Which electrons in an atom are involved in forming a chemical bond?

The electrons in the outermost electron shell are responsible for forming chemical bonds. These electrons are also known as valence electrons. Chemical bonds are formed when the outermost electron shells are not completely filled and atom needs more (or less) electrons to completely fill the outermost shell. For example, sodium has an atomic number of 11 and an electronic configuration of 1s2, 2s2, 2p6, 3s1. Thus, sodium has 1 valence electron and it readily loses it to gain a fully filled outermost shell (2p in this case). Chlorine, on the other hand, has 7 valence electrons and gains an electron to fully fill its outermost shell (3p). In some case, reacting atoms share their valence electrons (covalent bond, typical in case of non-metals), instead of losing or gaining them (as in ionic bonding) and achieve fully filled outermost electron shells.

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