Question 2 Page 49 (Bottom) - Chapter 4 Class 9 - Structure of Atom

Helium atoms have an atomic mass of 4 u and two protons in its nucleus. How many neutrons does it have?

Answer

We know that nearly all the mass of an atom is because of the nucleus.

The mass of the nucleus is due to the presence of protons and neutrons .

Mass of a proton = Mass of a neutron = 1u

Given, 

Atomic mass = 4u

Number of protons = 2 → Mass = 2u

Mass of neutrons = atomic mass - mass of protons

              = 4u - 2u

                             = 2u

2 u →  2 neutrons