Question #2e9c4

Number of Period = Row number in table, so it concerns Li to Ne...

4 Quantum numbers:
n = Principal number (basically row number);
`l` = Azimuthal (Orbital) number;
m = Magnetic number;
s = Spin number.

Maximum values:
n = 1,2,3,4,5 etc. (remember, this is the row number)
`l` ranges from 0 to n-1.
m ranges from `-l""` to `+l`

If n =1 (row 1) then `l` can only be 0. (n-1 = 1-1 = 0)
If `l`=0 then m can only be 0. (`-l""` to `+l` )
s is always either -1/2 or +1/2
This describes the `1S`-orbital, with max. 2 `e^-` in it.

If n=2, then `l` ranges from 0 to n-1, which is 1.

Once again:
If `l`=0 then m can only be 0. (`-l""` to `+l` )
s is always either -1/2 or +1/2
since n=2, this describes the `2S`-orbital, with max. 2 `e^-` in it.

If however `l`=1 then m can be `color(red)"-1, 0 or 1"` (`-l""` to `+l` )

`l` = 1 describes the `P`-orbital, or rather set of:
m=-1: `rarr P_x`
m=0: `rarr P_y`
m=+1: `rarr P_z`

Therefore, in period 2 there are 4 orbitals: `2S, 2P_x, 2 P_y` and `2P_z`

2 electrons in each orbital: `rarr` 4 x 2 = 8 `e^-`...

The `2S`-orbital (S for Spherical) has a larger diameter than the `1S`, and therefore completely envelopes it.

Hence the two `e^-` of the `1S` are sheltered and play no part in normal reactions. They are therefore not Valence Eectrons.