Why does the group of the atom determine how many electrons are lost or gained?

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  • 4 years ago
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    The group number, particularly the older Roman numeral approach, tells the number of valence electrons an element has. The alkali metals in group IA (1) have 1 valence electron and can form one bond*. The elements in group IVA (14) have four valence electrons and can form up to four bonds, take carbon for example.

    Keep in mind that atoms only gain or lose electrons when they form ions, as when a compound dissolves in water. The alkali metals and the alkaline earth metals lose 1 or 2 electrons, respectively, as they form ions in solution. The transition metals can lose 2 or 3 electrons as they form ions. Only some of the transition metals have agreement between the number of electrons lost and the group number.

    As for the formation of anions, despite the halogens being in group VIIA, they gain one electron to form an ion. Of the chalcogens, S^2-, Se^2- and Te2- ions only form in very basic solutions. There is no oxide ion since it is too basic and reacts with water to make OH- ions. Likewise, there are no N^3-, P^3-, etc ions.

    The bottom line is that only certain group numbers agree with the number of electrons gained or lost.

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  • 4 years ago

    Because the groups are organized that way.

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