atomic-models-from-thomson-to-bohr MCQs for UPSC Prelims

25 practice questions on atomic-models-from-thomson-to-bohr from the Journey Inside the Atom section of the UPSC Prelims syllabus. 25 come with a written explanation. Try the sample set below - the answer stays hidden until you ask for it.

8 Easy 12 Medium 5 Hard

Sample questions

Q1
medium
The gold sheet used in the scattering experiment was made extremely thin. Suppose a much thicker gold sheet had been used instead, with everything else kept the same. What change in the results would you expect?
  1. A A larger share of the alpha particles would be deflected, as each would meet more nuclei on its way through
  2. B Each gold nucleus would become larger and easier to hit
  3. C Every alpha particle would pass straight through without bending at all
  4. D The alpha particles would change into negatively charged particles
Show answer and explanation

Correct answer: A - A larger share of the alpha particles would be deflected, as each would meet more nuclei on its way through

A thicker sheet means more layers of atoms, so an alpha particle has more chances of passing close to a nucleus and being pushed off course. Making the sheet thicker cannot make deflection less likely. Passing through a metal does not change the charge of a particle, and the size of a nucleus is fixed by the atom, not by the thickness of the foil.
Q2
easy
When a narrow beam of alpha particles was fired at a very thin gold foil, what happened to most of the particles?
  1. A They were swallowed up by the foil and never came out
  2. B They were turned through large angles
  3. C They came straight back towards the source
  4. D They went straight through without being deflected
Show answer and explanation

Correct answer: D - They went straight through without being deflected

The great majority of alpha particles carried on in a straight line as though the foil were not there. Bouncing back happened to only a few particles and large deflections to some, and these rare events were exactly what made the result surprising. None of the particles were simply absorbed by the foil.
Q3
hard
Rutherford's model has the electrons revolving around the nucleus rather than resting quietly beside it. Why can an electron in this model not simply stay at rest a little away from the nucleus?
  1. A A resting electron would leave the atom positively charged overall
  2. B Being negative, it would be pulled by the positive nucleus and would fall into it
  3. C It would repel the nucleus and be thrown right out of the atom
  4. D It would lose its charge the moment it stopped moving
Show answer and explanation

Correct answer: B - Being negative, it would be pulled by the positive nucleus and would fall into it

Opposite charges attract, so a motionless electron would be dragged straight into the positive nucleus and the atom would not survive; revolving is the model's way of keeping the electron out there. Two opposite charges never repel. An electron keeps its charge whether it moves or not, and the atom's overall charge depends only on how many electrons it has, not on their speed.
Q4
medium
Alpha particles were used as the tiny bullets fired at the gold foil. What is an alpha particle?
  1. A A negatively charged particle far lighter than any atom
  2. B A positively charged particle given out by certain radioactive elements, made of two protons and two neutrons
  3. C An uncharged particle with a mass close to that of a proton
  4. D A ray of light given out when a metal is strongly heated
Show answer and explanation

Correct answer: B - A positively charged particle given out by certain radioactive elements, made of two protons and two neutrons

Alpha particles come from radioactive elements and each one is a helium nucleus, that is two protons and two neutrons, so it is positively charged and reasonably heavy. A light negative particle is an electron, an uncharged particle of about a proton's mass is a neutron, and light is not a particle beam of this kind.
Q5
hard
A student builds a model of Thomson's atom by pressing small beads, standing for electrons, into a ball of clay that stands for the positive charge. By mistake the clay is given less positive charge than the total negative charge on all the beads. What does the model now represent?
  1. A A neutral atom, because the two charges are still of opposite kinds
  2. B A particle with an overall negative charge, not a neutral atom
  3. C A particle with no charge anywhere inside it
  4. D A particle with an overall positive charge
Show answer and explanation

Correct answer: B - A particle with an overall negative charge, not a neutral atom

Neutrality needs the two amounts of charge to be equal, not merely opposite; if the negative charge is larger, what is left over is negative. Having opposite kinds of charge is not enough on its own. The excess cannot be positive when the positive part is the smaller of the two, and the charges inside have not gone away.
Q6
easy
Rutherford's nuclear model explained the gold foil results well, but it failed on one everyday fact about matter. Which fact was that?
  1. A The nucleus of an atom carries positive charge
  2. B Atoms are stable and do not collapse
  3. C Atoms of one element differ from atoms of another
  4. D Most of an atom is empty space
Show answer and explanation

Correct answer: B - Atoms are stable and do not collapse

The model could not say why a revolving electron does not lose energy and fall into the nucleus, so it could not account for atoms lasting at all. Most of the atom being empty and the nucleus being positive were successes of the model, not failures. Differences between elements were not what the model was struggling with.
Q7
medium
How does Bohr's model account for an atom not collapsing, even though its electrons keep revolving around the nucleus?
  1. A An electron is held completely still by the nucleus, so it never accelerates
  2. B An electron in an allowed shell is in a stationary state, keeping a fixed energy, so it does not spiral inwards
  3. C The nucleus gives up its positive charge once the electrons begin to revolve
  4. D Electrons move so fast that the pull of the nucleus cannot reach them
Show answer and explanation

Correct answer: B - An electron in an allowed shell is in a stationary state, keeping a fixed energy, so it does not spiral inwards

Bohr's answer was to lay down that certain shells are stationary states in which a moving electron keeps a constant energy and radiates none, so there is nothing to make it fall in. His electrons certainly do move, so they are not at rest. The nucleus keeps its positive charge, and its pull is what holds the electron in its shell, however fast the electron travels.
Q8
hard
An electron inside an atom shifts from a shell of lower energy to a shell of higher energy. According to Bohr's model, what must happen, and how much energy is involved?
  1. A It must absorb energy, and the amount is exactly the difference between the energies of the two shells
  2. B It must absorb energy, and any amount of energy at all will do
  3. C It must give out energy equal to the whole energy it has in the higher shell
  4. D It must give out energy, and the amount is exactly the difference between the energies of the two shells
Show answer and explanation

Correct answer: A - It must absorb energy, and the amount is exactly the difference between the energies of the two shells

Climbing to a higher energy level costs energy, so the electron has to take energy in, and Bohr allowed only the exact difference between the two levels, never a part of it or more than it. Giving out energy is what happens on the way down, not up. Accepting any amount at all would let an electron sit between shells, which Bohr's model forbids.

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