Why do some elements have multiple ionic charges? like the transition metal copper, why can it form both cu+ and cu2+ ions?

The skeletal system performs a variety of functions that are crucial to maintaining life processes. what function is performed in the bone marrow, but not in the ossified bones of the skeleton? a oxygen transportation c mineral storage b. muscle attachment d red blood cell production

Can someone me with questions 15 to 17 plz.

What material does not melt at any temperature