# Why does the third period contain only 8 electrons even though the electron capacity of the third shell is 18?

You are correct. The third shell has a capacity for electrons of 18.

If we look at quantum theory of anatom, we find sub-shells and orbitals. Spin of electrons is another topic. I will spare you the details, but one rule applies to this question: the Aufbau Principle or the n+l Rule.

Three sub-shells make up the third shell, 3s,3p, and 3d.

The fourth shell contains 4 sub-shells: 4s, 4p and 4d.

You can also determine the sequence of filling different sub-shells using (n+l). The electron enters the sub-shell that has the lowest value of (n+l). If (n+l is equal), then the electron will enter the sub-shell with the lowest value of n.

(n=no. (n=no.)

The values of l for subshells s,p.d.f are 0,1,2 and 3, respectively. They can accommodate electrons at 2,6,10 and 14, respectively.

Therefore, 3d (3+2=5) is more valuable than 4s (4+0=4). Instead of traveling to the 3d orbital the electron prefers being accommodated in 4s where energy is less.

The universe prefers to use the least amount of energy.