How can I find valence electrons of transition metals?

Most transition metals have 2 .

are the sum total of all the electrons in the highest energy level (principal quantum number n). Most transition metals have an 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 ##1s^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 ##[Ar] 4s^1 3d^5## 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^1 3d^10##. Having a filled 3d and a half fille 4s subshell is more stable than ##[Ar] 4s^2 3d^9##.