The Lewis symbols (one pair = 2 é, one point = 1 é) represent the number of →    outer electrons of neutral atoms:

In other words, if e.g. $ O $ has 8 outer electrons, it will have the charge of two excess electrons = 2 negative elementary charges ($ -2 \; \cdot \; 1,6 \cdot 10^{-19} \; C $) $ Cl $ has 6 outer electrons, it will have the charge of one excess proton = 1 positive elementary charge ($ + \; 1.6 \cdot 10^{-19} \; C $)