Question

atomic number of Lithium is 3 and its mass number is 7 then how many protons and neutrons are present in a Lithium atom

Answer 1

protons are = atomic number

=3

neutrons are 7 -3 = 4

Answer 2

Atomic no: = 3

No: of protons = atomic no: = 3
No: of electrons = 3

Mass no: = 7

No: of protons + NO: of neutrons = Mass no:
NO: of neutrons =  Mass number - No: of protons 
                          = 7- 3
                          = 4