First period
1
s
2
s
2
p
H
He
Principal quantum number (
n
) = 1
Hydrogen
Helium
Z
= 1
Z
= 2
1
s
1
1
s
2
HydrogenHeliumZ = 1Z = 21s 11s
2HydrogenHeliumZ = 1Z = 21s 11s
2