Revision notes for OCR GCSE Chemistry Bonding. Open the guide for explanations and worked examples. Written against the OCR GCSE Chemistry (J248) specification, so the content matches what's examinable rather than general Chemistry background.
Bonding
What you'll learn
- How electron shells link atomic number, group, period and reactivity.
- Why metals and non-metals behave differently.
- How ionic, covalent and metallic bonding work using electrons and electrostatic forces.
- How to draw and judge dot-and-cross diagrams and other bonding models.
The atomic structure you need first
Atoms are made from a tiny central nucleus containing protons and neutrons, with electrons arranged in electron shells around it. In GCSE bonding, an electron shell is a simple model for an energy level where electrons are found.
Definition
Atomic number
The atomic number of an element is the number of protons in the nucleus of one atom of that element. In a neutral atom, it is also the number of electrons.
For the first 20 elements, you normally fill shells as 2 in the first shell, then 8 in the second, then 8 in the third for simple GCSE diagrams. The electrons in the outer shell are called outer electrons or valence electrons.
Key Idea
Outer electrons control bonding
Chemical reactions usually involve atoms losing, gaining or sharing outer electrons. The nucleus does not change during ordinary chemical bonding.
This diagram shows how sodium’s electron arrangement links to its position in the Periodic Table.
Size and scale
Atoms are extremely small: a typical atomic radius is about 1×10−101 \times 10^{-10}1×10−10 m, which is 0.1 nm. A nanoparticle is usually between 1 nm and 100 nm in size. This size-estimation work is for separate Chemistry J248, not Combined Science.
Example
Estimating atomic scale
An atom has a diameter of about 0.1 nm. Estimate how many atoms would fit in a straight line across 1 mm.
- Convert the atom diameter into metres: 1 nm is 1×10−91 \times 10^{-9}1×10−9 m, so 0.1 nm is 1×10−101 \times 10^{-10}1×10−10 m.
- Convert 1 mm into metres: 1 mm is 1×10−31 \times 10^{-3}1×10−3 m.
- Divide the length by the atom diameter:
- So about ten million atoms would fit across 1 mm.
The Periodic Table as an electron map
The modern Periodic Table is arranged in order of increasing atomic number. This means the number of protons increases by one as you move from one element to the next.
For the main-group elements:
- The group number tells you the number of outer electrons.
- The period number tells you the number of occupied electron shells.
- Elements in the same group have similar chemical reactions because they have the same number of outer electrons.
Group 0 elements are the noble gases. They have full outer shells, so they are very unreactive. Helium is a special case: it has 2 electrons in its full first shell.
Example
Finding electron arrangement and position
Chlorine has atomic number 17. Work out its electron arrangement, group and period.
- A neutral chlorine atom has 17 electrons because its atomic number is 17.
- Fill the shells in order: 2 in the first shell, 8 in the second shell, leaving 7 in the third shell. The arrangement is 2,8,7.
- There are 3 occupied shells, so chlorine is in Period 3.
- There are 7 outer electrons, so chlorine is in Group 7 and tends to gain or share one electron in reactions.
Metals and non-metals
A metal is an element that usually conducts electricity and heat, is shiny when freshly cut, and can be bent or shaped. Metals are found on the left and centre of the Periodic Table.
A non-metal is an element that usually does not conduct electricity, is often dull or brittle when solid, and may be a gas at room temperature. Non-metals are found on the right-hand side of the Periodic Table.
Chemically, metals often form positive ions by losing electrons. Metal oxides are usually basic, and if they dissolve in water they form alkaline solutions. Non-metals often gain or share electrons. Non-metal oxides are usually acidic.
Tip
Fast metal/non-metal clue
If an element has only 1, 2 or 3 outer electrons, it is often a metal. If it has 5, 6 or 7 outer electrons, it is often a non-metal. Carbon and silicon are useful exceptions to think about carefully.
What is a chemical bond?
A chemical bond is not a tiny stick made of matter. It is a force of attraction that holds particles together.
Definition
Chemical bond
A chemical bond is a strong electrostatic force of attraction involving charged particles, such as oppositely charged ions or electrons attracted to nuclei.
Atoms often bond to get a more stable outer electron arrangement, usually a full outer shell.
Common Mistake
Bonding does not change the nucleus
When atoms lose, gain or share electrons, only the electrons are rearranged. The number of protons in the nucleus stays the same, so the element does not change into a different element.
Ionic bonding
Ionic bonding usually happens between a metal and a non-metal. The metal atom loses electron(s) and becomes a positive ion. The non-metal atom gains electron(s) and becomes a negative ion.
Definition
Ions and ionic bonds
An ion is an atom or group of atoms with an electric charge. An ionic bond is the strong electrostatic attraction between oppositely charged ions.
For example, sodium transfers one electron to chlorine. Sodium becomes Na⁺ and chlorine becomes Cl⁻. The ions attract each other and form a giant ionic lattice, not separate molecules.
Common Mistake
Ionic compounds are not molecules
A pair such as Na⁺ and Cl⁻ is not a molecule. Sodium chloride forms a giant lattice of many positive and negative ions.
Example
Predicting the formula of an ionic compound
Predict the formula formed between magnesium and chlorine.
- Magnesium is in Group 2, so it has 2 outer electrons. It loses both and forms Mg²⁺.
- Chlorine is in Group 7, so it has 7 outer electrons. Each chlorine atom gains 1 electron and forms Cl⁻.
- The total charge must balance to zero. One Mg²⁺ needs two Cl⁻ ions because +2+2(−1)=0+2 + 2(-1) = 0+2+2(−1)=0.
- The formula is MgCl₂.
Covalent bonding
Covalent bonding usually happens between non-metal atoms. Instead of transferring electrons, the atoms share pairs of electrons.
Definition
Covalent bond
A covalent bond is a strong electrostatic attraction between a shared pair of electrons and the nuclei of the bonded atoms.
A molecule is a small group of atoms held together by covalent bonds. Examples include hydrogen chloride, HCl, and water, H₂O.
The diagram below compares electron transfer in ionic bonding with electron sharing in covalent bonding.
Drawing dot-and-cross diagrams
A dot-and-cross diagram shows outer-shell electrons. Dots and crosses are used to show which atom each electron came from.
For binary ionic substances, meaning ionic compounds made from two different elements:
- show electron transfer from metal to non-metal
- put ions in square brackets
- show charges, such as Na⁺ or Cl⁻
For simple covalent substances:
- overlap the outer shells
- show one shared pair for each single covalent bond
- check that each atom has a stable outer shell in the simple model
Example
Drawing water
Draw the dot-and-cross diagram for water, H₂O.
- Oxygen is in Group 6, so it has 6 outer electrons. Each hydrogen atom has 1 outer electron.
- Oxygen shares one pair of electrons with each hydrogen atom. That makes two O–H covalent bonds.
- Each hydrogen now has 2 electrons in its first shell, which is full.
- Oxygen now has 8 electrons around it: two shared pairs and two lone pairs.
Comparing bonding structures
Different substances have different arrangements of particles and bonds.
Ionic compounds
Ionic compounds contain a giant lattice of positive and negative ions. The bonding is strong electrostatic attraction in all directions.
Simple molecules
Simple molecular substances contain separate molecules. Atoms within each molecule are joined by covalent bonds. The molecules themselves are separate particles.
Giant covalent structures
A giant covalent structure is a huge network of atoms joined by covalent bonds. Diamond and graphite are examples of carbon allotropes, meaning different structural forms of the same element.
Polymers
A polymer is a very large molecule made from repeating units. The atoms in the chain are joined by covalent bonds. Polymer diagrams often look flat, but real polymer chains are three-dimensional.
Metals
Metals contain positive metal ions in a regular lattice, surrounded by delocalised electrons. Delocalised electrons are electrons that are free to move through the structure. Metallic bonding is the electrostatic attraction between positive metal ions and delocalised electrons.
Limits of bonding models
Models are useful, but they are never perfect.
Dot-and-cross diagrams show outer electrons clearly, but they do not show the true size of atoms, the distance between atoms, or the 3D shape of a molecule.
Ball-and-stick models help you picture shape, but the “sticks” are not real bonds and the atom sizes are not always accurate.
Two-dimensional diagrams are convenient on paper, but many structures are three-dimensional. For example, diamond is a 3D giant covalent network, and simple molecules may have 3D shapes rather than being flat.
Common Mistake
Models are simplified
Do not assume a diagram shows everything. Always ask what the model is trying to show: electron transfer, electron sharing, particle arrangement, or 3D shape.
From Mendeleev to the modern Periodic Table
Mendeleev arranged elements mainly by atomic mass and chemical properties. He left gaps for undiscovered elements and predicted their properties.
The modern Periodic Table is arranged by atomic number instead. This fixed ordering problems because atomic number is the number of protons, which defines the element. Once elements are arranged by atomic number, repeating patterns in electron arrangements explain the repeating chemical properties of groups.
Key Idea
Why atomic number matters
Atomic number determines the number of electrons in a neutral atom, which determines the electron arrangement, which strongly influences bonding and reactions.
Exam technique
In the exam
- For bonding questions, start from the electron arrangement: identify outer electrons, then decide whether electrons are lost, gained or shared.
- For ionic dot-and-cross diagrams, always include square brackets and charges on the ions.
- For model questions, say both what the model shows well and what it leaves out, such as 3D shape or true particle size.
Self review
Check yourself
- Why do sodium and potassium have similar reactions?
- How is an ionic bond different from a covalent bond?
- What are two limitations of a dot-and-cross diagram?
Recap questions
Test yourself with 5 quick questions on this guide. Answer them all correctly to complete it.
Elements, compounds and mixtures
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