Given: Atomic number, Z = 3
The electronic configuration of the element = K-shell has 2 electrons; L-shell has 1 electron.
As we know that L-shell should have 8 electrons to attain stability of atom, so the atom tends to lose one electron from its L-shell rather than gaining 7 electrons which will consume more energy. As the atom loses one electron from its L-shell, so its last shell is the K-shell which should have 2 electrons for a stable atom. So after losing one electron from its L-shell, the atom becomes stable. As it loses one electron, so its valency is one.
Valency of the given atom= 1
The element with atomic number 3 is Lithium.