The number of chlorine atoms in \( 20 \ \text{mL} \) of chlorine gas at STP is ________ \( \times 10^{21} \). (Round off to the Nearest Integer) [Assume chlorine is an ideal gas at STP \( R = 0.083 \ \text{L bar mol}^{-1}\text{K}^{-1}, \ N_A = 6.023\times 10^{23} \)]
