ATOMS are the smallest individual parts of an element .They are tiny,with diameters in the order of one ten thousand millionth of a metre(10^-10).Two or more atoms join together (bond) to form a molecule of substance known as compound.For example,when atoms of the elements hydrogen and fluorine join together,they form a molecule of the compound hydrogen fluoride.So molecule are the smallest individual parts of a compound.Atoms themselves are not indivisible-they possess an internal structure.At their centre is a dense neucleus,consisting of protons,which have a positive electric charge.,and neutrons,which are uncharged.Arround the neucleus are the negatively charged electrons.It is the electrons that give most of its physical and chemical properties.They do not follow definite path arround the neucleus.Instead electrons are found with in certain regions.called orbits.These are arranged arround the neucleus in "shells",each containg electrons of particular energy.For example the first shell(1) can hold up to two electrons,in a 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 lose and electron it becomes positive iom(cation).If an electron is gained ,an atom becmes a negative ion(anion).Ions of opposite charges will atract and join together,in a type of bonding known as ionic bonding.In covalent bonding,the atoms bond by sharing of electrons in what become molecular orbitals.
