We are surrounded by thousands of different materials. By the action of chemical processes some of them could be broken down into substances completely different from the initial ones. But there are also some which couldn't be broken down by any chemical means. A 17th century scientist Robert Boyle tried to discover which of the known substances could be broken down and which not. The first ones were called compounds and the second ones were called by him simple bodies or as we say today-elements. Soon it emerged that all chemical compounds consist of some limited number of different elements. Water, for example, which is a compound could be broken down into two elements: oxygen and hydrogen. It must be accentuated here that the chemical characteristic of the compound is different from the characteristics of the elements it is bound of. Look at the chart and compare the chemical characteristics of water, oxygen, and hydrogen.

 
 

CHARACTERISTICS	WATER	OXYGEN	HYDROGEN
Density when liquid	1000 kg/m3	1141 kg/m3	71 kg/m3
Melting point	0 oC	-218,18 oC	-259,32 oC
Boiling point	99,974 oC	-189,96 oC	-252,88 oC
Refractive index of light of the length of 589,3 nm (in liquid)	1,33288	1,221	1,0974

 
 

In our everyday life we deal with both compounds (like water, table salt, or alcohol), and elements (like iron, silver, gold, nickel, oxygen or copper). In the beginning of the 19th century the great chemist John Dalton discovered a very important principle ruling the elements in the compounds. He demonstrated that if two elements make more than one combination, the weight amounts of one of them suiting the unchanging amount of the second one are staying in relations of small integers (the law of multiplied proportions). For example, for chlorine oxides (Cl₂O, Cl₂O₆, Cl₂O₇) the masses of oxygen belonging to the chlorine unit are staying in proportions 1:6:7. Dalton noticed that the results he got could be simply explained using the conception of the atom. It emerged that after breaking down a compound one always gets elements in the same, characteristic for each compound, weight ratio. For example, table salt always compounds of 23 parts by weight of sodium and 35 parts by weight of chlorine. John Dalton was explaining that principle with the atomic theory. He should form one particle of table salt from one atom of sodium and one atom of chlorine, that should be exactly 35/23 heavier than the atom of sodium. Soon he managed to determine the relative weights of many elements (that means the relation between the weights of different elements). Hydrogen was the lightest one. Its relative mass is equal to 1 (so it was logical to accept its mass for a unitary mass). Lithium is a bit heavier and weights 3 units. Oxygen's mass is equal to 8 units, Sodium's 23, and Chlorine's 35. Much heavier is Lead, which weights 82 units. The masses of the most important elements known nowadays are shown in the chart below.

After determining the atomic weights of different elements it became clear that many chemical compounds are formed not only from one atom of one kind and only one of the other kind. They could for example, be formed from two atoms of the first kind and three of the second kind. It is so, for example, with water, which is combined of one atom of oxygen and two atoms of hydrogen. And another example is ethyl alcohol consisting of two atoms of Carbon, six atoms of hydrogen, and one atom of oxygen.

With the researches on chemistry in the 18th and the 19th centuries very good arguments for the atomic theory appeared, but they still weren't proofs.