The Transition Metals
At first sight, they look like a confusing and irritating break in the
pattern. For one thing, they can't be divided neatly into groups;
all of them have very similar properties. Also, unlike the
elements we've been talking about, they don't always use the same number
of valence electrons in chemical reactions. Iron (Fe), for example, sometimes likes to give
away two electrons, and sometimes three.
The result is that all the transition metals
have the same arrangement of outer electrons; only the 3d orbitals,
lower down, are different.
And that's why all the transition metals are so much alike!
Exactly. You can also see why they might keep changing their minds about
how many valence electrons they have. Iron sometimes gives up just its
two 4s electrons, and sometimes throws in a 3d, since those are so close