Question

write all four quantum numbers for the galence electron of magnesium atom

Answer 1

The electronic configuration of magnesium is:

Mg= 1s^2, 2s^2, 2p^6, 3s^2

Clearly the last electron gets in the 3s orbital.
So, the quantum numbers of the last electrons will be:

n=3 (since the valence shell is the third shell)
l=0 (the s subshell signifies the azimuthal quantum number zero)
m=0 (Can not have any other value. Magnetic quantum number signifies the orbitals in a subshell. In the s-subshell, only one s-orbital is present)
s= +1/2 or -1/2 (depending upon whatever the spin of the electron is).