Electrons in Completely Filled Subshells (n = 4) | JEE Main PYQ | Structure of Atom
Q. The number of electrons present in all the completely filled subshells having n = 4 and s = +1/2 is ______.

(Where n = principal quantum number and s = spin quantum number)

Correct Answer: 16

Explanation

For n = 4, possible subshells are:

4s, 4p, 4d, 4f

Completely filled subshells contain maximum electrons:

  • 4s → 2 electrons
  • 4p → 6 electrons
  • 4d → 10 electrons
  • 4f → 14 electrons

In a completely filled subshell, half the electrons have spin +1/2.

So number of electrons with s = +1/2:

  • 4s → 1
  • 4p → 3
  • 4d → 5
  • 4f → 7

Total = 1 + 3 + 5 + 7 = 16

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