Doubt From Chemical Bonding

I was asked to draw a dot-and-cross diagram for chlorine dioxide, and I was wondering why Cl in ClO2 will form one double bond with one O atom and another dative bond with another O atom. (This was the answer my school gave.)

Why can't Cl form one double bond with one O atom and another double bond with another O atom? I learnt that when it comes to drawing dot-and-cross diagrams, we should always consider drawing single bond first, followed by double, triple and then finally, dative. Also, considering how Cl can expand its octet since its in Period 3...

Can anyone enlighten me as to why we can't draw 2 double bonds like O=Cl=O?
Thank you so so much!

Cl has valency 1,3,5,7

As we know that Cl has 7 Valence electrons in its outermost orbital thus it can possess valency of -1(with a less electronegative atom than Cl),+1,+3,+5,+7(with a more electronegative atom than Cl).
We also know that lone pairs always contains 2 electrons.
If you form O=Cl=O, then in this, the no.of electrons unused remains to be 3 which cannot be possible.