Dot-and-cross diagrams
Cambridge A-Level Chemistry (9701) · Unit 3: Chemical bonding · 8 flashcards
Dot-and-cross diagrams is topic 3.7 in the Cambridge A-Level Chemistry (9701) syllabus , positioned in Unit 3 — Chemical bonding , alongside Electronegativity and bonding, Ionic bonding and Metallic bonding.
Marked as AS Level: examined at AS Level in Paper 1 (Multiple Choice), Paper 2 (AS Structured Questions) and Paper 3 (Advanced Practical Skills). The same content may also be assumed in Paper 4 (A Level Structured Questions).
The deck below contains 8 flashcards — 4 definitions, 3 key concepts and 1 calculation — covering the precise wording mark schemes reward. Use the 4 definition cards to lock down command-word answers (define, state), then move on to the concept and calculation cards to handle explain, describe, calculate and compare questions.
Some cards in this deck are on other syllabus topics: 4.1 The gaseous state: ideal and real gases and pV = nRT and 3.4 Covalent bonding and coordinate (dative covalent) bonding. Each card is labelled with its topic.
What the Cambridge 9701 syllabus says
2025-2027 syllabus, in our words · AS LevelWhat the syllabus expects you to be able to do in this topic, in plain English. For the exact wording, see the official syllabus (PDF).
- Draw dot-and-cross diagrams of coordinate, covalent and ionic bonding, covering every compound named in 3.4 and 3.5; the species drawn may have an odd number of electrons or contain an atom with an expanded octet.
Also in this deck: 4.1 The gaseous state: ideal and real gases and pV = nRT
- Explain how gas pressure arises from gas molecules colliding with the walls of their container.
- Understand that in an ideal gas the particles take up no volume and there are no attractive intermolecular forces.
- State pV = nRT, the ideal gas equation, and use it in calculations, including to determine Mᵣ.
Also in this deck: 3.4 Covalent bonding and coordinate (dative covalent) bonding
- Define a covalent bond as the electrostatic attraction between a shared pair of electrons and the nuclei of the two atoms it joins. (a) Describe covalent bonding in molecules, which must include hydrogen (H₂), oxygen (O₂), nitrogen (N₂), chlorine (Cl₂), hydrogen chloride (HCl), carbon dioxide (CO₂), ammonia (NH₃), methane (CH₄), ethane (C₂H₆) and ethene (C₂H₄). (b) Understand that period 3 elements can expand their octet, including in sulfur dioxide (SO₂), phosphorus pentachloride (PCl₅) and sulfur hexafluoride (SF₆). (c) Describe coordinate (dative covalent) bonding, including in the ammonium ion, NH₄⁺, formed when gaseous ammonia reacts with gaseous hydrogen chloride, and in the Al₂Cl₆ molecule.
- (a) Describe covalent bonds as overlapping orbitals that give σ and π bonds: a σ bond forms when orbitals overlap directly between the two bonded atoms, and a π bond forms when neighbouring p orbitals overlap sideways, so the overlap lies on either side of the σ bond (above and below it). (b) For molecules including N₂, HCN, C₂H₄, C₂H₆ and H₂, describe the way their σ and π bonds are formed. (c) Describe sp, sp² and sp³ orbitals using the idea of hybridisation.
- (a) Define bond energy (the energy needed to break one mole of a given covalent bond, with the species in the gas phase) and bond length (the distance between the nuclei of two atoms joined by a covalent bond). (b) Compare how reactive covalent molecules are, using bond energy values and the idea of bond length.
Cambridge syllabus keywords to use in your answers
These are the official Cambridge 9701 terms tagged to this section. Mark schemes credit responses that use the exact term — weave them into your answers verbatim rather than paraphrasing.
Tips to avoid common mistakes in these topics
- › Represent lithium hydride in its ionic form as Li+ and H-, showing exactly four electrons total.
- › State that a gas behaves most ideally at high temperature and low pressure; do not list assumptions like 'negligible volume' unless asked.
- › Distinguish between mass and volume; ideal gas theory assumes molecules have negligible volume and no intermolecular forces, but they still possess mass.
- › Specify the exact atomic orbitals involved in hybridisation, such as the sp hybridised orbitals of nitrogen in the N2 molecule.
- › Describe hybridisation as the mixing of atomic orbitals of similar energies to form a new set of equivalent hybrid orbitals.
Topic 3.7 · Dot-and-cross diagrams
Draw a dot-and-cross diagram for sodium chloride (NaCl), clearly showing the charges on the ions.
Na has 1 valence electron, which it donates to Cl (7 valence electrons) to form Na⁺ (full outer shell) and Cl⁻ (full outer shell). The diagram should show the transfer of the electron and the resulting charges on each ion.
Topic 3.7 · Dot-and-cross diagrams
Draw a dot-and-cross diagram for carbon dioxide (CO₂), showing only the outer shell electrons.
Carbon shares two electrons with each oxygen atom forming two double covalent bonds. The diagram should show each oxygen with 8 electrons in its outer shell and carbon with 8 electrons in its outer shell.
Topic 4.1 · The gaseous state: ideal and real gases and pV = nRT
Explain, in terms of molecular collisions, how the pressure of a gas arises.
Gas pressure is caused by the continuous random motion of gas molecules colliding with the walls of the container. Each collision exerts a small force; the cumulative force over the area of the container walls is the pressure.
Topic 4.1 · The gaseous state: ideal and real gases and pV = nRT
State the two main assumptions of the kinetic theory of gases that are not valid for real gases.
Ideal gases are assumed to have zero particle volume, meaning the volume of the gas particles themselves is negligible compared to the volume of the container. Ideal gases also have no intermolecular forces of attraction between the gas particles.
Topic 4.1 · The gaseous state: ideal and real gases and pV = nRT
State the Ideal Gas Equation, defining each term.
The ideal gas equation is pV = nRT, where p is pressure (Pa), V is volume (m³), n is the number of moles, R is the ideal gas constant (8.31 J K⁻¹ mol⁻¹), and T is temperature (K).
Topic 4.1 · The gaseous state: ideal and real gases and pV = nRT
A gas occupies 10.0 dm³ at 27 °C and 100 kPa. Calculate the number of moles of gas present (R = 8.31 J K⁻¹ mol⁻¹).
Using pV = nRT, first convert to SI units: V = 0.010 m³, T = 300 K. Rearrange to n = pV/RT = (100000 x 0.010) / (8.31 x 300) = 0.401 mol.
Topic 3.7 · Dot-and-cross diagrams
Draw a dot-and-cross diagram for the ammonium ion (NH₄⁺), showing only outer shell electrons and indicating the coordinate bond.
Nitrogen shares electrons with three hydrogen atoms (covalent bonds). The nitrogen atom donates its lone pair to another hydrogen ion (H⁺) forming a coordinate bond, creating the NH₄⁺ ion with a +1 charge.
Topic 3.4 · Covalent bonding and coordinate (dative covalent) bonding
Define a coordinate bond and how it differs from a covalent bond.
A coordinate bond (also called a dative covalent bond) is formed when one atom provides both of the electrons for the shared pair in the bond. In a regular covalent bond, each atom provides one electron to be shared.
More Chemistry flashcards
Browse every 9701 flashcard topic by syllabus area.
All Chemistry FlashcardsMore topics in Unit 3 — Chemical bonding
Dot-and-cross diagrams sits alongside these A-Level Chemistry decks in the same syllabus unit. Each uses the same spaced-repetition system, so progress in one informs the next.
Key terms covered in this Dot-and-cross diagrams deck
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