Question

Total number of electrons present in "penultimate" potassium shell of potassium 19 is equal to

Answer 1

The penultimate shell of potassium (19) contains 8 electrons.

• Potassium has an atomic number of 19. Hence, its electronic configuration is $1s^{2}2s^{2}2p^{6}3s^{2}3p^{6}4s^{1}$ .
• Here, we note that the valence electron of potassium is in 4s subshell. This is because 3d has higher energy than 4s.
• The penultimate shell is M shell, containing 3s, 3p and 3d subshells. 2 electrons occupy 3s, 6 electrons occupy 3p, and no electrons occupy 3d.
• The total number of electrons in the penultimate shell (M shell) is equal to 2+6 = 8.