section — the model had to go. Not because it explained nothing, but because it forbade something that happens.
Thomson's model also said nothing about stability or about spectra. It offered no reason why the electrons stay where they are rather than falling to the centre, and no account of why an element emits light of particular colours. So even before the foil experiment it was an incomplete description — but it was the specific, measurable failure that settled the matter, and that is the pattern this chapter repeats.
Once electrons were known to exist — negatively charged particles far lighter than an atom — a model had to place them somewhere and still leave the atom electrically neutral.
Thomson's model does that:
- The atom is a sphere of positive charge
- Electrons are embedded in that sphere, spread through it
- The total negative charge equals the total positive charge, so the atom as a whole is neutral
The standard picture is a watermelon: the red flesh is the positive charge filling the whole volume, and the black seeds scattered through it are the electrons. A plum pudding with plums set into it is the same image.
What it explained. The atom's electrical neutrality, and the existence of electrons as a part of every atom. Both were real achievements, and neither had a place in Dalton's indivisible atom.
Where it failed. The model spreads the positive charge thinly through the whole atom. Fast, heavy, positively charged particles fired at a thin sheet of metal should therefore meet only a weak, diffuse repulsion everywhere and pass through with slight deflections at most. Nothing in the model could turn such a particle back the way it came.
When exactly that was observed — and it was, in the experiment described in the next section — the model had to go. Not because it explained nothing, but because it forbade something that happens.
Thomson's model also said nothing about stability or about spectra. It offered no reason why the electrons stay where they are rather than falling to the centre, and no account of why an element emits light of particular colours. So even before the foil experiment it was an incomplete description — but it was the specific, measurable failure that settled the matter, and that is the pattern this chapter repeats.
What did the gold foil experiment actually show?
That an atom is mostly empty space, with its mass and positive charge packed into a tiny central region.
The set-up. A beam of fast-moving alpha particles — helium nuclei, each carrying a charge of and a mass of about u — is directed at an extremely thin sheet of gold foil. A fluorescent screen around the foil records where the particles arrive, so their deflections can be measured.
The three observations.
- Most of the alpha particles passed straight through the foil, undeflected
- Some were deflected through small angles
- A very small fraction were turned back through large angles, a few of them almost straight back towards the source
What each observation implies.
- Most particles passing straight through means most of the atom is empty space
- Small deflections mean the positive charge inside the atom is concentrated, not spread out — only a close approach produces a noticeable push
- The few large-angle reversals mean there is something very small, very dense and positively charged for a particle to rebound from. A heavy fast particle only bounces back from something heavier than itself
Rutherford's nuclear model follows directly from those three conclusions:
- Nearly all the mass and all the positive charge are in a tiny central nucleus
- The nucleus is extremely small compared with the whole atom
- Electrons revolve around the nucleus in circular paths
- The rest of the atom is empty space
A scale comparison worth holding. The nucleus is so much smaller than the atom that if an atom were the size of a large sports stadium, the nucleus would be a small object at the centre of the pitch — and the electrons would be somewhere out at the stands. The thinness of the foil mattered for exactly this reason: a thick sheet would have deflected particles many times over and the pattern would have been impossible to read.
Where this model failed. An electron moving in a circle is constantly changing direction, and changing direction means accelerating — which is precisely the point the third part of the motion chapter established about uniform circular motion. An accelerating charged particle radiates energy. So a revolving electron should lose energy continuously, spiral inward and fall into the nucleus, and the atom should collapse.
Atoms plainly do not collapse. So the model, which explained the foil observations completely, was defeated by a different question altogether — and that is the third time on this page that a model has been broken by one specific thing it could not allow.
The set-up. A beam of fast-moving alpha particles — helium nuclei, each carrying a charge of and a mass of about u — is directed at an extremely thin sheet of gold foil. A fluorescent screen around the foil records where the particles arrive, so their deflections can be measured.
The three observations.
- Most of the alpha particles passed straight through the foil, undeflected
- Some were deflected through small angles
- A very small fraction were turned back through large angles, a few of them almost straight back towards the source
What each observation implies.
- Most particles passing straight through means most of the atom is empty space
- Small deflections mean the positive charge inside the atom is concentrated, not spread out — only a close approach produces a noticeable push
- The few large-angle reversals mean there is something very small, very dense and positively charged for a particle to rebound from. A heavy fast particle only bounces back from something heavier than itself
Rutherford's nuclear model follows directly from those three conclusions:
- Nearly all the mass and all the positive charge are in a tiny central nucleus
- The nucleus is extremely small compared with the whole atom
- Electrons revolve around the nucleus in circular paths
- The rest of the atom is empty space
A scale comparison worth holding. The nucleus is so much smaller than the atom that if an atom were the size of a large sports stadium, the nucleus would be a small object at the centre of the pitch — and the electrons would be somewhere out at the stands. The thinness of the foil mattered for exactly this reason: a thick sheet would have deflected particles many times over and the pattern would have been impossible to read.
Where this model failed. An electron moving in a circle is constantly changing direction, and changing direction means accelerating — which is precisely the point the third part of the motion chapter established about uniform circular motion. An accelerating charged particle radiates energy. So a revolving electron should lose energy continuously, spiral inward and fall into the nucleus, and the atom should collapse.
Atoms plainly do not collapse. So the model, which explained the foil observations completely, was defeated by a different question altogether — and that is the third time on this page that a model has been broken by one specific thing it could not allow.
How did Bohr's model fix the stability problem?
By allowing electrons only certain permitted orbits, in which they do not radiate energy.
Bohr's postulates:
- Electrons revolve around the nucleus only in certain permitted orbits, also called shells or discrete energy levels
- While revolving in a permitted orbit, an electron does not radiate energy — so the atom is stable
- Energy is absorbed or emitted only when an electron jumps from one orbit to another. It absorbs energy moving to a higher orbit and emits energy falling to a lower one
The shells are labelled K, L, M, N outwards from the nucleus, corresponding to , and each has a definite energy.
What this achieves. The second postulate is a direct answer to the objection that sank Rutherford's model. Bohr does not explain why an electron in a permitted orbit fails to radiate; he states it as a rule, and with that rule the atom is stable and the model works.
What it explains that the earlier models did not. Because only certain energies are allowed, the energy an atom can emit comes in definite amounts — which is why an element gives out light of particular colours rather than a continuous smear. Heating a compound of sodium turns a flame a characteristic yellow for exactly this reason, and every element has its own set of colours.
What Bohr kept and what he added. He kept Rutherford's nucleus entirely — the foil experiment's conclusions were not in dispute. What he replaced was the description of the electrons, swapping freely chosen orbits for a fixed set of permitted ones.
Bohr's model is not the final word either. It works well for the simplest atoms and less well for larger ones, and later models replace the neat orbit with a region in which the electron is likely to be found rather than a path it follows. But for the chemistry in this course the shells are exactly what is needed — they are what the electronic configurations in the third part of this chapter are built from.
So the sequence is not four guesses, one of which is right. Each model kept what its predecessor had established and repaired the one place it broke: Dalton's atoms, then electrons inside them, then a nucleus at the centre, then permitted shells for the electrons. That is how a scientific model develops — which is the law, theory and model idea from the opening chapter, shown working across a single topic.
Bohr's postulates:
- Electrons revolve around the nucleus only in certain permitted orbits, also called shells or discrete energy levels
- While revolving in a permitted orbit, an electron does not radiate energy — so the atom is stable
- Energy is absorbed or emitted only when an electron jumps from one orbit to another. It absorbs energy moving to a higher orbit and emits energy falling to a lower one
The shells are labelled K, L, M, N outwards from the nucleus, corresponding to , and each has a definite energy.
What this achieves. The second postulate is a direct answer to the objection that sank Rutherford's model. Bohr does not explain why an electron in a permitted orbit fails to radiate; he states it as a rule, and with that rule the atom is stable and the model works.
What it explains that the earlier models did not. Because only certain energies are allowed, the energy an atom can emit comes in definite amounts — which is why an element gives out light of particular colours rather than a continuous smear. Heating a compound of sodium turns a flame a characteristic yellow for exactly this reason, and every element has its own set of colours.
What Bohr kept and what he added. He kept Rutherford's nucleus entirely — the foil experiment's conclusions were not in dispute. What he replaced was the description of the electrons, swapping freely chosen orbits for a fixed set of permitted ones.
Bohr's model is not the final word either. It works well for the simplest atoms and less well for larger ones, and later models replace the neat orbit with a region in which the electron is likely to be found rather than a path it follows. But for the chemistry in this course the shells are exactly what is needed — they are what the electronic configurations in the third part of this chapter are built from.
So the sequence is not four guesses, one of which is right. Each model kept what its predecessor had established and repaired the one place it broke: Dalton's atoms, then electrons inside them, then a nucleus at the centre, then permitted shells for the electrons. That is how a scientific model develops — which is the law, theory and model idea from the opening chapter, shown working across a single topic.
Exam tip
Exam tip: give the observation and the conclusion as a pair
For the gold foil experiment, write each observation with the conclusion it leads to. Most particles passed straight through, therefore the atom is mostly empty space. The pairing is where the marks are.
Learn the three observations in order — most undeflected, some small deflections, a very small fraction turned back through large angles.
Alpha particles are positively charged and heavy, which is why a large deflection needs something dense and positive.
For Dalton, state the postulates you are asked for, and name the two that failed: the atom is not indivisible, and isotopes differ in mass.
For Thomson, the standard image is a watermelon — positive sphere, embedded electrons, overall neutral. Its limitation is the foil experiment.
For Rutherford, the limitation is instability: a revolving electron accelerates, so it should radiate energy and spiral in.
For Bohr, give all three postulates, and remember he kept the nucleus and changed only the electrons.
Label a diagram with nucleus, shells and K, L, M, N — diagrams carry their own marks in this chapter.
And when asked to compare two models, say what each explained and what defeated it — a list of features without the failure answers only half the question.
Learn the three observations in order — most undeflected, some small deflections, a very small fraction turned back through large angles.
Alpha particles are positively charged and heavy, which is why a large deflection needs something dense and positive.
For Dalton, state the postulates you are asked for, and name the two that failed: the atom is not indivisible, and isotopes differ in mass.
For Thomson, the standard image is a watermelon — positive sphere, embedded electrons, overall neutral. Its limitation is the foil experiment.
For Rutherford, the limitation is instability: a revolving electron accelerates, so it should radiate energy and spiral in.
For Bohr, give all three postulates, and remember he kept the nucleus and changed only the electrons.
Label a diagram with nucleus, shells and K, L, M, N — diagrams carry their own marks in this chapter.
And when asked to compare two models, say what each explained and what defeated it — a list of features without the failure answers only half the question.
Did you know
How empty an atom really is
The most surprising conclusion of the foil experiment is also the easiest to overlook: solid matter is almost entirely empty.
Most of the alpha particles went straight through a sheet of metal without being deflected at all. Not squeezed through gaps between atoms — through the atoms themselves. If atoms were solid throughout, a heavy fast particle could not have crossed even a thin foil unimpeded, and certainly not in such numbers.
The nucleus is minute compared with the atom it sits in, and the electrons are far lighter still and far out. Between them is space. So the gold foil that looks and feels completely solid is, to something small and fast enough, mostly a corridor.
Which raises an obvious question. If atoms are mostly empty, why can you not push your hand through a table?
The answer is that solidity is not about filled space. It is about force. The electrons in the surface of your hand and those in the surface of the table repel each other strongly, and that repulsion is what you feel as contact. You never touch the table in any literal sense; you are held a tiny distance away by a force between two clouds of electrons.
An alpha particle is unaffected by that, because it is positively charged and moving far too fast to be turned aside by electrons. It passes through the electron region as though it were not there and is deflected only if it comes near a nucleus — which is rare, because the nucleus is so small.
So the same atom is a solid barrier to a hand and an open corridor to an alpha particle, and both experiences are accurate. The experiment that revealed the nucleus also revealed that solid describes a force, not a filling.
Most of the alpha particles went straight through a sheet of metal without being deflected at all. Not squeezed through gaps between atoms — through the atoms themselves. If atoms were solid throughout, a heavy fast particle could not have crossed even a thin foil unimpeded, and certainly not in such numbers.
The nucleus is minute compared with the atom it sits in, and the electrons are far lighter still and far out. Between them is space. So the gold foil that looks and feels completely solid is, to something small and fast enough, mostly a corridor.
Which raises an obvious question. If atoms are mostly empty, why can you not push your hand through a table?
The answer is that solidity is not about filled space. It is about force. The electrons in the surface of your hand and those in the surface of the table repel each other strongly, and that repulsion is what you feel as contact. You never touch the table in any literal sense; you are held a tiny distance away by a force between two clouds of electrons.
An alpha particle is unaffected by that, because it is positively charged and moving far too fast to be turned aside by electrons. It passes through the electron region as though it were not there and is deflected only if it comes near a nucleus — which is rare, because the nucleus is so small.
So the same atom is a solid barrier to a hand and an open corridor to an alpha particle, and both experiences are accurate. The experiment that revealed the nucleus also revealed that solid describes a force, not a filling.
Exam relevance
Why do JEE and NEET keep returning to atomic models?
Because this page is where the vocabulary of the atom is fixed, and the two chapters that follow it in Class 11 assume every term here without redefining it.
This is the foundation for the Class 11 Chemistry chapter Structure of Atom, examined in both JEE Main and NEET. That chapter works through the same sequence of models and then goes considerably further: Bohr's postulates are made quantitative, with formulas for the radius and the energy of the th orbit, and the energy released in a jump is calculated and matched to the observed spectral lines of hydrogen. The Class 9 statement that an electron absorbs energy going outward and emits it coming inward becomes the equation that predicts those lines.
Where Rutherford's limitation is developed. The objection used above — that an accelerating charge radiates — is stated formally in Class 11 and is the reason the quantum model is needed. It also connects to Class 12 Physics Atoms, where the same gold foil reasoning gives an estimate of nuclear size, and to Dual Nature of Radiation and Matter.
Dalton's postulates feed into Class 11 Chemistry Some Basic Concepts of Chemistry, which opens with the laws of chemical combination that his theory explains, and then builds the mole concept on them. That chapter is common ground for JEE Main and NEET.
What the questions look like. For this particular page, assertion-reason items are the most common form in NEET, and the standard pairing offers a statement about the foil experiment with a reason about the nucleus. Match-the-column questions pair a model with its limitation, or a scientist's name with a feature of the atom. Observation-to-conclusion questions give one of the three foil observations and ask what it implies — which is why learning them as pairs matters. JEE Main moves quickly past the descriptive material to the numerical Bohr model, so the qualitative content here is the entry requirement rather than the destination.
How board and competitive emphasis differ. A board paper asks you to state Thomson's model with a diagram, list the foil observations and conclusions, and give Bohr's postulates — descriptive answers, marked for completeness. A competitive paper rarely asks for a description; it asks which model failed to explain a named observation, or which conclusion follows from a named result. So the failures matter more than the features, and a student who memorised four models without their limitations has prepared for the board paper only.
The single trap that costs the most marks. Attaching the wrong limitation to the wrong model. Thomson's model was defeated by the gold foil experiment; Rutherford's was defeated by the instability of a revolving electron. Swapping those two is the commonest error in the chapter, and the fix is to remember the order: the experiment created the nucleus, and the nucleus created the stability problem.
A second trap worth naming. Saying the foil experiment showed the atom has a nucleus because most particles were deflected. Most were not deflected, and that is the observation proving the atom is mostly empty. The nucleus follows from the few that came back — and reading the observation carefully is the whole of the question.
This is the foundation for the Class 11 Chemistry chapter Structure of Atom, examined in both JEE Main and NEET. That chapter works through the same sequence of models and then goes considerably further: Bohr's postulates are made quantitative, with formulas for the radius and the energy of the th orbit, and the energy released in a jump is calculated and matched to the observed spectral lines of hydrogen. The Class 9 statement that an electron absorbs energy going outward and emits it coming inward becomes the equation that predicts those lines.
Where Rutherford's limitation is developed. The objection used above — that an accelerating charge radiates — is stated formally in Class 11 and is the reason the quantum model is needed. It also connects to Class 12 Physics Atoms, where the same gold foil reasoning gives an estimate of nuclear size, and to Dual Nature of Radiation and Matter.
Dalton's postulates feed into Class 11 Chemistry Some Basic Concepts of Chemistry, which opens with the laws of chemical combination that his theory explains, and then builds the mole concept on them. That chapter is common ground for JEE Main and NEET.
What the questions look like. For this particular page, assertion-reason items are the most common form in NEET, and the standard pairing offers a statement about the foil experiment with a reason about the nucleus. Match-the-column questions pair a model with its limitation, or a scientist's name with a feature of the atom. Observation-to-conclusion questions give one of the three foil observations and ask what it implies — which is why learning them as pairs matters. JEE Main moves quickly past the descriptive material to the numerical Bohr model, so the qualitative content here is the entry requirement rather than the destination.
How board and competitive emphasis differ. A board paper asks you to state Thomson's model with a diagram, list the foil observations and conclusions, and give Bohr's postulates — descriptive answers, marked for completeness. A competitive paper rarely asks for a description; it asks which model failed to explain a named observation, or which conclusion follows from a named result. So the failures matter more than the features, and a student who memorised four models without their limitations has prepared for the board paper only.
The single trap that costs the most marks. Attaching the wrong limitation to the wrong model. Thomson's model was defeated by the gold foil experiment; Rutherford's was defeated by the instability of a revolving electron. Swapping those two is the commonest error in the chapter, and the fix is to remember the order: the experiment created the nucleus, and the nucleus created the stability problem.
A second trap worth naming. Saying the foil experiment showed the atom has a nucleus because most particles were deflected. Most were not deflected, and that is the observation proving the atom is mostly empty. The nucleus follows from the few that came back — and reading the observation carefully is the whole of the question.
Key takeaways
Atomic models and the gold foil experiment: quick revision
- Each model was replaced by one observation it could not explain — that is the pattern to carry through the chapter.
- The idea of indivisible particles is old: anu and parmanu in early Indian thought, atomos meaning uncuttable in Greek. Both were philosophical, not testable.
- Dalton's postulates: matter is made of atoms; atoms are indivisible and not created or destroyed in a reaction; atoms of the same element are identical; atoms of different elements differ; atoms combine in small whole-number ratios.
- It explains the law of conservation of mass and fixed composition — g of carbon and g of oxygen give exactly g of carbon dioxide.
- Two postulates failed: the atom is divisible, and isotopes of one element differ in mass.
- Thomson's model: a sphere of positive charge with electrons embedded in it, overall neutral — the watermelon picture.
- It explained neutrality and the presence of electrons, but forbade a fast positive particle from being turned back — which the foil experiment observed.
- Gold foil experiment: fast alpha particles (charge , mass about u) fired at a very thin gold foil.
- Observations and conclusions: most passed straight through, so the atom is mostly empty space; some deflected slightly, so the positive charge is concentrated; a very small fraction came back through large angles, so there is a tiny dense positive nucleus.
- Rutherford's nuclear model: nearly all the mass and all the positive charge in a very small nucleus, electrons revolving around it, the rest empty.
- Its limitation: a revolving electron changes direction, so it accelerates, and an accelerating charge radiates energy — the electron should spiral in and the atom collapse.
- Bohr's postulates: electrons revolve only in permitted orbits; while in a permitted orbit they do not radiate, so the atom is stable; energy is absorbed or emitted only on jumping between orbits.
- Shells are labelled K, L, M, N for .
- Discrete energy levels explain why each element emits light of particular colours.
- Bohr kept the nucleus and changed only the electrons, and later models replace the orbit with a region of probability.
- An atom is mostly empty, and solidity is a matter of electron repulsion, not filled space.
Write the three foil observations down the left of a page and their conclusions down the right, then cover one side and reproduce the other — that pairing is what nearly every question on this topic asks for.
- The idea of indivisible particles is old: anu and parmanu in early Indian thought, atomos meaning uncuttable in Greek. Both were philosophical, not testable.
- Dalton's postulates: matter is made of atoms; atoms are indivisible and not created or destroyed in a reaction; atoms of the same element are identical; atoms of different elements differ; atoms combine in small whole-number ratios.
- It explains the law of conservation of mass and fixed composition — g of carbon and g of oxygen give exactly g of carbon dioxide.
- Two postulates failed: the atom is divisible, and isotopes of one element differ in mass.
- Thomson's model: a sphere of positive charge with electrons embedded in it, overall neutral — the watermelon picture.
- It explained neutrality and the presence of electrons, but forbade a fast positive particle from being turned back — which the foil experiment observed.
- Gold foil experiment: fast alpha particles (charge , mass about u) fired at a very thin gold foil.
- Observations and conclusions: most passed straight through, so the atom is mostly empty space; some deflected slightly, so the positive charge is concentrated; a very small fraction came back through large angles, so there is a tiny dense positive nucleus.
- Rutherford's nuclear model: nearly all the mass and all the positive charge in a very small nucleus, electrons revolving around it, the rest empty.
- Its limitation: a revolving electron changes direction, so it accelerates, and an accelerating charge radiates energy — the electron should spiral in and the atom collapse.
- Bohr's postulates: electrons revolve only in permitted orbits; while in a permitted orbit they do not radiate, so the atom is stable; energy is absorbed or emitted only on jumping between orbits.
- Shells are labelled K, L, M, N for .
- Discrete energy levels explain why each element emits light of particular colours.
- Bohr kept the nucleus and changed only the electrons, and later models replace the orbit with a region of probability.
- An atom is mostly empty, and solidity is a matter of electron repulsion, not filled space.
Write the three foil observations down the left of a page and their conclusions down the right, then cover one side and reproduce the other — that pairing is what nearly every question on this topic asks for.