Elements and Compounds

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 is 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

Roert Boyle was a subscriber for the atomic theory. He did the research on the effect of air resistance, its pressure and on changes of its volume while changing its pressure. He discovered the law describing dependence between these two quantities (Boyle's law). You surely remember it from school (it should be somewhere at the beginning of your notes from lessons about thermodynamics). This law was experimentally discovered by Boyle but formulated later by Marriotte. The experiments with pressure were guiding later scientists, who were researching air structure. The experiments proved that air is composed of separate, moving atoms, which stay, in quite a distance from each other. Thanks to such structure, air can change its volume considerably. Pressure, which Boyle researched, is caused by the movement of particles, which collide with other things influencing them with some force. When air's volume is smaller, there are more collisions in each square centimetre of surface (what Bernoullie's researches showed later). This interdependence Boyle discovered in an experimental way.

Chemistry as a science, at the beginning of the 18th century, was an assembly of different, chaotic rules. The same was with chemical nomenclature. The great French chemist Antoine Laurent Lavoisier developed chemical nomenclature based on the names of simple substances while giving names to complicated substances composed of those simple ones. But before he was able to systematise chemical nomenclature he had to make changes in chemistry. He proved that all elements could occur in three states of aggregation: gaseous, liquid and solid. He showed that while burning, substances combine with oxygen. He also proved that water could not change into other substances as many scientists thought. After a longer research he managed to prove that water consisted of oxygen and hydrogen. By decomposing water he discovered that the weight ratio between oxygen and hydrogen was always 8:1. It was the direction indicating that world consisted of atoms. Lavoisier believed in it, but he didn't develop his study of atom. The one who did it was John Dalton.

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 to 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 and 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 by 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.

 ELEMENT SYMBOL ATOMIC WEIGHT ELEMENT SYMBOL ATOMIC WEIGHT HYDROGEN H 1 PHOSPHOROUS P 31 HELIUM He 4 SULFUR S 32,06 LITHIUM Li 6,91 CHLORINE CL 35,45 BERYLIUM Be 9,01 POTASSIUM K 39,09 BORON B 10,81 CALCIUM Ca 40,07 CARBON C 12,01 IRON Fe 55,84 NITROGEN N 14 COPPER Cu 63,54 OXYGEN O 16 ZINC Zn 65,59 FLUORINE F 19 SILVER Ag 107,86 NEON Ne 20,17 IODYNE I 126,9 SODIUM Na 23 GOLD Au 197 MAGNESIUM Mg 24,3 MERCURY Hg 200,59 ALUMINIUM Al 27 LEAD Pb 207,2 SILICON Si 28,08 URANIUM U 238,02

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.