How do you calculate the number of valence electrons in an atom?
1 Answer
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.
