Physics · Atoms · NEET
Spread out evenly over the whole atom. This is the single most important point. In Thomson's model the positive charge (the pudding) fills the entire sphere uniformly, and the mass is spread out too. There is NO tiny central nucleus. The nucleus idea came later from Rutherford. If a question says the positive charge is at the centre, that is Rutherford, not Thomson.
Because it looks like an English dessert called plum pudding, where dried fruits (plums) are stuck all over inside a soft cake. The positive sphere is the pudding, and the electrons are the plums stuck inside. In India teachers often say watermelon model: red flesh is positive charge, black seeds are electrons.
No. There is no nucleus in Thomson's model. The positive charge is not packed into a small dense point. Because of this, an alpha particle passing through such an atom feels only a weak, spread-out push and cannot be turned back. The nucleus was discovered later by Rutherford's alpha scattering experiment.
It could not explain Rutherford's alpha-particle scattering experiment. Thomson's spread-out positive charge is too weak to bounce heavy, fast alpha particles backward. But some alpha particles did bounce back almost straight (angle near 180 degrees). This is only possible if all the positive charge and mass are packed into a tiny hard nucleus, which Thomson's model did not have.
The electrons (negative charge) are the plums. The positive charge sphere is the pudding. Students often flip this. Remember: plums are small pieces stuck inside, just like the small electrons stuck inside the big positive sphere.
Given below are two statements: Statement I: Atoms are electrically neutral as they contain equal number of positive and negative charges. Statement II: Atoms of each element are stable and emit their characteristic spectrum. In the light of the above statements, choose the most appropriate answer.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
J. J. Thomson proposed it in 1898, a few years after he discovered the electron in 1897 using cathode ray experiments. It was the first real model of atomic structure.
One, the atom is a positively charged sphere. Two, electrons are stuck inside this sphere. Three, the positive charge and mass are spread uniformly. Four, the total negative charge equals the total positive charge, so the atom is neutral.
It is also called the watermelon model, the raisin pudding model, or simply Thomson's atomic model. All describe the same idea: small negative electrons stuck inside a large positive sphere.
Yes. This was its main success. Since the positive charge of the sphere exactly balances the negative charge of all the electrons, the atom has zero net charge. It correctly explained why atoms are electrically neutral.
Rutherford's alpha-particle scattering (Geiger-Marsden) experiment. A few alpha particles bounced back at large angles, which a spread-out positive charge could never cause. This led to Rutherford's nuclear model with a tiny dense nucleus.
