Chemical patterns

The atomic model consists of a nucleus containing protons and neutrons, surrounded by electrons in shells. The numbers of particles in an atom can be calculated from its atomic number and mass number.

Mendeleev made an early periodic table. In the modern periodic table, elements are arranged in order of atomic number in periods and groups. Electronic arrangements model how electrons are arranged.

The group 0 elements, the noble gases, are all unreactive non-metal gases. They show trends in their physical properties. Their uses depend on their inertness, low density and non-flammability.

All substances are described by their formulae, which are used to write balanced chemical equations. Writing the formula for an ionic compound requires knowledge of the charges on its ions.