Question

The scientist who is credited with the discovery of ‘nucleus’ in an atom is:
(A) Niels Bohr
(B) Rutherford
(C) J.J. Thomson
(D) Balmer

Answer 1

Hint Atoms are electrically neutral, the positive charge on the nucleus is equal to the total negative charge of all the orbiting electrons. Geiger and Marsden did experiments and found that some of the alpha particles are deviated rather than going straight. This led to the well-known gold foil experiment, which led to the discovery of the nucleus.

Complete Step-by-step solution
The concept of the nucleus in an atom is the most important contribution of scientist Rutherford and his model is a landmark in the development of the modern atomic theory.
An experiment was performed by Geiger and Madsen. In this experiment, a narrow beam of alpha particles from a radioactive source was incident on the gold foil. The scattered alpha particles were detected by a detector fixed at a rotating stand. The whole setup is evacuated in the chamber. They observed the number of alpha particles as a function of the scattering angle. The deviation of alpha particles from their original direction is the scattering angle. It is observed that most of the alpha particles passed undeviated and only a few are scattering. This anomalous behavior seemed interesting to Rutherford and based on observations made by Geiger and Madsen, he proposed his atomic model. He said that a large scattering angle is possible only because alpha particles are repelled by a massive positive charge in a very small space. He called this massive center a nucleus.
By studying the scattering of alpha particles and the trajectory of the scattered alpha particle, he determined the size of the nucleus which is ${10}^{-15}$ to ${10}^{-14}$ meter. The size of the nucleus is found $100000$ times smaller than the size of an atom. According to Rutherford, the atom has a tiny positively charged core called a nucleus. The total charge and mass are included in the nucleus only. The nucleus is surrounded by negatively charged electrons orbiting around the nucleus in circular orbits.

Hence, the correct option is (B) Rutherford.

Note Up to $1897$, it was believed that atoms were invisible until J.J. Thomson discovered electrons. He showed that electrons have a negative charge on it. After that Rutherford came with the nucleus theory. But his theory was unable to explain the hydrogen spectrum. One of the series of hydrogen spectrum was explained by Balmer. Neils Bohr, in $1913$ modified the theory of Rutherford.