The metals will lose electrons while the non metals will gain electrons in order to attain octet structure.
An ion can be cation (positively charged) or anion (negatively charged).
Cations attain octet structure (8) by losing electron(s) while anions become stable or attains octet structure (8) by gaining electron(s).
The remaining elements are completed as follows to attain octet structure;
Element--valence electron--electrons to gain--electrons to lose--ion formed
O ------------ 6 ---------------------- 2 ------------------------ none -------------- [tex]O^{2-}[/tex]
Ca -------- 2 ----------------------- none ---------------------- 2 ------------------ [tex]Ca^{2+}[/tex]
Br ----------- 7 --------------------- 1 ------------------------ none --------------- [tex]Br^{-}[/tex]
S ------------ 6 ----------------------- 2 ------------------------ none --------------- [tex]S^{2-}[/tex]
Cl ------------ 7 ----------------------- 1 ------------------------ none ----------------[tex]Cl^{-}[/tex]
K -------------- 1 ----------------------- none ----------------------- 1 ------------------ [tex]K^{+}[/tex]
Mg ------------ 2 ---------------------- none ---------------------- 2 ---------------- [tex]Mg^{2+}[/tex]
Be ------------- 2 ---------------------- none ---------------------- 2 ---------------- [tex]Be^{2+}[/tex]
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