Where each of the twenty sits

Apply both rules to every one of the twenty and this is the result — the whole of what you need to know about their positions, on one page.

Atomic number	Element	Electron configuration	Period	Group
1	H	1	1	I
2	He	2	1	VIII / 0
3	Li	2, 1	2	I
4	Be	2, 2	2	II
5	B	2, 3	2	III
6	C	2, 4	2	IV
7	N	2, 5	2	V
8	O	2, 6	2	VI
9	F	2, 7	2	VII
10	Ne	2, 8	2	VIII / 0
11	Na	2, 8, 1	3	I
12	Mg	2, 8, 2	3	II
13	Al	2, 8, 3	3	III
14	Si	2, 8, 4	3	IV
15	P	2, 8, 5	3	V
16	S	2, 8, 6	3	VI
17	Cl	2, 8, 7	3	VII
18	Ar	2, 8, 8	3	VIII / 0
19	K	2, 8, 8, 1	4	I
20	Ca	2, 8, 8, 2	4	II

Table 3.3 — where the elements with atomic numbers 1 to 20 sit in the periodic table.

Do not memorise this table

Every column of it can be worked out from the atomic number alone. Learn the two rules — count the levels for the period, count the last level for the group — and you can rebuild any row of this table whenever you need it.