Chemical formula

Help with Chemical formula

There are many different types of chemical formula.

Chemical formula are commonly used as a shorthand way to show

1. the number and type atoms in a compound and
2. the number of atoms in a molecular element.

The chemical symbols of the elements are shown in the Periodic table. For example the element hydrogen is given the chemical symbol H and the element oxygen the chemical symbol O.

In a chemical formula the chemical symbol of each element is shown with subscript numbers which tell us the numbers or ratio of atoms in the compound or molecular element.

For example the compound water has the chemical formula H₂O. This tells us water is made up of two elements, hydrogen and oxygen. The subscript 2 in H₂O tells us there are two parts hydrogen to one part oxygen. Note: The subscript 1 is never but taken for granted. That is why water has the formula H₂O and not H₂O₁

Some elements also exist as molecules. Molecules are groups of atoms joined together by chemical bonds. Molecular elements like compounds also have a chemical formula. Examples of molecular elements include hydrogen, H₂ and oxygen O₂.

The following image shows the relationship between atoms and molecules.

Hydrogen atoms are too unstable to exist by themselves as free atoms. They pair up to with other hydrogen atoms to form hydrogen molecules which are much more stable. The chemical formula of the hydrogen molecule is therefore H₂. This means two hydrogen atoms are joined together. The molecule O₂ is also much more stable than single oxygen atoms.

The combining power or valency of an element determines the ratio that atoms or chemical groups will combine with one another. Knowledge of valencies can help us predict the chemical formula of a substance. Atoms achieve a stable outer shell of electrons by transferring or sharing electrons. They always combine with one another in simple whole number ratios.

Compounds between metals and non-metals.

These types of compounds are called ionic compounds.

Electrons are transferred between chemical species producing charged particles called 'ions'.

The electrostatic attraction between a positive ion (cation) and a negative ion (anion) produces an ionic bond.

Metals lose electrons producing positive ions. eg. Na Na⁺ + e^-.

Non-metals gain electrons producing negative ions. Cl + e^- Cl^-

The charge of the ion determines the combining power of a chemical species.

Magnesium has a combining power of 1.

Chlorine has a combining power of 1.

In the following 'arm and link' model the combining power of a chemical species is shown by the number of 'arms'.

Magnesium combines with other chemicals in fixed whole number ratios. When the arms are all linked the chemical formula can quickly be determined.

Model of compound
Name of compound	sodium chloride	magnesium carbonate	magnesium carbonate	magnesium hydroxide
Chemical formula	NaCl	MgCl2	MgCO3	Mg(OH)2

In the atomic world, the electrostatic attraction between positive and negative ions produces an ionic compound and crystalline structure.

The chemical formula of ionic compounds shows the simplest ratio of ions in the crystal.

In sodium chloride, NaCl the ratio of ions is 1:1 as seen in the diagram.

Compounds between non-metals and non-metals.

These types of compounds are called covalent compounds.

Electrons are shared between chemical species producing covalent bonds.

There are two types of covalent compounds that are formed.

1. Molecular covalent compounds or molecules

A molecular formula is used which shows the number and type of particles in the molecule.

For example in water, H₂O two hydrogen atoms are covalently bonded to an oxygen atom producing a discrete molecule as shown by the image below.

The chemical formula of water is H₂O

2. Network covalent compounds

These types of compounds produced a continuous network structure.

A quartz crystal can greatly vary in size from a sand grain to a crystal so large that you cannot lift. Quartz has the chemical formula of SiO₂. This formula is also called an empirical formula as it shows the ratio of the elements in quartz. For every silicon atom (symbol Si) there are two oxygen atoms (symbol O).

The chemical formula of silicon dioxide is SiO₂

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