Chemistry · Structure Of Atom · NEET
Thomson (1898) said positive charge is spread evenly over the whole atom, with electrons stuck inside it like seeds in a watermelon or plums in a pudding. There is no nucleus in Thomson's model. Rutherford (1911) said the positive charge and almost all the mass sit in a very tiny centre called the nucleus, and electrons move in the large empty space around it. So the key change is: Thomson = charge spread out everywhere; Rutherford = charge concentrated in a tiny nucleus.
Rutherford, with Geiger and Marsden, fired positive alpha-particles at a very thin gold foil. Three results: (1) Most alpha-particles passed straight through, so the atom is mostly empty space. (2) A few were deflected by small angles, so there is a positive charge inside that pushes them. (3) A very few (about 1 in 20,000) bounced almost straight back, so the positive charge and mass are packed in a very small, dense region. This dense centre is the nucleus.
By classical physics, any charged particle moving in a circle is accelerating, and an accelerating charge must give off energy (radiation). So the electron would keep losing energy and spiral into the nucleus in about 10 to the power minus 8 seconds. This means Rutherford's atom should collapse and cannot be stable, but real atoms are stable. This is its main drawback. It also could not explain the line spectrum (the fixed colours) of atoms or say anything about how electrons are arranged. This matters for NEET because these gaps are exactly why the Bohr model was made next.
In the Rutherford picture they should fall in, and that is the flaw. The real answer came later from Bohr, who said electrons can only stay in fixed energy levels (orbits) and do not radiate energy while in them. So no energy is lost and the electron does not spiral in. Remember: Rutherford's model cannot answer this, and that failure is the reason NEET asks about Bohr's postulates next.
The radius of a whole atom is about 10 to the power minus 10 metres, while the radius of the nucleus is about 10 to the power minus 15 metres. So the nucleus is roughly 100,000 times smaller than the atom in radius. This is why almost all alpha-particles missed it and passed straight through the empty space.
The number of protons, neutrons and electrons in Lu-175 (atomic number 71, mass number 175), respectively, are:
Dalton's atomic theory could not explain which of the following?
Try the real previous-year questions from this chapter — each with the answer and a full solution.
J. J. Thomson proposed it in 1898. It is also called the raisin pudding or watermelon model, because the positive charge is like the pudding and the electrons are like the plums or seeds spread inside it.
It correctly introduced the nucleus: a tiny, dense, positively charged centre that holds almost all the atom's mass, with electrons moving in the empty space around it. This nuclear picture is still accepted today.
No. In Thomson's model the positive charge is spread evenly over the whole atom. The idea of a concentrated nucleus came only after Rutherford's gold foil experiment.
By classical physics, only about 10 to the power minus 8 seconds. Since real atoms do not collapse in that time, this proves the Rutherford model is wrong about stability.
The Bohr model. It fixes Rutherford's stability problem by allowing electrons only in fixed energy levels where they do not lose energy.
