CHEM Chemical Formulae and Equations

A chemical formula is like a recipe showing the ratio of elements in a substance.
- It consists of chemical symbols indicating elements present and subscripts indicating the number of atoms of each element.
Elements can exist as monoatomic, diatomic, or polyatomic molecules.
- Monoatomic elements exist as uncombined atoms (e.g., Group 18 elements).
- Diatomic molecules consist of two chemically combined atoms (e.g., hydrogen $H<em>2$ , oxygen $O</em>2$ ).
- Polyatomic molecules consist of three or more chemically combined atoms (e.g., sulfur $S<em>8$ , phosphorus $P</em>4$ ).
Compounds are made of chemically combined atoms of elements and have fixed formulae.

Valency is the number of electrons that an element loses, gains, or shares to achieve a noble gas electronic configuration.
Valencies can be determined from group numbers in the periodic table.
Transition metals have variable valencies indicated by Roman numerals.
Polyatomic ions are ions made of multiple covalently bonded atoms with an overall charge.
The crossing method is used to construct chemical formulas by cross-multiplying valencies of chemical species.

Word equations use chemical names of reactants and products.
Chemical equations show the ratio of reactants and products using chemical formulas and state symbols.
State symbols indicate the physical state of substances: (s) solid, (l) liquid, (g) gas, (aq) aqueous.

A balanced chemical equation has an equal number of each type of atom on both sides of the equation.
Balance equations by adding appropriate coefficients to chemical formulas.