The atom - whose name comes from a Greek word meaning "indivisible" - is the basic unit of matter and the smallest thing that can take part in chemical reactions.
It was originally though that the atom was a solid, uniform sphere. However, in 1897, J J Thomson discovered that atoms can emit negatively-charged particles, which he called electrons; he imagined atoms to consist of electrons embedded in positively-charged matter (The plum pudding model). Later, in 1911, Ernest Rutherford asked two of his students - Geiger and Marsden - to fire alpha particles at a very thin sheet of gold foil. Although most of the alpha particles passed through the foil, some were deflected significantly and a few were deflected back towards the source; Rutherford concluded that most of the mass and all of the positive charge of an atom are contained in a nucleus, with the electrons orbiting. Rutherford subsequently discovered that the nucleus contains positively-charged particles whose charge is equal in magnitude to that of the electron but whose mass is 1846 times as much; he called them protons. Later still, it was discovered that atoms of every element except hydrogen also contain another type of particle in their nuclei, one with approximately the same mass as a proton but no charge; these were named neutrons.
The electrons orbit the nucleus in a series of orbits or shells, filling the shells in order from the innermost. The names of the elements are shown in what is termed the periodic table, with the location of each element depending on the number of shells its atoms normally have and the number of electrons its atoms normally have in their outer shells. Those at the top have only one shell, and those in the leftmost column have only one electron in their outer shells.
Atoms are most stable if they have a full outer shell. The noble gases, which occupy the rightmost column of the periodic table, already have a full outer shell and are therefore unreactive. Other atoms obtain a full outer shell by the transfer or sharing of the outermost electrons, known as valence electrons. This creates a bond between the atoms involved, and the atoms form a molecule. Interatomic bonds are strong and are termed primary bonds, while intermolecular bonds are weaker and are termed secondary bonds.
Ionic bonds are formed between atoms on opposite sides of the periodic table. Electrons are transferred from metallic atoms, which have few valence electrons, to non-metallic atoms, which have few vacancies in their outer shells. The atoms involved become ions and are bound together by opposing electric charges.
Covalent bonds are formed between non-metallic atoms when they share electrons. The shared electrons orbit both atoms' nuclei and are considered part of both atoms. Many non-metallic atoms combine in this way to form molecules consisting of two identical atoms.
Metallic bonds, as the name suggests, occur between metallic atoms. As these atoms have few valence electrons, they cannot form ionic or covalent bonds between themselves. Therefore, they share electrons in a complex system of orbitals. This arrangement can be considered an array of positive ions in a cloud of electrons.
A fourth type of bond is the van der Waals bond. A comparatively weak bond, it occurs between all atoms and molecules. It occurs because as electrons move in their orbits, asymmetric charge distributions arise across an atom. Positive and negative charges are therefore not balanced across a molecule, and bonds are formed between opposing charges.
The atoms from which a material is formed, and the way those atoms are bonded together, decide the properties of the material.