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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?

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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.

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