Question

If an atom ‘A’ has atomic number 19 and mass number 39, state –

1. Its electronic configuration.
2. The number of valence electrons it possesses.

Answer 1

Atom ‘A’ has mass number A = 39 and atomic number Z = 19 = p
∴ A = Z + n
A = p + n
39 = 19 + n
n = 39 – 19 = 20
But e = p = 19

1. A (K, L, M, N)
   19 = 2, 8, 8, 1
   There will be 2 electrons in K-shell or 1st shell
   8 electrons in 2nd shell or L-shell
   8 electrons in 3rd shell or M-shell
   1 electron in 4th shell or nth-shell
2. The number of valence electrons i.e. in outer most shell = 1 electron.