The first twenty elements

The relative atomic mass of an element does not have to be calculated every time — it is a value you look up. This one table is what the next three sections keep coming back to.

Atomic number	Element	Symbol	Relative atomic mass
1	Hydrogen	H	1
2	Helium	He	4
3	Lithium	Li	7
4	Beryllium	Be	9
5	Boron	B	11
6	Carbon	C	12
7	Nitrogen	N	14
8	Oxygen	O	16
9	Fluorine	F	19
10	Neon	Ne	20
11	Sodium	Na	23
12	Magnesium	Mg	24
13	Aluminium	Al	27
14	Silicon	Si	28
15	Phosphorus	P	31
16	Sulphur	S	32
17	Chlorine	Cl	35.5
18	Argon	Ar	40
19	Potassium	K	39
20	Calcium	Ca	40

Two things in the table are worth a look

Chlorine is not a whole number

Every other value is a whole number, but chlorine is 35.5. The reason is isotopes, from chapter 3: natural chlorine is a mixture of two kinds of atom with different masses, so its relative atomic mass is their average.

Argon and potassium are out of order

Argon has atomic number 18 and a relative atomic mass of 40; potassium has atomic number 19 and a mass of 39. So mass does not always rise with atomic number — because mass depends on the number of neutrons as well as the number of protons.