Fundamental chemical concepts

Atoms are joined together by chemical bonds, which involve the electrons found around the nucleus of the atom. Covalent bonds and ionic bonds are two types of chemical bond.

During a chemical reaction, reactants are changed into products. Chemical symbols and formulae are used to represent substances in symbol equations. Molecular formulas show the number of each type of atom in a molecule, while displayed formulas show the atoms and bonds in a molecule.

Atomic structure

All substances are made from atoms [atoms: An atom is the smallest part of an element that still has the properties of that element, comprising electrons surrounding a nucleus of protons and neutrons. ].


Atoms have a small central nucleus [nucleus: The central part of an atom. It contains protons and neutrons, and has most of the mass of the atom. ] surrounded by even smaller particles called electrons [electrons: Sub-atomic particles, with a negative charge and a negligible mass relative to protons and neutrons. ]. The nucleus is positively charged, and the electrons are negatively charged.

atoms have a small central nucleus surrounded by electrons

The structure of the atom


Any given element [element: A substance made of one type of atom only. ] is made of atoms of just one particular sort. The atoms of any element are different to the atoms of any other element. So iron is made from a different sort of atom to sulfur, and carbon atoms are different to oxygen atoms.

Chemical symbols

The atoms of each element are represented by chemical symbols. These usually consist of one or two different letters but three letters are sometimes used for newly discovered elements.

The first letter in a chemical symbol is always an UPPER CASE letter. The other letters are always lower case. So the symbol for magnesium is Mg, and not mg, MG or mG.

Every element has its own chemical symbol. For example, iron is Fe, sulfur is S, sodium is Na and oxygen is O.

Chemical formulas

A compound [compound: A compound is a substance formed by the chemical union (involving bond formation) of two or more elements. ] consists of two or more atoms from different elements chemically joined together. There are two types of chemical bond:

  • covalent bonds
  • ionic bonds.

Covalent bonds

A covalent bond [covalent bond: A covalent bond between atoms forms when atoms share electrons to achieve a full outer shell of electrons. ] forms when two atoms share a pair of electrons. The atoms involved are non-metals such as carbon, oxygen and hydrogen. They join together to form molecules [molecule: A molecule is a collection of two or more atoms held together by chemical bonds. It is the smallest part of a substance that displays the properties of the substance. ].

The animation shows a covalent bond being formed between a hydrogen atom and a chlorine atom to form a molecule of hydrogen chloride:

Ionic bonds

Atoms can lose or gain electrons in chemical reactions. When they do this they form charged particles called ions [ions: Electrically charged particles, formed when an atom or molecule gains or loses electrons. ].

  • hydrogen atoms and metal atoms can lose electrons to form positively charged ions.
  • non-metal atoms can gain electrons to form negatively charged ions.

Ionic bonds are the forces of attraction between oppositely charged ions in a compound.

Chemical formulas

The chemical formula of a compound tells you how many atoms of each element the molecule contains.

Formulas of some compounds, with the number of atoms of each element in the molecule

nameformulasodium atomshydrogen atomscarbon atomssulfur atomsoxygen atoms
carbon monoxideCO11
carbon dioxideCO212
sulfur dioxideSO212
sulfuric acidH2SO4214
sodium carbonateNaCO3113

The small 2 after an element tells you there are two atoms of that particular element in each molecule. For example, the water molecule H2O has two hydrogen atoms. Notice that you do not write a number 1 if there is only one atom of an element in a molecule.


Some formulas have brackets in them. For example, sodium hydroxide is NaOH, but magnesium hydroxide is Mg(OH)2. The 2 outside the brackets tells you that you have two of each atom inside the bracket. So in Mg(OH)2 you have one magnesium atom, two oxygen atoms and two hydrogen atoms. The table shows some more examples of these formulas.

Examples of formulas with brackets

nameformulairon atomsoxygen atomshydrogen atoms
iron(III) hydroxideFe(OH)311 × 3 = 3
iron(II) nitrateFe(NO3)211 × 2 = 23 × 2 = 6

Displayed formulas

A chemical formula, such as H2O, shows the number and type of each atom in a molecule. A displayed formula shows more information.

Displayed formulas

In a displayed formula, the symbols for each atom are joined by straight lines. Each line represents a covalent bond (a shared pair of electrons).

The table shows some examples of displayed formulas:

Examples of displayed formulas

substancechemical formuladisplayed formulamolecular model
hydrogenH2H - Htwo atoms joined with a straight horizontal line
waterH2OH - O - Hthree atoms joined
ammoniaNH3H - N - H (with a line down from the N to an H) four atoms joined
methaneCH4H - C - H in a row, line from above the C to an H, line from below the C to an H five atoms joined

You should be able to write the molecular formula of a compound when given its displayed formula. Compare each displayed formula in the table above with its corresponding chemical formula: check that you can see the connection between them.

Word and balanced equations

Chemical equations show what happens in a reaction. In general, we write:

reactants → products

The reactants are the substances that react together. The products are the substances produced in the reaction. Individual substances are separated by a plus sign.

Word equations

A word equation gives the names of the substances involved in a reaction. For example:

copper + oxygen → copper(II) oxide

Copper and oxygen are the reactants, and copper(II) oxide is the product.

Balanced equations

Balanced equations give the symbols and formulas of the substances involved in a reaction. In the example above, if we just replace the words shown above with the correct chemical formulas, we will get an unbalanced equation, as shown here:

Cu + O2    →    CuO

To make things equal, we need to adjust the number of units of some of the substances until we get equal numbers of each type of atom on both sides of the arrow.

Here is the balanced symbol equation:

2Cu + O2    →    2CuO

You can see that we now have two copper atoms and two oxygen atoms on each side. This matches what happens in the reaction.

Two atoms of copper react with two atoms of oxygen to form two molecules of copper oxide

Here are some other examples of balanced equations. Check that you understand why they are balanced.

  • Mg + Cl2    →    MgCl2
  • 2Na + Cl2    →    2NaCl
  • 4Fe + 3O2    →    2Fe2O3
  • 4Na + O2    →    2Na2O
  • 2Na + 2H2O    →    2NaOH + H2.

Formulas and equations

You should recall certain chemical formulas. You should also be able to write balanced symbol equations if you are given the formulas of the reactants and products.

Specified formulas- both tiers

Formulas of substances

carbon monoxideCO
carbon dioxideCO2
calcium oxideCaO
calcium carbonateCaCO3
hydrochloric acidHCl
sodium chlorideNaCl
potassium chlorideKCl

Balancing equations

Example: write the balanced equation for the reaction between hydrogen and oxygen to produce water.

Step 1

Write the formulae for each substance:

H2 + O2 → H2O

Step 2

Check for an unbalanced element, for example, O. Adjust the number of each atom or molecule needed, but never change a formula. We need two O atoms on each side:

H2 + O2 → 2H2O

Step 3

Check for another unbalanced element. In this example, there are 2 H atoms on the left and 2 × 2 = 4 atoms on the right. So we need to double the number of hydrogen molecules:

2H2 + O2 → 2H2O

Formulas and equations- higher tier

You should recall certain chemical formulas in addition to the ones on the previous page. You should also be able to write balanced symbol equations, given the names or formulas of the reactants and products (with brackets in the formulas).

Specified formulas- higher tier

Speficied formulas

ammonium sulfate(NH4)2SO4
calcium chlorideCaCl2
copper(II) oxideCuO
copper(II) sulfateCuSO4
magnesium chlorideMgCl2
magnesium sulfateMgSO4
nitric acidHNO3
potassium hydroxideKOH
potassium sulfateK2SO4
sodium carbonateNa2CO3
sodium hydrogencarbonateNaHCO3
sodium hydroxideNaOH
sodium sulfateNa2SO4
sulfur dioxideSO2
sulfuric acidH2SO4

Balancing equations

Example: Write the balanced equation for the reaction between calcium and water to produce calcium hydroxide and hydrogen.

Step 1

Write the formulae for each substance:

Ca + H2O → Ca(OH)2 + H2

Step 2

Check for one unbalanced element, for example, O. Adjust the number of each atom or molecule needed, but never change a formula. We need two Os on each side:

Ca + 2H2O → Ca(OH)2 + H2

Step 3

Check for another unbalanced element. In this example, the equation is now balanced.

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