Calcium (atomic number 20) has electron configuration 1s2,2s2,2p6,3s2,3p6,4s2. the atomic number of scandium is 21. which orbital does the 21st electron enter? a.3d b.4p c.4d d.4f

If you look at the periodic table, you can see that Sc is in the transitional group metals, so it should have d orbital,
also it is in 4th period, so it has 3d orbital 
and 21 st electron enters 3d orbital
Answer a.3d
after 4s electrons go into 3d -orbital

1. o2(g)as a product

They are all solids.