Atoms, Elements, Compounds and Chemical Formulae

Atoms are the building blocks of substances

All substances are made of atoms. An atom is the smallest part of an element that can exist. An element is a substance containing only one type of atom: a sample of sodium contains sodium atoms, while a sample of oxygen contains oxygen atoms.

There are about 100 different elements, displayed in the periodic table. These elements provide the building blocks for a much larger range of substances because atoms of different elements can chemically combine.

Chemical symbols identify elements

Each element has a chemical symbol. O represents oxygen and can represent one oxygen atom; Na represents sodium and can represent one sodium atom. Some symbols resemble the English name, while others do not, so you need to recognise the name–symbol pairs rather than guess them.

The first letter is always a capital. If there is a second letter, it is lower case. For example, Co means cobalt, whereas CO is a formula containing carbon, C, and oxygen, O.

You will be supplied with a periodic table in the exam. Being comfortable with it means you can move between an element's name and its symbol when reading or naming a compound.

The first 20 elements and other useful symbols

The first 20 elements, in periodic-table order, are:

A group is a vertical column in the periodic table. Using the GCSE group numbering:

• Group 1 metals: lithium, Li; sodium, Na; potassium, K; rubidium, Rb; caesium, Cs; francium, Fr.
• Group 7: fluorine, F; chlorine, Cl; bromine, Br; iodine, I; astatine, At.

Hydrogen, H, is often shown above Group 1, but it is not a Group 1 metal.

Other useful name–symbol pairs encountered in GCSE chemistry include iron, Fe; copper, Cu; zinc, Zn; silver, Ag; gold, Au; and lead, Pb. Use the supplied periodic table to check other elements when they appear in a question.

Chemical reactions form compounds

A compound contains two or more different elements chemically combined in fixed proportions. Compounds are formed from elements by chemical reactions.

A chemical reaction always produces one or more new substances. It often also produces a detectable energy change, such as a temperature change. The formation of a new substance is the essential distinction: heating or mixing substances does not necessarily mean that a chemical reaction has occurred.

The new compound usually has properties quite different from those of its constituent elements. Sodium and chlorine, for example, combine to form sodium chloride, the compound commonly called salt. Sodium chloride is a new substance, not simply sodium and chlorine placed side by side.

In a mixture, substances are not chemically combined. By contrast, separating a compound into its elements requires a chemical reaction. Physical separation methods such as filtration cannot undo the chemical combination.

Formulae show the fixed proportions

A chemical formula uses element symbols to show which elements are present and the relative numbers of their atoms. A small number written below and to the right of a symbol is called a subscript. It gives the number of atoms of that element. If there is no subscript, the number is one.

For compounds made of molecules, the formula gives the atoms in each molecule:

• HCl: one hydrogen atom and one chlorine atom per molecule.
• H₂O: two hydrogen atoms and one oxygen atom per molecule.
• NH₃: one nitrogen atom and three hydrogen atoms per molecule.

The proportions are fixed. Every water molecule has two hydrogen atoms for each oxygen atom. A larger sample contains more molecules, but the hydrogen-to-oxygen ratio remains 2 : 1.

Not every compound consists of separate molecules. For compounds such as sodium chloride, NaCl, the formula shows the simplest ratio of the elements: one sodium for each chlorine. You can read this ratio without needing to describe the compound as a molecule.

Some formulae contain brackets. In Ca(OH)₂, the subscript 2 applies to the whole OH group, so the formula contains one calcium, two oxygen and two hydrogen atoms. In Na₂CO₃, there are two sodium, one carbon and three oxygen atoms.

Naming compounds from their formulae

First identify the element symbols. Then use the appropriate naming pattern or familiar compound name.

For many compounds containing a metal and one non-metal, name the metal first and change the non-metal ending to -ide:

A compound's name does not always state the number of atoms. Magnesium chloride is MgCl₂, even though the name does not include ‘two’. The formula gives the proportions.

For some compounds of non-metals, prefixes indicate numbers of atoms. CO₂ is carbon dioxide: ‘di-’ indicates two oxygen atoms. CO is carbon monoxide: ‘mono-’ indicates one oxygen atom. These are different compounds, so reading the subscript matters.

Names ending in -ate often indicate an oxygen-containing compound. Useful examples are Na₂CO₃, sodium carbonate; NaNO₃, sodium nitrate; and CuSO₄, copper sulfate. The ending alone does not tell you the number of oxygen atoms: read that from the formula.

Some compounds have familiar names that you need to recognise directly:

The same naming approach works when a formula appears within a symbol equation. Identify each formula as a substance in its own right: MgCl₂ still means magnesium chloride wherever it appears in the equation.

Elements and symbols

• All substances are made of atoms; an atom is the smallest part of an element that can exist.
• An element contains one type of atom. About 100 elements are displayed in the periodic table.
• Symbols: capital first letter, lower-case second letter, if present.
• Be able to use the first 20 elements, Groups 1 and 7, and other elements encountered in the specification. A periodic table is supplied in the exam.

Compounds and reactions

• Compound: different elements chemically combined in fixed proportions.
• Chemical reaction: always makes new substances; often involves a detectable energy change.
• Compounds can be separated into elements only by chemical reactions, not physical separation.

Reading and naming formulae

• Symbols identify elements; subscripts give atom numbers or ratios. No subscript means one.
• A number outside brackets multiplies the whole bracketed group.
• Metal + one non-metal: metal name first, non-metal ending -ide.
• Prefixes such as mono- and di- indicate atom numbers in some non-metal compounds.
• Recognise oxygen-containing names such as carbonate, nitrate and sulfate, and familiar names such as water, ammonia, methane and the common acids.