How can I find valence electrons of transition metals?
Valence electrons are the sum total of all the electrons in the highest energy level (principal quantum number n). Most transition metals have an electron configuration that is
$$ ns^2 (n-1) d $$, so those $$ ns^2 $$ electrons are the valence electrons.
For example. How many valence electrons are there in Fe?
Solution: 2 valence electrons.
Reason: The electron configuration of Fe is $$ 1 s^2 2s^2 2p^6 3s^2 3p^6 4s^2 3d^5 $$. The two 4s electrons are in the highest principal quantum number, n = 4, so they are the valence electrons.
Copper and chromium have one valence electron (they are exceptions), because they have one 4s electron. Chromium has an electron configuration of [Ars] because having a half filled ^3d subshell is more stable, so it has one valence electron. Copper has one valence electron (the 4s electron) because it has electron configuration of $$ [ Ar]4s^13d^5 $$having filled and a half fille 4s subshell is more stable than $$ [Ar]4s^2 3d^9 $$