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You have now worked through the whole of Topic C1 of your OCR Gateway Combined Science course — the particle model and states of matter, changes of state and heating curves, the structure of the atom, atomic number, mass number and isotopes, the development of the atomic model, electronic structure, and relative atomic mass. This final lesson pulls it all together. It shows how the C1 ideas connect, drills the maths skills that earn reliable marks (standard form, reading curves, weighted averages), gathers the misconceptions that catch students out, and finishes with a synoptic model answer. Treat it as a revision and exam-technique session rather than as new content.
By the end of this lesson you should be able to see how the C1 topics connect, perform every C1 maths skill confidently, avoid the common C1 errors, and structure a top-band particle-model answer.
This lesson draws on all three assessment objectives together: AO1 (recalling the key C1 facts), AO2 (performing every C1 maths skill — standard form, reading curves and weighted-mean Ar) and AO3 (analysing and evaluating in the synoptic model answers).
It helps to see C1 as one connected story. Everything is made of particles; whether those particles form a solid, liquid or gas depends on their arrangement and energy; and each particle is itself an atom with a definite internal structure that determines the element's place in the periodic table.
flowchart TD
A["All matter is made of particles"] --> B["States of matter<br/>solid / liquid / gas"]
A --> C["Each particle is an atom"]
B --> D["Changes of state<br/>+ heating curves"]
C --> F["Inside the atom<br/>nucleus + electrons"]
F --> G["Atomic number,<br/>mass number, isotopes"]
F --> H["Development of<br/>the atomic model"]
F --> I["Electronic structure<br/>→ periodic table"]
G --> J["Relative atomic mass<br/>(weighted mean)"]
Notice the links: the arrangement and energy of particles explains both the states of matter and the changes of state; looking inside the particle gives the atom, whose electronic structure sets its position in the periodic table and whose isotopes lead to relative atomic mass. Seeing these connections is exactly the kind of synoptic thinking that lifts an answer.
The topic also builds naturally towards what comes next in the course. The electronic structure you learned in C1 — and in particular the number of electrons in the outer shell — is the foundation of the whole of bonding in Topic C2: whether an atom loses, gains or shares electrons depends entirely on how many outer electrons it starts with. In the same way, the idea of relative atomic mass leads on to relative formula mass and to the mole. So although this is the last lesson of C1, none of it is a dead end; each idea is a tool you will reach for again and again in later chemistry.
Several maths skills recur in C1. Here is each one again, with a fresh worked example so you can check your method.
To compare sizes, divide the front numbers and subtract the powers of ten.
Worked example: An atom has radius 1×10−10m; its nucleus has radius 2×10−14m. How many times larger is the atom's radius?
2×10−141×10−10=21×10(−10)−(−14)=0.5×104=5000
The atom's radius is about 5000 times larger than the nucleus's.
The flat sections are changes of state: the lower plateau is the melting point, the upper plateau is the boiling point.
Worked example: A substance has a heating curve with plateaus at −7°C and 59°C. What state is it at 20°C?
Melting point =−7°C; boiling point =59°C. Since 20°C lies between the two, the substance is a liquid (this is bromine).
For ZAX: protons =Z, electrons =Z, neutrons =A−Z.
Worked example: How many protons, neutrons and electrons are in 1939K? Protons =19; electrons =19; neutrons =39−19=20.
Ar=100∑(isotope mass×% abundance)
Worked example: An element is 60% mass-69 and 40% mass-71. Find Ar. Ar=100(69×60)+(71×40)=1004140+2840=1006980=69.8 (this is gallium).
Exam Tip: Always show your working: for standard form, divide-and-subtract-powers; for curves, read across from the plateau to the temperature axis; for Ar, multiply–add–divide by 100. The method earns marks even if the final figure slips.
These combine ideas from across C1, which is what the hardest exam questions do.
A pure solid is heated steadily. Its heating curve shows a flat section at 801°C. State what is happening to the substance at 801°C, and explain, in terms of particles and energy, why the temperature stays constant.
At 801°C the substance is melting (the lower plateau is the melting point), changing from solid to liquid. The temperature stays constant because the energy supplied is used to overcome the forces of attraction holding the particles in the fixed solid lattice, so the particles can break free and move around as a liquid; this energy does not increase the particles' average kinetic energy, so the temperature does not rise.
An atom has a diameter of 2×10−10m and a nucleus of diameter 2×10−14m. Show that the nucleus is about 1/10 000 of the diameter of the atom.
atomnucleus=2×10−102×10−14=1×10(−14)−(−10)=1×10−4=100001
So the nucleus is about 1/10 000 of the atom's diameter — confirming that the atom is mostly empty space.
Element Z has two isotopes: 63Z (69%) and 65Z (31%). Explain why both isotopes react in the same way, and calculate the relative atomic mass of Z.
Both isotopes have the same number of protons (and so the same number of electrons, arranged the same way), and chemical reactions depend on electrons, so the extra neutrons make no difference to how they react. The relative atomic mass is:
Ar=100(63×69)+(65×31)=1004347+2015=1006362=63.6
(Z is copper.)
OCR uses specific command words that tell you exactly what kind of answer to give. Reading them correctly is worth easy marks.
| Command word | What it asks for |
|---|---|
| State / Name / Give | A short fact, no explanation (e.g. "Name the change of state from liquid to gas") |
| Describe | Say what happens or what you see, with no need for reasons (e.g. describe the arrangement of particles in a solid) |
| Explain | Give reasons why — use "because", "so that" (e.g. why a gas can be compressed) |
| Calculate | Work out a number — show working and give a unit (e.g. a relative atomic mass or a size ratio) |
| Predict / Deduce | Use the rules to work something out (e.g. the state at a given temperature, or the group from an electronic structure) |
| Evaluate | Give both sides and a judgement (e.g. the usefulness and limitations of the simple particle model) |
Exam Tip: The difference between describe and explain decides many marks. "The particles are far apart" describes; "...so the gas can be compressed because there are gaps between them" explains. If the command word is explain, you must give the reason.
Many C1 questions ask you to explain an observation in terms of particles. A reliable structure is: (1) name the arrangement of the particles in the relevant state(s); (2) describe their movement and energy; (3) refer to the forces of attraction between them; and (4) link each of these back to the property or observation in the question. Working through arrangement → movement → forces → link makes sure you hit every marking point rather than just restating the observation.
Use this as a final recall list. Cover the right-hand column and test yourself.
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