Compounds and Elements

Today more than 100 elements are known to science. Some of them exist in large quantities. Others such as uranium and radium are relatively rare. Some elements do not exist at all in nature, they are created by atomic physicists using special equipment.

All the substances, which occur in nature, consist of one or more basic elements; a substance containing more than one element is known as a compound. An element is a substance that can neither be decomposed by ordinary chemical action, nor made by a chemical union of a number of other substances. A compound consists of two or more different elements in combination and has properties different from the properties of its constituent parts. Water, for example, is a compound of oxygen and hydrogen. A molecule is the smallest amount of a substance that can retain the characteristic properties of that substance.

An atom is the smallest particle of an element, which has all the properties of the element and can take part in chemical reactions.

In each atom there is a nucleus, containing a number of protons (each proton has a positive electrical charge) and a number of neutrons, having no electrical charge. The nucleus is surrounded by a number of electrons, which orbit around the nucleus. Each electron has a negative electrical charge. An atom has as many electrons as it has protons. Besides the electron has the same magnitude of charge as the proton and this results in a zero electrical charge of the atom.

Other charged particles called ions may exist. They are usually created from a normal atom or molecule by the addition or removal of one or more electrons or protons from its structure.

Electrons orbit around the nucleus in one of the several possible orbits. These orbits are called shells. The outer shell of any atom is called its valence shell, it can hold other electrons. The electrons of the outer shell are called valence electrons and the number of electrons in the valence shell is known as the valency of the atom.

Each valence electron orbits not only around the parent atom but also around the neighbouring atom. This bond is called a covalent bond.

When an electron leaves the covalent bond, it leaves behind it an “absence of an electron”. This absence of an electron consists of a missing negative charge “e”, and it is equivalent to a positive charge called “a hole”.

Each hole is able to attract electrons and can be filled by a passing electron. This process is called recombination. Recombination causes a continual loss of holes and free electrons. The total number of free electrons and holes is constant but their movement is random.

Дата добавления: 2015-07-18