Structure Of The Atom

What you will learn

• Credit for the discovery of electron and proton goes to J.J.Thomson and E.Goldstein, respectively.

• J.J. Thomson proposed that electrons are embedded in a positive sphere.

• 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.

• Shells of an atom are designated as K,L,M,N,....

• Valency is the combining capacity of an atom.

• The atomic number of an element is the same as the number of protons in the nucleus of its atom.

• The mass number of an atom is equal to the number of nucleons 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.

• Elements are defined by the number of protons they possess.