The fact that a limited number of chemical elements compose all the materials people find on earth shows the importance of each of them. Some of the elements are very common on the earth, others are very rare. Many of the elements are never seen in pure form by most chemists, so rare are they. In fact, some seventy-five of the elements make up less than one per cent of the earth. However, the future may discover important uses for some of these little-known and rare elements.
Elements are characterized by some properties distinguishing them from other substances. The main of these properties are, their colour, hardness, odour, taste, density, solubility in water and other liquids, ability to conduct electricity and others. The ability to burn in air and the manner in which an element enters into chemical reactions with other substances are also properties that help to distinguish one element from another. For example, oxygen is a colourless, odourless, tasteless gas with a density at standard ‘conditions of pressure and temperature of about one seven-hundredth that of water. It is soluble in water, and it becomes liquid at about 180 below zero on the centigrade thermometer.
Similarly, these properties determine whether a substance is necessary or useless industrially. Copper is of great value, being a good »conductor of electricity. Carbon andihydrogen are also useful because they combine readily with oxygen, i.e. burn, and in so doing liberate great quantities of heat. Tungsten is valuable because of the several important properties it possesses. It is a hard metal which does not melt, when it is heated to high temperatures producing white light. It does not rust and is not affected by cold acids. Thus it is very useful and is now considered one of the very important metals of industry.
The particles all the different atoms consist of are neutrons, protons (positively charged particles) and electrons (negatively charged particles); therefore, the only way in which the chemical elements differ from each other in their composition is in the number and arrangement of these particles in their atoms. Similarly, many of the properties of the elements, both physical and chemical, are determined by the number of these particles.