[ToxicIphone]

On the periodic table, the period number should be equal to the number of energy shells. For example, period 4 should have four energy shells. I can't get proof of this though with several elements in period 4. Take Calcium for example, when I make a Bohr model I get Calcium having 3 energy shells. I use 2n^2 to see the max number of electrons per shell. 2 for shell 1, 8 for shell 2 and 18 for shell 3. The 2 first shells for Calcium become full and 10 go into shell 3. What am I doing incorrectly?

[Hunter2]

Calcium has 20 electrons. 2 on first shell, 8 on second shell, again 8 on 3rd. shell and remaining 2 on 4th. shell.  1s², 2s², 2p⁶, 3s², 3p⁶, 4s²

[ToxicIphone]

Thanks! But, why can't it have more than eight on the third shell? I thought 2n^2 would yield the maximum number of electrons on any given shell (n being the shell's number). So 2(3^2) would be 18. Does this apply to only certain atoms?

[Hunter2]

The Bohr system doesn't describe it. But there is some more restrictions.

The last shell can only carry max 8 Electrons, it doesn't matter what is the max. according 2n². If it has to carry more then 8 the next shell already has to be started. This is the case at Element Scandium Sc.

1s², 2s², 2p⁶, 3s², 3p⁶, 4s² 3 d¹. The 18 electron will be reached at Zinc 1s², 2s², 2p⁶, 3s², 3p⁶, 4s² 3 d¹⁰

Although the 4th. shell what can carry max 32 electron can only filled if the outer shell already carry 3 electrons see Lanthanum La. Look in an periodic table of elements you see the jumps.