Atoms are assumed to have a definite weight. Hydrogen is the lightest element and has therefore been selected as the unit of weight; all other elements are measured in terms of hydrogen. For example: If equal volumes of hydrogen and oxygen are weighed, oxygen is found to weigh sixteen times as much as hydrogen. Hence the atomic weight of oxygen is 16.

The atomic weights of the commonest elements are given in the following table:

Table of Atomic Weights

Element

Symbol

Approximate

Element

Symbol

Approximate

Aluminum

Al

27

Magnesium

Mg

24

Antimony

Sb

120

Manganese

Mn

55

Argon

A

40

Mercury

Hg

200.5

Arsenic

As

75

Nickel

Ni

58.5

Barium

Ba

137

Nitrogen

N

14

Bismuth

Bi

208

Oxygen

O

16

Boron

B

11

Phosphorus

P

31

Bromine

Br

80

Platinum

Pt

195

Cadmium

Cd

112

Potassium

K

39

Calcium

Ca

40

Radium

Ra

226.5

Carbon

C

12

Silicon

Si

28

Chlorine

Cl

35.5

Silver

Ag

108

Chromium

Cr

52

Sodium

Na

23

Cobalt

Co

59

Strontium

Sr

87.5

Copper

Cu

63.5

Sulphur

S

32

Fluorine

F

19

Tin

Sn

119

Gold

Au

197

Titanium

Ti

Helium

He

4

Tungsten

W

Hydrogen

H

1

Uranium

U

Iodine

I

127

Vanadium

V

Iron

Fe

56

Zinc

Zn

65

Lead

Pb

207