One element in the third period of the periodic table that has three valence electrons is aluminum (Al).
To understand why aluminum has three valence electrons, we need to take a look at its position in the periodic table. The third period includes the elements sodium (Na), magnesium (Mg), aluminum (Al), silicon (Si), phosphorus (P), sulfur (S), chlorine (Cl), and argon (Ar). Aluminum is the third element in this row.
Aluminum is in group 13 (IIIA) of the periodic table, which is significant because the group number corresponds to the number of valence electrons for the elements in that group. Since aluminum is in group 13, it has three electrons in its outermost shell, which are responsible for bonding and chemical reactions. Therefore, aluminum is correctly identified as having three valence electrons.