Question

The electronic configuration of calcium is 2,8,8,2. Why isnt it 2,8,9,1 when the third shell can hold upto 18 electrons?

Answer 1

according to neil bohr's theory,
the outermost shell should only contain 8 electrones.if it is more than 8,then it will move to the next shell.so it cant be 9

Answer 2

1)the capacity of the 3rd shell is only 8 electrons.it can hold only 8 electrons.
2)also according to neir bohr's octet rule the last shell can hold only 8 electrons
no shell has the capacity to hold 9 electrons.thus 2,8,8,2 is only the correct one.

hope tis helps u...