1.4 Element

Atoms that have the same number of protons are grouped together and constitute a chemical element. All known substances on earth are formed from a sort of the periodic table consisting of 111 known elements. An element is a fundamental or elementary substance that can not be broken down by chemical means. Elements are the basic building blocks of all substances.

In this section，we will introduce these important elements，including H，C，N，O and S.

1.4.1 Hydrogen

Hydrogen，the lightest of the elements，possesses interesting properties in its own right，and forms important compounds with nearly all of the other elements in a variety of bonding situations. The element exists as the diatomic gas，H₂，which forms the simplest example of a molecular covalent bond. Hydrogen exists in nature in three isotopic forms. The most abundant，with nuclear mass one，is commonly referred to as hydrogen （symbol H）. The isotope of mass two is called deuterium （symbol D），and that of mass three is called tritium （symbol T）.

1.4.2 Carbon

Carbon exists in two crystalline structures—diamond and graphite. The diamond structure，based on tetrahedral bonding of hybridized sp³ orbital，is encountered among Group ⅣA elements. In fact，graphite is so soft that it exists in the form of a microcrystal or merely as a powder. Typical of the Group ⅣA diamond-type crystalline elements，it is a nonconductor and shows other nonmetallic properties.

1.4.3 Nitrogen

The element is the major constituent of the earth’s atmosphere. Nitrogen is found in inorganic materials in the form of nitrate （NO₃^-） compounds. It is widely and heavily distributed in plants and animals. The pure element is conveniently obtained by distillation of liquid air. Among the most important considerations in nitrogen chemistry are the nitrogen cycles in plants，animals，and in the earth’s crust. The nitrogen of the atmosphere is converted in various ways into nitrogen compounds，such as NO，NO₂. The dioxide is washed down by rain，and then enters into the plant cycle. Rain may also contain ammonia，which enters the air as a result of decomposition of plant and animal material.

1.4.4 Oxygen

Oxygen is the most abundant element in the earth’s crust，and is second only to iron in abund-ance in the earth as a whole. In addition，it presents itself in the earth’s atmosphere as molecular oxygen and to a less extent as O and O₃.Oxygen is covalently bonded to hydrogen in water，which is very abundant on the earth’s surface. It is also associated with a great many metals in the form of oxides，such as corundum，Al₂O₃; hematite，Fe₂O₃. With many non-metals and metals，it forms oxy-anions which are common in nature，such as sulfate，SO₄^2-; phosphate，PO₄^3-; iodate，IO₃^-; chromate，CrO₄^2-. The industrial preparation of oxygen involves liquefaction of air and subsequent fractional distillation.

1.4.5 Sulfur

Sulfur is a fairly abundant element in the earth’s crust，it is estimated to average a little over one pound per ton. It exists mainly in the form of solid mineral sulfides. There are some metals which are found in nature mainly as oxides，and others largely as sulfides. The sulfate ion，SO₄^2-，is the second most abundant anion in sea water. In addition，a number of solid sulfate minerals are known. Calcium sulfate，the best known of these，is found as the hydrous salt，CaSO₄·2H₂O. Hydrogen sulfide may occur as a by product of the formation of petroleum. There is one exceptional petroleum gas field in southern France which contains up to 18 percent hydrogen sulfide by volume. The hydrogen sulfide is removed by absorption in a solvent under pressure，then recovered from solvent and catalytically decomposed to yield sulfur. The operation produces about 4000 tons of sulfur per day.