MYP Chemistry · Topic 4
Matter
Everything is made of particles in constant motion. Their arrangement explains the states; their structure explains the elements.
Topic: Matter (states and properties of matter; particle/kinetic theory, diffusion; atomic structure [including isotopes]; electron configuration and valency). Part 1 uses the particle model to explain states, changes of state, gas behaviour and diffusion. Part 2 looks inside the atom: protons, neutrons and electrons, isotopes, electron arrangements and the combining power of elements.
Prior knowledge: elements, compounds and mixtures; the periodic table (Topic 1).
Learning objectives
Objectives and contextOverview
By the end of this chapter you should be able to:
- describe the arrangement and movement of particles in solids, liquids and gases, and draw them correctly A
- explain changes of state and the shape of heating and cooling curves in terms of particles and energy A
- explain diffusion, and the effects of temperature and pressure on gases, using kinetic theory A B
- determine the numbers of protons, neutrons and electrons in atoms, isotopes and ions A
- deduce electron configurations for the first 20 elements and relate them to valency A
A tea bag placed in hot water colours the water within a minute; in cold water the colour creeps out slowly. A sealed bag of sweets carried up a mountain puffs up. Molten lead poured into a mould stays at exactly the same temperature for several minutes while it solidifies. None of these everyday observations makes sense until we accept that matter is made of tiny particles in constant motion — and once we do, all three follow from the same few ideas.
02 / The three states
States and propertiesStates and properties of matter
The kinetic particle theory states that all matter is made of particles (atoms, molecules or ions), that these particles are always moving, and that there are attractive forces between them. The state of a substance depends on the balance between two things: the kinetic energy of the particles, which tends to move them apart, and the attractive forces between them, which hold them together.
| Solid | Liquid | Gas | |
|---|---|---|---|
| Arrangement | Regular, closely packed, touching | Irregular, closely packed, touching | Random, far apart |
| Movement | Vibrate about fixed positions | Move around, sliding past each other | Move quickly and randomly in straight lines between collisions |
| Forces between particles | Strong | Weaker than in solid, still significant | Negligible |
| Shape and volume | Fixed shape and volume | Takes the shape of the container; fixed volume | Fills the container; no fixed volume |
| Compressible? | No | Hardly | Yes — mostly empty space |
| Density | High | High (usually a little lower than the solid) | Low |
A drawn liquid must look different from both a solid and a gas. Its particles are touching (most of them in contact with neighbours), irregularly arranged, and gathered towards the bottom of the container. Widely spaced particles or isolated pairs look like a gas and were not credited. A solid needs a regular pattern of touching particles.
03 / Changing state
Changes of stateParticle/kinetic theory
Heating a substance gives its particles more kinetic energy. In a solid they vibrate more strongly until, at the melting point, they have enough energy to break out of their fixed positions: the solid melts. In a liquid, heating makes the particles move faster until, at the boiling point, they have enough energy to overcome the attractions completely and move far apart: the liquid boils. Cooling reverses each step — condensing (gas → liquid) and freezing (liquid → solid). Evaporation also turns liquid into gas, but only from the surface and at any temperature below the boiling point.
Melting point: the temperature at which a solid turns into a liquid (the same temperature as the freezing point of the liquid).
Boiling point: the temperature at which a liquid turns into a gas throughout the liquid.
Sublimation: a change directly from solid to gas without melting (for example solid carbon dioxide).
While a substance is changing state, its temperature stays constant even though heat is still being supplied or removed. The energy is being used to overcome the attractive forces between particles (melting, boiling), or is released as the attractions form again (freezing, condensing), instead of changing the particles' kinetic energy. That produces the flat section on a heating or cooling curve.
Given: Figure 4.2. Find: the melting point of lead and the state at A and at B.
Reasoning: the temperature is constant only while the state is changing, so the flat section marks the melting (freezing) point: 327 °C. A is above this temperature, before freezing: liquid. B is below it, after freezing is complete: solid.
Check: the curve falls again only after the plateau — the solid cannot cool until all the liquid has frozen.
Particles and temperature
Change the temperature of a substance with melting point 0 °C and boiling point 100 °C (water). The model compares the temperature with the melting and boiling points to set the state, then shows the particles with a speed that increases with temperature.
04 / Gases and kinetic theory
Gas pressure, temperature and volumeParticle/kinetic theory
Gas particles collide with the walls of their container; each collision exerts a force, and the collisions together produce the pressure. Three connected ideas follow.
- Heating a gas in a rigid container raises the pressure: faster particles collide with the walls more often and with more force.
- Heating a gas in a flexible container makes it expand, so the same mass occupies a larger volume and its density decreases. The warm air inside a hot-air balloon is less dense than the cooler air around it, so the balloon rises.
- Reducing the pressure outside a sealed flexible container makes it expand, because the gas inside now pushes out harder than the surroundings push in. A sealed bag of sweets swells as a balloon or aircraft climbs, because atmospheric pressure decreases with altitude while the pressure inside the bag is unchanged.
Pressure also controls how much gas stays dissolved in a liquid. A fizzy drink is bottled under high pressure of carbon dioxide. When the cap is removed, the pressure above the liquid falls, and dissolved gas comes out of solution as bubbles.
Build the chain: temperature ↑ → kinetic energy of the particles ↑ → they move faster / collide more often and harder → observed effect. “Hot air rises” is only the starting point and earns one mark at most; the explanation through kinetic energy, volume and density is what earns the rest.
05 / Diffusion
DiffusionDiffusion
Diffusion is the net movement of particles from a region where they are at a higher concentration to a region where they are at a lower concentration, as a result of their random motion. No stirring is needed. The colour spreading from a tea bag, the smell of perfume crossing a room and pollutants spreading from a chimney are all diffusion. It is fastest in gases (particles far apart and fast), slower in liquids, and effectively absent in solids.
Two factors change the rate of diffusion:
- Temperature. At a higher temperature the particles have more kinetic energy and move faster, so they mix more quickly. Tea colours hot water faster than cold water.
- Mass of the particles. At the same temperature, lighter particles move faster and diffuse faster. Ammonia (M = 17) diffuses faster than hydrogen chloride (M = 36.5): in a long tube with ammonia at one end and hydrogen chloride at the other, the white ring of ammonium chloride forms nearer the hydrogen chloride end.
Investigation: does the temperature of the water affect the time taken for tea to diffuse completely?
If the temperature of the water increases, then the time taken for diffusion to be complete will decrease, because the particles have more kinetic energy, so the tea particles and water molecules move faster and mix more quickly.
The “because” gives the particle-level mechanism; restating “because diffusion is faster” only repeats the prediction.
06 / Inside the atom
Atomic structureAtomic structure
An atom has a tiny, dense, positively charged nucleus containing protons and neutrons, surrounded by electrons arranged in shells. Almost all the mass is in the nucleus; almost all the volume is the space occupied by the electrons.
| Particle | Where | Relative mass | Relative charge |
|---|---|---|---|
| Proton | Nucleus | 1 | +1 |
| Neutron | Nucleus | 1 | 0 |
| Electron | Shells around the nucleus | about 1/1840 (negligible) | −1 |
Two numbers describe an atom. The atomic number (proton number), Z, is the number of protons; it identifies the element, and it is the number used to order the periodic table. The mass number, A, is the total number of protons and neutrons. In a neutral atom the number of electrons equals the number of protons, so the charges cancel.
number of neutrons = mass number − atomic number N = A − Z
An atom is written with the mass number at the top left and the atomic number at the bottom left of the symbol: 2311Na has 11 protons, 11 electrons and 23 − 11 = 12 neutrons.
Given: 21084Po and 21082Pb. Find: protons, neutrons and electrons in each atom.
Po: protons = Z = 84; electrons = 84 (neutral atom); neutrons = 210 − 84 = 126.
Pb: protons = 82; electrons = 82; neutrons = 210 − 82 = 128.
Point to notice: the same mass number does not mean the same element. The atomic number decides the element.
07 / Isotopes
Isotopes and relative atomic massIsotopes
Isotopes are atoms of the same element (same number of protons) with different numbers of neutrons, and so different mass numbers.
Magnesium has three stable isotopes, 24Mg, 25Mg and 26Mg. All have 12 protons and 12 electrons, so they have the same chemical properties — chemistry depends on electrons. They have 12, 13 and 14 neutrons, so their masses differ. Oxygen likewise occurs as a mixture of isotopes with 8, 9 and 10 neutrons: their mass numbers are 8 + 8 = 16, 8 + 9 = 17 and 8 + 10 = 18. Some isotopes are unstable and radioactive, such as radium isotopes and polonium-210; radioactivity can damage living tissue.
Because an element is a mixture of isotopes, its relative atomic mass, Ar, is a weighted average of the isotope masses, taking account of how abundant each one is. That is why the periodic table gives magnesium as 24.31 rather than a whole number.
Ar = Σ(mass number × percentage abundance) ÷ 100
Given: magnesium is 79.0% 24Mg, 10.0% 25Mg and 11.0% 26Mg (illustrative abundances, rounded). Find: Ar.
Calculation: (24 × 79.0 + 25 × 10.0 + 26 × 11.0) ÷ 100 = (1896 + 250 + 286) ÷ 100 = 24.32.
Check: the answer lies between 24 and 26 and close to 24, because 24Mg is by far the most abundant.
For “the number of neutrons in an atom of magnesium-24”, use the mass number of that isotope: 24 − 12 = 12. Subtracting from the relative atomic mass (24.31 − 12 = 12.31) is wrong — an atom cannot contain a fraction of a neutron.
Isotope mixer
Set the percentage abundance of each isotope. The model calculates the weighted mean, Ar = Σ(A × %)/100, and marks it on the scale. Abundances are rescaled if they do not add up to 100%.
08 / Electrons in shells
Electron configurationElectron configuration
Electrons occupy energy levels, or shells, around the nucleus. The first shell, closest to the nucleus, holds up to 2 electrons. For the first twenty elements the second and third shells fill to 8, and the fourth shell then starts. Electrons fill the lowest available shell first.
The configuration links structure to the periodic table (Topic 1). Neon, 2,8, has a full outer shell; that is why it is unreactive. Sodium, 2,8,1, has one electron more than neon and loses it easily. Chlorine, 2,8,7, is one electron short of argon and gains one easily.
Ions
An ion is an atom (or group of atoms) that has gained or lost electrons and so carries a charge. Losing electrons gives a positive ion; gaining electrons gives a negative ion. Only the number of electrons changes — the numbers of protons and neutrons stay the same.
Given: 24Mg2+. Find: protons, neutrons and electrons.
Protons = 12 (atomic number). Neutrons = 24 − 12 = 12. Electrons = 12 − 2 = 10, because the 2+ charge means two electrons have been lost. The configuration of Mg2+ is 2,8 — the same as neon.
09 / Combining power
ValencyValency
The valency of an element is its combining power: the number of electrons an atom loses, gains or shares when it forms bonds, so that it reaches a full outer shell. It can be read from the outer-shell electrons.
| Group | 1 | 2 | 13 | 14 | 15 | 16 | 17 | 18 |
|---|---|---|---|---|---|---|---|---|
| Outer electrons | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 (He 2) |
| Valency | 1 | 2 | 3 | 4 | 3 | 2 | 1 | 0 |
| Typical ion | +1 | +2 | +3 | — | −3 | −2 | −1 | none |
Metals on the left lose their outer electrons; their valency equals the number of outer electrons. Non-metals on the right gain electrons (or share them); their valency equals 8 minus the number of outer electrons. Carbon, in the middle, shares all four. Valencies predict formulas (Topic 6): magnesium (valency 2) and chlorine (valency 1) combine as MgCl2; carbon (4) and hydrogen (1) as CH4; aluminium (3) and oxygen (2) as Al2O3.
Atom and ion builder
Choose an element (Z = 1–20), a mass number and a charge. The model counts protons, neutrons and electrons, writes the electron configuration of the atom or ion (shells fill 2, 8, 8, 2), and gives the valency from the outer shell of the neutral atom.
10 / Practice
Examination practicePast examination tasks
These tasks are adapted from past on-screen examinations; the chemistry, data and marks are unchanged. Attempt each one before opening the marking guidance.
States and kinetic theory
A Criterion A · Knowing and understanding
Toy figures can be made by pouring molten lead into clay moulds and letting it cool. A scientist recorded the temperature of cooling lead; the graph is shown in Figure 4.2.
Marking guidance
(a) Solid.
(b) Point A (liquid): an irregular arrangement of at least 6 particles with at least 4 in contact, towards the base of the container. Completely dispersed particles, or pairs of particles suggesting gas molecules, are not accepted. Point B (solid): a regular arrangement of at least 6 particles at the base of the container.
(c) 327 °C (± 1 °C).
Marking guidance
(a) Four points:
- hot air rises, or warm air has a lower density than colder air;
- because the kinetic energy of the molecules is greater in warm air;
- so the same mass of gas has a bigger volume;
- it is the lower density of the warm air inside the balloon that makes it float up.
(b) The bag increases in size / inflates, because the external pressure decreases while the internal pressure is unchanged.
Hard water contains ions such as Mg2+. State the number of protons and neutrons in a 24Mg2+ ion.
Marking guidance
Protons = 12; neutrons = 12. (The charge changes only the number of electrons, which is 10.)
Magnesium is added to fireworks to produce white sparks. It has three stable isotopes, with mass numbers 24, 25 and 26.
Marking guidance
(a) Atoms of the same element with different numbers of neutrons (or different mass numbers); or atoms with the same number of protons and different numbers of neutrons (or mass numbers).
(b) 12. The answer 12.31 is not accepted.
Oxygen occurs as a mixture of three isotopes, which have 8, 9 and 10 neutrons. Calculate the mass number of the isotope that has 9 neutrons.
Marking guidance
- Mass number = protons + neutrons (seen or implied).
- Mass number = 17. Two marks for 17 alone; maximum 1 mark if a unit such as g is added — a mass number is a count, not a mass.
Neon exists as a mixture of isotopes; the most abundant are 20Ne, 21Ne and 22Ne.
Marking guidance
(a) Protons 10; neutrons 12; electrons 10.
(b) A diagram showing only 2 electrons in the inner shell (1 mark) and only 8 electrons in the outer shell (1 mark); each mark is awarded independently.
An old “atomic energy” toy set contained radioactive polonium-210 and lead-210. Determine the numbers of protons, neutrons and electrons in an atom of 21084Po and in an atom of 21082Pb.
Marking guidance
Po: p = 84, n = 126, e = 84. Pb: p = 82, n = 128, e = 82. One mark for each correctly completed set.
B Criterion B · Inquiring and designing
A student investigates whether the temperature of the water affects the time taken for tea to diffuse completely from a tea bag. Complete the hypothesis: If the temperature of the water increases, then the time taken for diffusion to be complete … because …
Marking guidance
Marks are awarded independently (words to that effect accepted):
- the time taken for diffusion to be complete decreases;
- the kinetic energy increases with increasing temperature;
- so the tea “particles” mix with the water molecules more quickly, or diffusion occurs more quickly.
A student has noticed that when making a cup of tea by placing a tea bag in boiling water, the colour of the water changes. This is due to the diffusion of the tea from the tea bag. The student wanted to know whether there is a relationship between the temperature of the water and the time for the diffusion to be complete.
The variables for this experiment are given below.
| Independent variable | Dependent variable | Control variable 1 | Control variable 2 |
|---|---|---|---|
| Temperature | Time taken for diffusion to be complete | Type of tea | Same type of tea bag used |
Suggest another control variable for this investigation.
Marking guidance
- Any one of the following [1 max]:
- constant colour (of solution)
- volume or amount of water
- type of glass or cup
A student knows that water freezes at 0°C but notices that other liquids freeze at different temperatures when they are put into a freezer. When chemicals are dissolved in water, the freezing point of the solution will become lower than the freezing point of water. The student wanted to know if there was a relationship between the solute and the freezing point of the solution.
The student placed salt water, sugar water, carbonated water, tap water and pure water into a freezer to investigate their freezing points. State the variables in the student’s experiment.
Independent variable: ____________
Dependent variable: ____________
Suggested answer
Not from an official marking scheme — a worked answer written for these notes.
Independent variable: type of liquid (salt water, sugar water, carbonated water, tap water, pure water). Dependent variable: freezing point — the temperature at which each liquid freezes.
Suggest two control variables for this investigation. Control variable one: Control variable two:
Suggested answer
Not from an official marking scheme — a worked answer written for these notes.
Any two, for example: volume of each liquid; type and size of container; position in the freezer or freezer temperature setting; starting temperature of the liquids; concentration of the dissolved substance.
Formulate a research question for the student’s investigation.
Suggested answer
Not from an official marking scheme — a worked answer written for these notes.
For example: “How does the substance dissolved in water (salt, sugar or carbon dioxide) affect the freezing point of water?” A research question names both the independent variable (what is dissolved) and the dependent variable (freezing point).
C Criterion C · Processing and evaluating
State of matter is a physical characteristic which allows materials to be classified and grouped together. Melting point is a useful property which is an important characteristic of materials. Two students wanted to check the melting point of the metal gallium, symbol (Ga). One student used a thermometer and the other used a temperature probe. The samples of gallium are placed in boiling tubes which As the temperature of the water increased, the students observed the change in state of the gallium.
Student B used a temperature probe and recorded the following results.
| Time / min | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 |
|---|---|---|---|---|---|---|---|---|
| Temperature / °C | 28 | 29 | 29.5 | 29.7 | 29.7 | 29.7 | 30 | 32 |
Using student B’s data above, determine the melting point of gallium.
Suggested answer
Not from an official marking scheme — a worked answer written for these notes.
29.7 °C. The temperature stays constant at 29.7 °C from 4 to 6 minutes while the gallium changes from solid to liquid; that constant temperature is the melting point.
State and justify whether student A in part (a) or student B in part (b) obtained the correct melting point of gallium.
Suggested answer
Not from an official marking scheme — a worked answer written for these notes.
Student B. The temperature probe recorded the temperature throughout heating, and the melting point is identified from the section where the temperature stays constant during the change of state. Student A relied on a single reading taken by eye at the moment melting appeared to start, which depends on judgement and is less reliable.
Chocolate is widely consumed around the world. The chemical composition of cocoa beans depends on the climate and location where the beans are grown. Cocoa beans are processed to produce cocoa butter and cocoa powder from which chocolate can be made. During the manufacturing process, chocolate is melted and solidified a number of times in a process called tempering. This processing results in changes to the arrangement of the molecules in the cocoa butter. The different arrangements of molecules affect the taste of the chocolate.
A student wanted to know if the melting point of a chocolate bar was influenced by the percentage of cocoa it contained. They tested several chocolate bars and produced the following results.
| Percentage of cocoa / % | Melting point / °C (trial 1) | Trial 2 | Trial 3 | Average melting point / °C |
|---|---|---|---|---|
| 30.0 | 69.0 | 68.0 | 70.0 | 69.0 |
| 37.0 | 50.0 | 51.0 | 51.0 | 51.0 |
| 60.0 | 37.0 | 36.0 | 37.0 | |
| 70.0 | 32.4 | 31.6 | 32.0 | 32.0 |
| 85.0 | 28.0 | 27.7 | 27.8 | 27.8 |
Calculate the average melting point for the chocolate with the composition 60 % cocoa and add your value to the table. Show your working and give your final value to three significant figures.
Suggested answer
Not from an official marking scheme — a worked answer written for these notes.
Average = (37.0 + 36.0 + 37.0) ÷ 3 = 110.0 ÷ 3 = 36.666… = 36.7 °C (three significant figures).
After further research, the student determined that the relationship in the graph should have been non-linear. Outline how the method could be improved to confirm that the relationship is non-linear.
Marking guidance
- record data points at intermediate temperatures
- carry out more than one trial
- calculate an average
D Criterion D · Reflecting on the impacts of science
Scientists studying space wonder whether life would be possible on planets other than the Earth. Most believe that, if molecular oxygen is present on a planet, then life is possible on this planet. But molecular oxygen itself is not a sign of life on a planet, because it might have formed from water. Samples collected from the Moon contain several isotopes of oxygen.
Several nations are showing renewed interest in going back to the Moon. So far, they have sent un-manned probes to orbit or land on the Moon. Future missions could involve building a space station or mining the Moon’s resources. Suggest two additional reasons why nations are interested in going back to the Moon and are launching these missions.
Marking guidance
- Accept any two reasonable suggestions, for example [max 2]
- collect materials that may be useful on the Earth
- collect materials that may give information about the origin of the structure or the atmosphere of the Moon or the universe
- search for water
- enable possible human settlement in the future
- collect materials to look for signs of life
- political control of the moon
- the Moon could be used as a base for exploring other planets
Atoms, isotopes and ions
Retrieval: matter and atoms
Original practice questions for retrieval — not past examination items.
11 / Examination feedback
Examiner's overall observationEvidence from examination feedback
Examiner's overall observation
Explaining macroscopic observations at the particle level is the main difficulty in this topic. Students struggled to draw the particles in a liquid — in contact with one another, irregularly arranged and towards the bottom of the container — and did not always draw a solid as a regular arrangement of touching particles. In questions on distillation, most could name the changes of state but could not describe them in terms of particles moving apart or closer together and the energy changes involved. When describing how gas particles move, most said “randomly” but few added the kinetic-energy part of the description. Pressure changes caused real difficulty: many could not explain why bubbles appear when a pressurised drink is opened, even though this is a familiar experience. In hypotheses, the “because” was often missing or did not give a scientific reason. Finally, converting units and writing values in standard form were weak, so practise these step by step.
12 / Summary
Summary and knowledge organiserRevision
Essential knowledge
- Solid: regular, touching, vibrating. Liquid: irregular, touching, moving past each other. Gas: far apart, fast, random.
- During a change of state the temperature is constant; energy goes into overcoming (or is released by forming) the attractions between particles.
- Higher temperature → more kinetic energy → faster particles → faster diffusion, higher gas pressure or larger volume.
- Lighter particles diffuse faster at the same temperature.
- Proton +1, mass 1; neutron 0, mass 1; electron −1, negligible mass. Z = protons; A = protons + neutrons.
- Isotopes: same protons, different neutrons. Ar is the abundance-weighted mean of isotope masses.
- Shells fill 2, 8, 8, 2 (Z ≤ 20). Valency = outer electrons (groups 1–14) or 8 − outer electrons (groups 15–17).
Definitions
- Diffusion — net movement from high to low concentration by random motion
- Atomic number — number of protons
- Mass number — protons + neutrons
- Isotopes — same element, different numbers of neutrons
- Ion — charged particle formed by losing or gaining electrons
Relationships
- neutrons = A − Z
- electrons in ion = Z − charge
- Ar = Σ(A × %) ÷ 100
Must-remember distinctions
- Boiling (throughout, at b.p.) vs evaporation (surface, any temperature)
- Mass number (whole number, no unit) vs Ar (average)
- Ion charge changes electrons only
- Liquid particles touch; gas particles do not
Examination checklist
- Draw liquids touching, irregular, at the base
- Explain with kinetic energy, not just “moves faster”
- Give a scientific “because” in a hypothesis
- Never swap protons and neutrons
Other chapters: Criteria A–D · 1 · Periodic table · 2 · IUPAC naming · 3 · Atmosphere · 4 · Matter · 5 · Pure and impure · 6 · Bonding · 7 · Types of reaction
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