Explanation:
Write the electronic configuration and then count number of paired electrons in valence shell.
Atomic number $17=1 \mathrm{~s}^2, 2 \mathrm{~s}^2, 2 \mathrm{p}^6, 3 \mathrm{~s}^2, 3 \mathrm{p}^5$
Valence shell has $3 \mathrm{~s}^2$ and $3 \mathrm{p}^5$ orbitals

$\therefore$ Valence shell has 3 paired electrons.