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Atomic structure that is common to noble gases, explaining a common property:
Electrons fill all the states in the outermost shell. For example, helium has two electrons that fill its 1s shell. For the other elements in Group VIII, the six electrons in the p subshells fill them to capacity. Any additional electrons would have to be added to a higher-energy s subshell, and any electrons taken away would cause the p subshell to be unfilled. Since the noble gases are satisfied with their outermost filled shells, they do not easily react with other elements.
Atomic structure that is common to halogens, explaining a common property:
Group VII halogens need just one electron to have a filled shell, i.e., they have 5 electrons in the outer p subshell. They would love to accept an electron from another atom, and are quite reactive. In particular, they prefer atoms that have one extra electron in their outer shells (such as alkali metals).
Atomic structure that is common to alkali metals, explaining a common property:
The alkali metals, in Group I, have a donor electron in their outer s subshell. This electron has significantly more energy than the electrons in the filled shells beneath it, so the alkali metal is eager to donate the excess electron. Alkali metals are particularly reactive, especially with halogens.
