The number of chlorine atoms in 20mL of chlorine gas at STP is...

$PV = nRT$

$1 . 0 \times \dfrac{20}{1000} = \dfrac{N}{6 . 023 \times {10}^{23}} \times 0 . 083 \times 273$

$∴$ Number of ${Cl}_{2}$ molecules, $N = 5 . 3 \times {10}^{20}$

Hence, Number of $Cl$ -atoms $= 1 . 06 \times {10}^{21}$

$\approx 1 \times {10}^{21}$