Question

Lithium atom has atomic mass 6u and 3 protons in its nucleus how many neutrons does it have

Answer 1

3 neutrons.
Mass no. - 6 u.
No. of protons- 3
No. of neutrons- Mass no. - no. of protons
6 - 3 = 3 neutrons...
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Answer 2

obviously
atomic mass= NO.Proton + NO.Neutron

6 = 3 +NO. N
NO. N = 6 -3
= 3