1s

2s 2p

3s 3p 3d

4s 4p 4d 4f

5s 5p 5d 5f

6s 6p 6d 6f

7s 7p 7d 7f

You fill the electron shells according to Aufbau Principle, with the lower shells first

that are closest to the nucleus.

Remember that:

s-shell only holds 2 electrons

p-shell only holds 6 electrons

d-shell only holds 10 electrons

f-shell only holds 14 electrons

Examples:

H¹

1s¹

Sr³⁸

1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶ 5s²