Question

An atom contains 31 protons, 39 neutrons and 31 electrons.Identify the mass number of this atom. 70 *

Answer 1

Atomic Mass is equal to mass of p,e,n
But mass is electron is very law that is 1/1837 of mass of proton Hence we consider only mass of protons and neutrons.

Answer 2

The mass number of the atom is 70.

• The atomic number of an atom is defined as the number of protons present in the atom.
• Here the atomic number of the atom is 31.
• Again the mass number of an atom is the sum of the number of protons and the number of neutrons in that atom.
• Hence the mass number of the atom is 39+31=70.