[cliffclof]

Why can atoms only have a max of 3 valence electrons?   Why can't Cobalt take one each of the two paired electrons from 3d orbital and the 4s orbital and give 4 valence electrons up ending up looking like [Ar] 3d5?

Hope this is the right section to ask.  Thanks.

[Borek]

Why can atoms only have a max of 3 valence electrons?

They can have more, so question doesn't make sense.

Why can't Cobalt take one each of the two paired electrons from 3d orbital and the 4s orbital and give 4 valence electrons up ending up looking like [Ar] 3d5?

Several compounds of tetravalent cobalt are known, so obviously it is possible.

[cliffclof]

Thanks for taking your time to answer my question.   It was not obvious to me.  IT is now.   The book I am reading states, "...the loss or gain of one, two or sometimes even three electrons can occur, but an element doesn't lose more than three electrons." -John T. Moore

I found this statement odd especially with Tin.

[AWK]

Look at electronegativities. Sn form SnH₄, but in this compound electrons are moved towards Sn.