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Meave60
Dec 4, 2015

C) The number of valence electrons.

Explanation:

Valence electrons are in the highest energy #"s"# and #"p"# sublevels. The number of valence electrons is the same for the elements in the same group. For example, all of the alkali metals in group 1 have 1 valence electron in their highest energy sublevel. We can see this by looking at their electron configurations.

Electron Configurations of Alkali Metals Using Noble Gas Notation
#"Li":##"[He]2s"^1"#
#"Na":##"[Ne]3s"^1"#
#"K":##"[Ar]4s"^1"#
#"Rb":##"[Kr]5s"^1"#
#"Cs":##"[Xe]6s"^1"#
#"Fr":##"[Rn]7s"^1"#

By using the noble gas notation, it is very easy to see that all of the alkali metals have the same number of valence electrons, which is #"ns"^1"#.

The diagram below shows the valence electrons for the main group elements (representative elements) in periods 2 and 3. You can see that the members of each group have the same number of valence electrons.
http://www.chem.latech.edu/~upali/chem481/slides/Chemical%20Bonding.htm

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