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Why do many Group 16 atoms tend to gain two electrons?

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A color-coded periodic table of the elements displays each element's symbol and atomic number, with categories such as alkali metals, noble gases, halogens, transition metals, and actinoid elements distinguished by different background colors. — oxygen group element Modern version of the periodic table of the elements (printable).

Atoms in Group 16 of the periodic table possess six electrons in their outermost electron shell. According to the octet rule, atoms with atomic numbers close to those of the noble gases tend to gain, lose, or share electrons to achieve a stable configuration of eight valence electrons.123

Because Group 16 elements already have six valence electrons, they require two additional electrons to complete this stable octet. By gaining two electrons, these atoms form ions with a double negative charge, effectively attaining the stable electronic structure characteristic of the noble gases. This tendency is a fundamental aspect of their chemical behavior, though it is most pronounced in lighter elements like oxygen; as one moves down the group, the elements increase in size and atomic number, and their tendency to gain electrons decreases, with the heaviest members often favoring positive oxidation states instead.12