The electronic configurations of the first 20 elements:
H: 1
He: 2
Li: 2, 1
Be: 2, 2
B: 2, 3
C: 2, 4
N: 2, 5
O: 2, 6
F: 2, 7
Ne: 2, 8
Na: 2, 8, 1
Mg: 2, 8, 2
Al: 2, 8, 3
Si: 2, 8, 4
P: 2, 8, 5
S: 2, 8, 6
Cl: 2, 8, 7
Ar: 2, 8, 8
K: 2, 8, 8, 1
Ca: 2, 8, 8, 2