ATOMS AND MOLECULES.

Atoms are the smallest individual parts of an element. They are tiny,with diameter in the order of one ten -thousand -millionth of a metre. Two ore more atoms join together (bond) to form a molecule of a substance known as  a compound. For example when atoms of elements hydrogen and fluorine join together they form a molecule of the compound hydrogen fluoride.  So molecules are the smallest individual parts of a compound .Atoms  themselves are not individual they posses an internal structure. At their center is a dense nucleus, consisting of protons , which have a positive electric charge, and neutrons which are uncharged. Around the nucleus are the negatively charged electrons.It is the electron that  give a substance most of its physical and chemical properties. They do not follow definite paths around the nucleus. Instead, electrons are said to be found within certain regions, called orbitals. These are arranged around the nucleus in 'shells' , each containing electrons  of a particular energy. For example , the first shell (1) can hold up to two electrons, in so called s-orbital.(1s). The second shell(2) can hold up to eight electrons in s orbitals (2s) and p orbitals (2p). If an atom loses an electron , it becomes a positive ion (cation). If an electron is gained, an atom becomes a negative ion (anion). Ions of  positive charges will attract and join together , in a type of bonding known as ionic bonding. In covalent bonding, the atoms bond by sharing their electrons in what becomes molecular orbitals.