Sample Material of Our Online Coaching Programme
Subject: General Science
Topic: Gist of Physics from NCERT Books
Atomic Physics
- An atom is the smallest particle of the element that can exist independently and retain all its chemical properties.
Dalton’s atomic theory, which suggested that the atom was indivisible and indestructible. But the discovery of two fundamental particles (electrons and protons) inside the atom, led to the failure of this aspect of Dalton’s atomic theory.
- Thomson proposed that:
i. An atom consists of a positively charged sphere and the electrons are embedded in it.
ii. The negative and positive charges are equal in magnitude. So, the atom as a whole is electrically neutral. - Rutherford’s alpha-particle scattering experiment led to the discovery of the atomic nucleus. Rutherford’s model of the atom proposed that a very tiny nucleus is present inside the atom and electrons revolve around this nucleus. The stability of the atom could not be explained by this model.
- Neils Bohr’s model of the atom was more successful. He proposed that electrons are distributed in different shells with discrete energy around the nucleus. If the atomic shells are complete, then the atom will be stable and less reactive.
- J. Chadwick discovered presence of neutrons in the nucleus of an atom. So, the three sub-atomic particles of an atom are: (i) electrons, (ii) protons and (iii) neutrons. Electrons are negatively charged, protons are positively charged and neutrons have no charges. The mass of an electron is about 1/2000 times the mass of an hydrogen atom. The mass of a proton and a neutron is taken as one unit each.
- We know that protons are present in the nucleus of an atom. It is the number of protons of an atom, which determines its atomic number. It is denoted by ‘Z’. All atoms of an element have the same atomic number, Z. In fact, elements are defined by the number of protons they possess.
- Mass of an atom is practically due to protons and neutrons alone. These
are present in the nucleus of an atom. Hence protons and neutrons are also
called nucleons. Therefore, the mass of an atom resides in its nucleus.
Isotopes are atoms of the same element, which have different mass numbers. - Isobars are atoms having the same mass number but different atomic numbers.
