Representing Elements and Simple Molecules

Atoms are represented by a single letter or two letters, known as a symbol, that typically originate from their English or Latin names. For example, hydrogen is H and sodium is Na from its Latin name ‘natrium’.
An element on its own will typically exist as individual atoms. This is true for noble gases like Helium (He) and elements such as Carbon (C).
Some non-metal elements form molecules by bonding together. These include hydrogen (H2), oxygen (O2), and nitrogen (N2). The number (subscript) indicates how many atoms of that element are involved.
Some other non-metal elements form molecules with multiple elements. Common examples include water (H2O) and carbon dioxide (CO2).
In a chemical formula, the number of atoms of each constituent element, if more than one, is shown by a small number (subscript) after the element symbol. If there is no number, it is assumed to be one.
Diatomic molecules are molecules composed of only two atoms, of the same or different chemical elements. The prefix ‘di’ means two.
A molecule may be non-polar or polar. Polarity depends on the difference in electronegativity between the bonded atoms.
Hydrocarbons are simplest of organic compounds containing only hydrogen and carbon atoms. The simplest hydrocarbon, methane, is CH4.
Structural formulas show the arrangements of atoms in a molecule. In full structural formulas, every atom and bond is shown. In skeletal formulas, carbon and hydrogen are not shown, and instead, each corner or end of line represents a carbon atom.
Molecular formulas give the actual numbers of atoms in a molecule.
Empirical formulas give the simplest whole-number ratio of atoms in a molecule.

When revising these points, practise by writing out the chemical formula for simple molecules and ensure you understand the differences between molecular, empirical, and structural formulas.