CBSE Class 9 Answered

(a) Rutherford's Atomic Model

• Based on the results of the α-particle scattering experiments, Rutherford put forth his atomic model.
• An atom contains a positively charged centre called the nucleus of the atom. Almost all the mass of the atom is concentrated in the nucleus.
• The electrons of the atom revolve around the nucleus in fixed, circular orbits.
• The size of the nucleus is many times smaller than the size of the atom. The nucleus of an atom is 10,000 times smaller than the atom.

(b) Valency is the combining capacity of an atom. The number of electrons which are present in the last shell and take part in chemical bond formation are known as valence electrons. The number of electrons which an atom has in its last orbits can be found out by writing the electronic configuration of the atom. So, electronic configuration expresses the valence electrons which are related to the valency of an atom.

(c) Atomic numbers and electronic configuration.