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. In one line: 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.
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.
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.
What the Cambridge 9701 syllabus says
Official 2025-2027 spec · AS LevelThese are the exact learning outcomes Cambridge sets for this topic. The candidate is expected to be able to do each of these on the relevant paper.
- use dot-and-cross diagrams to illustrate ionic, covalent and coordinate bonding including the representation of any compounds stated in 3.4 and 3.5 (dot-and-cross diagrams may include species with atoms which have an expanded octet or species with an odd number of electrons)
- explain the origin of pressure in a gas in terms of collisions between gas molecules and the wall of the container
- understand that ideal gases have zero particle volume and no intermolecular forces of attraction
- state and use the ideal gas equation pV = nRT in calculations, including in the determination of Mr 4.2 Bonding and structure Learning outcomes
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 Dot-and-cross diagrams
- › Only use 'intermolecular' when referring to simple molecules; use 'electrostatic attraction' for ionic, metallic, or giant covalent lattice forces.
- › Describe hybridisation as the mixing of atomic orbitals of similar energies to form a new set of equivalent hybrid orbitals.
- › Remember that a water molecule can form four hydrogen bonds: two through its lone pairs and two through its hydrogen atoms.
- › Count all single bonds and one bond from every double/triple bond as sigma bonds; ensure every C-H bond is included.
- › Recall that an octahedral structure has both 90 and 180 degree bond angles; visualize trans atoms to identify the 180 degree angles.
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.
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.
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.
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.
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).
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.
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.
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
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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
Every term below is defined in the flashcards above. Use the list as a quick recall test before your exam — if you can't define one of these in your own words, flip back to that card.
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