Explanation:
2, 8, 8, 2 is the electron arrangement of Calcium.
The way that the "orbitals", the shells, fill up is given as
[tex]1s^{2} 2s^{2} 2p^{6} 3s^{2} 3p^{6} 4s^{2} 3d^{10}[/tex] . . . etc etc.
It is more stable for the fourth shell to start filling up with an electron pair, and then subsequent electrons filling up the third shell again.
So while potassium 19, is 2,8,8,1
And calcium 20, is 2,8,8,2
Scandium 21 will fill up as, 2,8,9,2
This will continue until we reach Zinc 30, which will fill up as 2,8,18,2. The third shell is now "full", and the fourth shell will now being to fill up.
The fourth orbital starts getting filled up again, Gallium 31, is 2,8,18,3
