Knowledge database: Basics: Atoms and molecules

An atom is the smallest particle to which it is possible to divide an elementary substance, i.e. element. An atom consists of a nucleus and an electron cloud where electrons spin in specific pathways. The nucleus is composed of two types of subatomic particles - protons and neutrons. Of these particles, protons have a positive charge, electrons are negatively charged, and neutrons have no charge. Neutrons and protons are approximately of the same size and weight, while electrons are much smaller and lighter than neutrons and protons. Electrons are so light and small, that virtually all the mass of an atom is concentrated in the nucleus.

Above one can see bromine gas that comes in form of molecules composed of two bromine atoms, and argon gas which comes in monoatomic form.

Each particle consisting of several atoms is called a molecule. The atoms may be of the same element (such as diatomic molecules, which many elements that come in a gaseous state at normal conditions, form (oxygen, nitrogen, chlorine...)), or more often, the atoms can be of different elements. Atoms in molecules can be linked with different types of bonds. Basic types are: covalent, ionic and metallic bond. There are also specific intermolecular bonds that hold the molecules of a substance together, and thus allow formation of crystals. More about the various chemical bonds can be found in a separate chapter.