Answer
Position of an element is due to its atomic number, i.e, number of protons. Elements with same number of valence electrons are placed in same group and exhibit similar chemical properties and reactions.
Examples
Na & Li ar metals of Group-1. They have valancy of +1 and exhibit similar chemical properties.
2Na + O$_{2}$ = 2Na$_{2}$O
2Li + O$_{2}$ = 2Li$_{2}$O
Ca & Mg ar elements of Group-2. They have valancy of +2 and exhibit similar chemical properties
2Ca + O$_{2}$ = 2CaO
2Mg + O$_{2}$ = 2MgO
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