Selina ICSE Solutions for Class 10 Physics Chapter 12 Radioactivity Exercise 12(A) Solution 1. Three constituent of an atom are: Electrons: mass is 9.1 x 10 -31 kg, charge is -1.6 x 10 -19 C Neutron: mass is 1.6749 x 10 -27 kg, charge is zero. Protons: mass is 1.6726 x 10 -27 kg, charge is +1.6 x 10 -19 C Solution 2. Atomic number – the number of protons in the nucleus is called atomic number. Mass number – the total number of nucleons in the nucleus is called mass number. Solution 3. The nucleus at the centre of atom, whose size is of the order of 10 -15 m to 10 -14 m. The size of a nucleus is 10 -5 to 10 -4 times the size of an atom. It consists of protons and neutrons. If Z is the atomic number and A is the mass number of an atom, then the atom contains Z number of electrons; Z number of protons and A – Z number of neutrons. The atom is specified by the symbol Z X A where X is the chemical symbol...
