Question

How many electrons are present in an uncharged atom with its 1s, 2s, 2p, 3s, 3p and 4s subshells filled and with five electrons in the 3d subshell?Express your answer as an integer?

Answer 1

25

Explanation

The orbital quantum number of an atomic subshell determines the maximum number of electrons it can contain. For example:

• Each $s$ orbital $(l = 0)$ holds up to $4 \; l + 2 = 2$ electrons
• Each $p$ orbital $(l = 1)$ holds up to $4 \; l + 2 = 6$ electrons
• Each $d$ orbital $(l = 2)$ holds up to $4 \; l + 2 = 10$ electrons

The atom in question here contains four fully-occupied $s$ orbitals, two fully-occupied $p$ orbitals along with $5$ electrons in a partially-occupied $d$ orbital. It thus contains

$4 \times 2 + 2 \times 6 + 5 = 25 \; \text{electrons}$