Question

Three elements 'X', 'Y' and 'Z' have atomic numbers 18, 19 and 20 respectively. How many electrons are present in the M shells of these elements?

• A. 8, 9, 10
• B. 8, 10, 13
• C. 8, 8, 8
• D. 8, 9, 12

Answer 1

The correct option is C 8, 8, 8

Electronic configuration of X = 2, 8, 8 = $1s^{2}2s^{2}2p^{6}3s^{2}3p^6$
Electronic configuration of X = 2, 8, 8, 1 = $1s^{2}2s^{2}2p^{6}3s^{2}3p^{6}4s^1$
Electronic configuration of X = 2, 8, 8, 2 = $1s^{2}2s^{2}2p^{6}3s^{2}3p^{6}4s^2$
M shell is the third orbit of any element i.e. n = 3 and it contains maximum of 18 electrons
In element X, Y and Z, M-shell contains 8 electrons each.