Worked examples: Matter and the Atomic Structure

Work through each example by covering the answer, attempting it yourself, then checking your method against the solution. Every example flags the slip that most often costs marks. These are original questions in SPM style, not past-year papers.

Example 1, Particle arrangement in the three states

Question. Explain, in terms of particles, why a gas fills any container it is placed in but a solid keeps a fixed shape.

Solution. In a gas, the particles are far apart with negligible forces of attraction between them, so they move freely and randomly at high speed and spread out to occupy the whole container. In a solid, the particles are held closely together in a fixed, orderly arrangement by strong forces of attraction, so they can only vibrate about fixed positions and cannot move from place to place, the shape stays fixed.

Common slip: describing what happens without mentioning the forces of attraction. The forces are the reason, and the reason carries the marks.

Example 2, Reading a heating curve

Question. A solid is heated at a steady rate. Its temperature rises, then stays constant for a period, then rises again. Explain why the temperature stays constant.

Solution. The constant temperature is the melting point. During this time the heat supplied is used to overcome the forces of attraction holding the particles in the solid lattice so that they can move more freely, rather than to increase their kinetic energy. Because the average kinetic energy does not change, the temperature does not rise until melting is complete.

Common slip: writing “the heat is stored” or “no heat is added”. Heat is still being supplied; it is being used to weaken the forces, not to raise the temperature.

Example 3, Cooling curve

Question. When a liquid is cooled, its temperature falls and then remains constant while it freezes. Explain the flat part of the cooling curve.

Solution. As the liquid freezes, the particles come closer and forces of attraction form between them, releasing heat energy to the surroundings. This released energy balances the heat being lost, so the temperature stays constant at the freezing point until all the liquid has solidified.

Common slip: saying the temperature is constant “because it is freezing” without mentioning that energy is released as the forces form.

Example 4, Diffusion

Question. A few drops of bromine liquid are placed at the bottom of a gas jar. After a while the brown colour spreads throughout the jar. Name the process and explain it using the kinetic theory. State one factor that would make it faster.

Solution. The process is diffusion. Bromine evaporates to form a gas whose particles are in constant, random motion; they move from the region of high concentration at the bottom to regions of lower concentration, spreading the brown colour throughout the jar. Raising the temperature would make the diffusion faster, because the particles gain kinetic energy and move faster.

Common slip: calling the movement “spreading” without naming diffusion or linking it to moving particles.

Example 5, Development of the atomic model

Question. State the contribution of Rutherford to the atomic model, and name the scientist who discovered the neutron.

Solution. Rutherford proposed the nuclear model: the atom has a small, dense, positively charged nucleus at its centre, with electrons moving in the mostly empty space around it. The neutron was discovered by James Chadwick.

Common slip: mixing up the scientists, for example crediting Rutherford with the electron (that was Thomson) or with fixed shells (that was Bohr).

Example 6, Subatomic particles from the symbol

Question. An atom is represented with nucleon number 23 and proton number 11. State the number of protons, neutrons and electrons in a neutral atom of this element.

Solution. The proton number is 11, so there are 11 protons. In a neutral atom the number of electrons equals the number of protons, so there are 11 electrons. The number of neutrons equals the nucleon number minus the proton number: 23 − 11 = 12 neutrons.

Common slip: subtracting the wrong way round, or forgetting that electrons equal protons only in a neutral atom (not in an ion).

Example 7, Identifying isotopes

Question. Three atoms have these compositions: P has 17 protons and 18 neutrons; Q has 17 protons and 20 neutrons; R has 18 protons and 17 neutrons. Which two are isotopes, and why? State one property they share and one that differs.

Solution. P and Q are isotopes, because they have the same proton number (17), so they are atoms of the same element, but different nucleon numbers (35 and 37) because they have different numbers of neutrons. They share the same chemical properties, because they have the same electron arrangement; they differ in a physical property such as density, because they have different masses. R is a different element (18 protons).

Common slip: choosing atoms with the same nucleon number instead of the same proton number.

Example 8, Electron arrangement

Question. An atom has 17 electrons. Write its electron arrangement, state the number of valence electrons, and give its group and period in the periodic table.

Solution. Fill the shells from the inside: 2 in the first shell, 8 in the second, leaving 7 for the third. The electron arrangement is 2.8.7. There are 7 valence electrons (the outermost shell). The number of valence electrons gives the group, so it is in Group 17; the number of occupied shells gives the period, so it is in Period 3.

Common slip: over-filling the second shell or writing the shells in the wrong order, always fill from the innermost shell first, and never exceed 2, 8, 8 for the first twenty elements.

Example 9, Sublimation

Question. When solid iodine crystals are heated gently, a purple vapour forms and no liquid is seen. Name this change of state and describe the change in the arrangement of the particles.

Solution. The change is sublimation, a solid changing directly into a gas without passing through the liquid state. On heating, the particles gain enough kinetic energy to overcome the forces of attraction and escape directly from the fixed, orderly, closely-packed solid arrangement into the far-apart, randomly-moving gas arrangement.

Common slip: naming it “evaporation” or “melting”. Sublimation goes straight from solid to gas, with no liquid stage.

Example 10, Compressing a gas

Question. Explain, in terms of particles, why a gas can be compressed into a much smaller volume but a liquid cannot.

Solution. In a gas the particles are far apart with large empty spaces between them, so applying pressure pushes the particles closer together and greatly reduces the volume. In a liquid the particles are already close together with very little space between them, so they cannot be pushed significantly closer, and the liquid strongly resists compression.

Common slip: saying a gas compresses “because it is light”, compressibility depends on the spacing between particles, not on how heavy they are.

Using these examples

Notice that every answer either describes particles and their forces, works cleanly from the two numbers, or fills shells in strict order, the habits this chapter rewards. Once the method feels automatic, move on to the practice questions and mark yourself the same way. A one-to-one teacher can check that your heating-curve and isotope answers are complete, which is exactly where marks are gained or lost in the SPM Chemistry written papers.