# Question #ac1e7

Dec 4, 2015

C) The number of valence electrons.

#### Explanation:

Valence electrons are in the highest energy $\text{s}$ and $\text{p}$ 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
$\text{Li} :$$\text{[He]2s"^1}$
$\text{Na} :$$\text{[Ne]3s"^1}$
$\text{K} :$$\text{[Ar]4s"^1}$
$\text{Rb} :$$\text{[Kr]5s"^1}$
$\text{Cs} :$$\text{[Xe]6s"^1}$
$\text{Fr} :$$\text{[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 $\text{ns"^1}$.

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.