# How do you calculate the number of valence electrons in an atom?

Apr 15, 2014

For elements in Groups 1, 2, and 12 to 18, a valence electron is an electron that has highest principal quantum number n.

Example:

For example, how many valence electrons are in arsenic?

Solution: Arsenic is in the fourth row of the Periodic Table, so we count from left to right starting with K. From K to Ca, we are filling 4s orbitals. From Sc to Zn, we are filling 3d orbitals, but these are not valence electrons. They do not have the highest principal quantum number (n = 3 instead of 4). From Ga to As, we are putting electrons into 4p orbitals, and these are valence electrons.

Arsenic has five valence electrons.

Transition Metals

Transition metals (Groups 3 to 11) have incomplete d subshells. These atoms can use their d electrons for bonding. So the valence electrons for a transition metal are the ns and (n-1)d electrons.

This means that manganese (Mn) has the electron configuration [Ar]4s²3d⁵ and 7 valence electrons.