Some abnormalities in the Periodic Table

I observe some elements like $\ce{Cr}$ and $\ce{Cu}$. Some of them have strange oxidation numbers that don't really fit the purpose of making the element as stable as possible. Just Chromium, for example, it has one valence electron in 4s, the oxidation tells that it is gonna lose 2 electrons. Which obviously breaks the stability of half filled 3p orbitals. 


If this can be explained by the fact that $\ce{Cr}$ has weak electronegativity, then what about $\ce{Cu}$? It is among the most electronegative transition metals, but still might lose two electrons despite having fully filled 4d orbitals.


