atomic structure

1.Why does the 4s orbital system fills before the 3d orbital?
In a multi-electron system, the 4s orbital is lower in energy than 3d.
2.What's the difference between two electrons in the same orbital?
We use 4 quantum numbers to describe an electron,
principle quantum number --- n
angular momentum quantum number --- l
magnetic quantum number --- m
spin quantum number --- s
for 2 electrons in the same orbital, they have the same values of n,l and m but differ in s.
3. Does Ba have a lower first I.E. energy than Ca's? Why?/ why not?
Yes, Ba has lower first I.E. than Ca, because the outermost electron in Ba is further away from the nucleus and thus feel weaker attraction force from the nucleus.