Why are first period elements unusual?

The first question on my test was; why are H and He separated from the rest of the elements in the periodic table of elements. The answer is not because they are the first 2 elements and it's not because of their atomic weight.

Each row (or "period") on the periodic table corresponds to filling a different "shell" of electrons. (A shell is a group of electrons with the same principal quantum number, n). H and He are filling the first shell, with n = 1. This shell holds two electrons at most (fewer than any other shell) and H and He are the only elements in the first period. Because there aren't any shells below the first, hydrogen and helium are the only elements without core electrons. That gives them some unusual properties.

Hydrogen's single electron configuration makes it unique. It can lose the electron to form H+, or gain an electron to become H-. It doesn't belong with the alkali metals, and it doesn't belong with the halogens. We don't know quite what to do with hydrogen, and on some periodic tables it floats in the space above the rest of the table- the element without a family.

But helium should not be considered an orphan. It is clearly a noble gas, and belongs above Ne on the table. Although it's different from the others because of its smaller size, the same can be said of the first element in every family.

Author: Fred Senese senese@antoine.frostburg.edu

General Chemistry Online! Why are first period elements unusual?

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