An atom has an atomic mass of 7u. It has 2 electrons in K-shell and one electron in L-shell. How many neutrons does it have?
Atomic mass of atom = 7u
Electrons in K shell = 2
Electrons in L shell = 1
Total electrons = 2 + 1 = 3
Total protons = 3
The no. of electrons and proton and neutrons in any neutral atom are equal.
Total neutrons = atomic mass – protons
= 7 – 3 = 4
Atom has 4 neutrons.