Chemical formulae

A chemical formula represents a substance using the symbols of the elements it contains. For a compound, it shows which elements are chemically combined and the fixed proportions of their atoms.

For example, water has the formula $$\ce{H2O}$$. It contains hydrogen and oxygen in a ratio of two hydrogen atoms to one oxygen atom. Carbon dioxide, $$\ce{CO2}$$, contains carbon and oxygen in a ratio of one carbon atom to two oxygen atoms.

Reading a chemical formula

Each element has its own symbol. The first letter is a capital; if there is a second letter, it is lower case. For example, $$\ce{Co}$$ is the symbol for cobalt, whereas $$\ce{CO}$$ contains carbon and oxygen.

A small number written after a symbol is a subscript. It shows how many atoms of that element are represented. If there is no subscript, the formula represents one atom of that element. In $$\ce{CO2}$$, there is one carbon atom for every two oxygen atoms.

A subscript after brackets applies to everything inside them. In $$\ce{Ca(OH)2}$$, the 2 applies to both oxygen and hydrogen. The formula therefore represents one calcium atom, two oxygen atoms and two hydrogen atoms.

Some formulae describe the atoms in a molecule. Others, such as $$\ce{NaCl}$$ for sodium chloride, show the simplest ratio of atoms in the compound: one sodium atom for each chlorine atom. They do not necessarily represent separate molecules.

Writing and using formulae

A formula provides a compact way to identify a substance and connect it with its name. To write the correct formula for a compound, use the correct element symbols and the known proportions in which its atoms combine. For example, carbon dioxide has one carbon atom for every two oxygen atoms, so its formula is $$\ce{CO2}$$, not $$\ce{CO}$$. The latter represents a different compound, carbon monoxide.

When writing formulae for a reaction, use the correct formula for each substance. For example, hydrogen and oxygen react to form water: their formulae are $$\ce{H2}$$, $$\ce{O2}$$ and $$\ce{H2O}$$ respectively. A number placed before a formula counts whole units of that substance; it is not part of the formula. Changing a subscript instead changes the substance represented.

Chemical symbols and the periodic table

Each element has a chemical symbol: a standard shorthand used in chemistry. For example, oxygen is represented by $$\ce{O}$$ and sodium by $$\ce{Na}$$.

A symbol has one or two letters. Its first letter is always a capital; if there is a second letter, it is lowercase. The symbol does not always match the beginning of the element’s English name, as $$\ce{Na}$$ for sodium shows.

The periodic table organises the elements. You can use it to look up an element’s name from its symbol, or its symbol from its name. The elements below are listed in the order in which they appear in the table.

The first 20 elements

• Hydrogen $$\ce{H}$$, helium $$\ce{He}$$
• Lithium $$\ce{Li}$$, beryllium $$\ce{Be}$$, boron $$\ce{B}$$, carbon $$\ce{C}$$, nitrogen $$\ce{N}$$, oxygen $$\ce{O}$$, fluorine $$\ce{F}$$, neon $$\ce{Ne}$$
• Sodium $$\ce{Na}$$, magnesium $$\ce{Mg}$$, aluminium $$\ce{Al}$$, silicon $$\ce{Si}$$, phosphorus $$\ce{P}$$, sulfur $$\ce{S}$$, chlorine $$\ce{Cl}$$, argon $$\ce{Ar}$$
• Potassium $$\ce{K}$$, calcium $$\ce{Ca}$$

Elements in Groups 1 and 7

A group is a vertical column in the periodic table. Some elements in Groups 1 and 7 are among the first 20; others appear further down the table.

• Group 1: hydrogen $$\ce{H}$$, lithium $$\ce{Li}$$, sodium $$\ce{Na}$$, potassium $$\ce{K}$$, rubidium $$\ce{Rb}$$, caesium $$\ce{Cs}$$ and francium $$\ce{Fr}$$.
• Group 7: fluorine $$\ce{F}$$, chlorine $$\ce{Cl}$$, bromine $$\ce{Br}$$, iodine $$\ce{I}$$, astatine $$\ce{At}$$ and tennessine $$\ce{Ts}$$. This column is also called Group 17 in some periodic tables.

Use the periodic table in the same way to find the names and symbols of other elements named in the course.

Compounds

A compound is a substance made when atoms of two or more different elements are chemically joined. The atoms are held together by chemical bonds, so the compound is a new substance, not simply a mixture of its elements.

The elements in a compound are joined in fixed proportions. For example, water has the chemical formula $$\ce{H2O}$$. The small 2 shows that there are two hydrogen atoms for every oxygen atom. A given compound always has the same composition; every sample of pure water has this same ratio of hydrogen to oxygen atoms.

In a physical mixture, substances are together but are not chemically joined to form a new substance, and their proportions can vary. By contrast, separating a compound back into its elements requires chemical reactions that break and rearrange chemical bonds. Physical separation alone cannot do this.

Atoms and elements

All substances are made from atoms. An atom is the smallest part of an element that can exist as that element.

An element is a pure substance made from only one type of atom. For example, a sample of the element carbon contains only carbon atoms. The atoms in an element may be joined together, but they are still all the same kind of atom.

There are about 100 different elements. Their atoms are the basic building blocks from which other substances are made.

Balanced chemical equations

A symbol equation shows the substances that react and the substances they form. The substances on the left are the reactants; those on the right are the products.

Atoms are rearranged in a chemical reaction, not created or destroyed. A balanced chemical equation therefore has the same number of atoms of each element on both sides.

Coefficients and subscripts

A coefficient is a whole-number multiplier written in normal script before a chemical formula. It changes the number of whole particles represented. For example, $$\ce{2H2O}$$ represents two water molecules, containing four hydrogen atoms and two oxygen atoms in total.

A subscript is the small number within a formula. It shows how many atoms of an element are in one particle. In $$\ce{H2O}$$, the subscript 2 means that each water molecule contains two hydrogen atoms; there is one oxygen atom because no subscript is written after O.

To balance an equation, change the coefficients, not the subscripts. Changing a subscript changes the formula and represents a different substance.

Balancing an equation for water

Consider the equation for hydrogen reacting with oxygen:

The hydrogen atoms balance: there are two on each side. The oxygen atoms do not: there are two on the left but only one on the right.

Place a coefficient of 2 before $$\ce{H2O}$$. The product side now has two oxygen atoms, but it also has four hydrogen atoms. Place a coefficient of 2 before $$\ce{H2}$$ to give four hydrogen atoms on the reactant side:

The equation is now balanced: each side has four hydrogen atoms and two oxygen atoms. The formulas have not changed. No coefficient is written before $$\ce{O2}$$ because its coefficient is 1.

Chemical reactions

A chemical reaction changes starting substances into one or more new substances. The starting substances are called reactants, and the new substances are called products. The products have different properties from the reactants.

During a reaction, atoms are rearranged as chemical bonds change. The atoms are not used up: they become part of the products in new arrangements.

Elements can react chemically to form a compound, a substance containing atoms of different elements chemically bonded together. For example, hydrogen and oxygen can react to form water. The water contains hydrogen and oxygen atoms bonded in a different arrangement from those in the reactants.

Chemical reactions often transfer energy to or from their surroundings. This may be detectable as a change in the surroundings, such as a rise or fall in temperature. An energy change is not necessarily detectable in every reaction.

Word equations

A word equation describes a chemical reaction using the names of the substances involved. The substances that react are called reactants; the substances formed are called products.

Reactant names are written on the left of an arrow, and product names are written on the right. The arrow means that the reactants form the products. A plus sign separates names when there is more than one reactant or more than one product.

In this reaction, hydrochloric acid and sodium hydroxide are the reactants. Sodium chloride and water are the products. The plus signs separate the two reactant names and the two product names.

To write a word equation, identify the substances that react and the substances formed. Place the reactant names to the left of the arrow and the product names to the right, using plus signs where needed.

A word equation uses substance names, not chemical formulae. Unlike a symbol equation, it does not use formulae to show the composition of each substance.