Question

What is the nucleus of an atom? Compare its size with that of the atom. Name its constituents. How is the number of these constituents determined by the atomic number and mass number of the atom?

Answer 1

The nucleus is at the centre of an atom, consists of protons and neutrons and thus has positive charge. The nucleus of an atom is about ${10}^{-15}$ m in size. It is ${10}^{-5}\text{to}{10}^{-4}$ times the size of an atom.

The total number of protons and neutrons in the nucleus is called the mass number (denoted with symbol, A). The total number of protons in the nucleus of an atom is called the atomic number (denoted with symbol, $Z$).
If Z is the atomic number and A is the mass number of a neutral atom, then
Number of proton = Z
Number of electrons = Z
Number of neutrons = A - Z