Question

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

Answer 1

Formula used:
           No. of Protons + No. of Neutrons = (No. of electrons) = Mass number.

Mass number of He = 4
No.of Protons in He = 2

Using the formula,    
              No. of Protons + No. of Neutrons = Mass number.
              No. of Neutrons = Mass number - No.of Protons
              No. of Neutrons = 4-2 = 2

Therefore, No.of Neutrons in Helium atom = 2

Answer 2

HI !

We know that ,

No. of Protons + No. of Neutrons = Mass number.

Mass number of Helium = 4

No.of Protons in Helium = 2

No. of Protons + No. of Neutrons = Mass number

Then ,
No. of Neutrons = Mass number - No.of Protons
= 4 - 2
= 2
Hence ,
Number of Neutrons in Helium atom = 2