The particle model — OCR GCSE Physics
Test yourself on The particle model with OCR GCSE practice questions.
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The particle model explained
Density tells you how much mass is packed into each cubic metre of a material, so it links mass and volume in one equation: density = mass ÷ volume.
Read the full explanation
In symbols, ρ = m / V, with density in kg/m³, mass in kg and volume in m³. To apply it, identify the mass and the volume, convert them to kg and m³, then divide. A block of mass 2.4 kg and volume 0.003 m³ has density 2.4 ÷ 0.003 = 800 kg/m³. To find mass, rearrange to m = ρ × V; to find volume, use V = m / ρ. A larger volume of the same material has more mass but the same density, because density depends on the material, not the size of the sample.
describe how and why the atomic model has changed over time (to include: the Thomson, Rutherford (alongside Geiger and Marsden) and Bohr models)
The atomic model developed as new evidence challenged old ideas. Thomson proposed that atoms contain small negative electrons embedded in a spread of positive charge, often pictured as a plum-pudding model, after electrons were discovered. Rutherford, working with Geiger and Marsden, fired alpha particles at thin gold foil; most passed straight through but some were deflected strongly, so he concluded that positive charge and most mass sit in a tiny central nucleus with mostly empty space around it. Bohr then suggested that electrons orbit the nucleus in fixed shells or energy levels, explaining why atoms emit and absorb particular energies. Each change happened because experimental evidence did not fit the previous model.
describe the atom as a positively charged nucleus surrounded by negatively charged electrons, with the nuclear radius much smaller than that of the atom and with almost all of the mass in the nucleus
Build a mental picture of the atom as mostly empty space. At the centre sits a tiny nucleus carrying positive charge; around it, negatively charged electrons occupy the surrounding region. The nucleus is far smaller than the whole atom, so the atom's radius is set by where the electrons are, not by the nucleus. Despite its small size, the nucleus holds almost all of the atom's mass, because protons and neutrons are far heavier than electrons. A useful check: if the atom were a sports hall, the nucleus would be a speck near the centre. In a multiple-choice question, test each option against all three ideas — charge arrangement, relative size and mass distribution — and reject any option that puts most mass in the electrons or makes the nucleus comparable in size to the atom.
recall the typical size (order of magnitude) of atoms and small molecules (to include: knowledge that it is typically 1 × 10⁻¹⁰m)
Atoms and small molecules are extremely small, and physicists describe their typical size as an order of magnitude rather than an exact figure. The value to recall is about 1 × 10⁻¹⁰ m, which is 0.1 nm, or one ten-billionth of a metre. An order of magnitude means the nearest power of ten, so a diameter of roughly 10⁻¹⁰ m tells you the scale: about ten million atoms would fit across a one-millimetre gap. Small molecules such as water are of a similar order, typically a few times 10⁻¹⁰ m across. Use this figure to judge whether a quoted size is sensible: a value like 10⁻³ m describes a grain of sand, not an atom.
define density
Density tells you how much mass is packed into a given volume of a material. The definition is mass per unit volume, written as density = mass ÷ volume, with units such as kg/m³ or g/cm³. A material with high density, such as iron, has a lot of mass in each cubic metre; a low-density material, such as air, has little. Density depends on both the mass of the particles and how closely they are packed, which links to the particle model: solids are usually denser than gases because their particles are closer together. In a multiple-choice question, pick the option that states mass per unit volume, not mass alone or volume alone.
explain the differences in density between the different states of matter in terms of the arrangements of the atoms and molecules
Density differs between solids, liquids and gases because of how the particles are arranged and how closely they sit. In a solid, particles are packed tightly in a regular arrangement and only vibrate about fixed positions, so a given volume holds many particles and the density is high. In a liquid, particles are still close together but are irregularly arranged and can slide past one another, so the density is usually slightly lower than the solid but still high. In a gas, particles are far apart and move randomly at high speed, so the same volume contains far fewer particles and the density is much lower. The key idea is that density depends on how much mass is squeezed into a volume, which follows from particle spacing. In a multiple-choice question, match each state to its spacing and arrangement.
apply the relationship between density, mass and volume to changes where mass is conserved
Density links mass and volume through ρ = m ÷ V, so a fixed mass squeezed into a smaller volume has a higher density and the same mass spread through a larger volume has a lower density. Because mass is conserved, only volume changes in these situations: heating a gas in a sealed rigid container raises pressure but keeps both mass and volume, while compressing a gas in a syringe keeps mass constant and reduces volume, raising density. Worked method: a 0.60 kg sample occupies 200 cm³, so ρ = 0.60 kg ÷ 0.000200 m³ = 3000 kg/m³; halving the volume to 100 cm³ doubles density to 6000 kg/m³ with mass still 0.60 kg. Convert cm³ to m³ by dividing by 1 000 000, and check that the mass value you use is the same before and after the change.
Your focus
- State the equation density = mass ÷ volume with correct units.
- Calculate density from a given mass and volume in kg and m³.
- Rearrange the equation to find mass or volume when density is known.
Show all 21 objectives
- Describe the Thomson, Rutherford and Bohr models of the atom.
- Explain how the Geiger and Marsden scattering evidence led to the nuclear model.
- Explain why the atomic model changed as new evidence emerged.
- Identify the nucleus as positively charged and electrons as negatively charged.
- Compare the nuclear radius with the radius of the whole atom.
- Locate almost all of the atom's mass in the nucleus.
- State the typical order of magnitude of an atom as 1 × 10⁻¹⁰ m.
- Explain that an order of magnitude gives the nearest power of ten rather than an exact size.
- Judge whether a quoted length is a sensible size for an atom or small molecule.
- Define density as mass per unit volume.
- State the equation density = mass ÷ volume with correct units.
- Relate density to the mass and spacing of particles in a material.
- Describe the arrangement and spacing of particles in solids, liquids and gases.
- Explain how particle spacing and arrangement account for differences in density between states.
- Select the correct particle-model explanation for a given state of matter.
- Apply ρ = m ÷ V to a situation in which mass is conserved and volume changes.
- Convert between cm³ and m³ correctly before calculating density.
- Predict and justify how density changes when a fixed mass occupies a different volume.
The particle model exam tips
Marking Points
- Selects the relationship density = mass ÷ volume and substitutes mass in kg and volume in m³.
- Converts grams to kilograms or cubic centimetres to cubic metres before dividing.
- Rearranges the equation to mass = density × volume when the question supplies density and volume.
- Rearranges the equation to volume = mass ÷ density when the question supplies mass and density.
- States the unit of density as kg/m³ when the mass is in kg and the volume is in m³.
- Recognises that two samples of the same material have equal density even when their volumes differ.
- Describes the Thomson model as negative electrons set in a spread of positive charge.
- Explains that Thomson's model followed the discovery of the electron.
- Describes the Geiger and Marsden alpha-particle scattering result, with most particles passing through and a few deflected strongly.
- Explains that Rutherford interpreted the scattering as a tiny, dense, positive nucleus surrounded by mostly empty space.
- Describes the Bohr model as electrons in fixed shells or energy levels around the nucleus.
- Explains that each model changed because new experimental evidence did not fit the earlier model.
- States that the nucleus is positively charged and the electrons are negatively charged.
- Explains that the nuclear radius is much smaller than the radius of the whole atom.
- States that almost all of the mass of the atom is concentrated in the nucleus.
- Rejects an option that places most of the atom's mass in the electrons or makes the nucleus the same size as the atom.
- Selects 1 × 10⁻¹⁰ m as the typical order of magnitude of an atom.
- Recognises that an order of magnitude is the nearest power of ten, not an exact measurement.
- Applies the same order of magnitude to small molecules as well as single atoms.
- Rejects values many powers of ten larger, such as 10⁻³ m, as the size of an atom.
- Defines density as mass per unit volume.
- States the relationship density = mass ÷ volume.
- Identifies suitable units such as kg/m³ or g/cm³.
- Links density to how closely particles are packed and the mass of those particles.
- States that solid particles are closely packed in a regular arrangement, giving high density.
- States that liquid particles are close together but irregularly arranged and able to move past each other.
- States that gas particles are far apart and move randomly, giving much lower density.
- Links the density of each state to the spacing and arrangement of its particles.
- Selects the form ρ = m ÷ V and substitutes a mass that stays unchanged across the change.
- Converts a volume in cm³ to m³ by dividing by 1 000 000 before substituting.
- States that halving the volume of a fixed mass doubles the density.
- Explains that a change in density arises from a change in volume because mass is conserved.
- Rejects an option that treats mass as changing when only the volume changes.
Examiner Tips
- 💡Write the equation first, then substitute numbers with units so the conversion step is visible.
- 💡Check the unit asked for in the answer before calculating, and convert mass and volume to kg and m³.
- 💡Use a quick sense check: dividing a small mass by a small volume should give a plausible density such as several hundred or thousand kg/m³.
- 💡Name each scientist with the key feature of their model so the examiner can match them quickly.
- 💡Use the gold-foil scattering result as the evidence that supports the nuclear model.
- 💡State the reason for change: new evidence contradicted the previous model.
- 💡Read all options before choosing, since several may contain one true idea and one false one.
- 💡Check each option against three separate facts: charge, relative size and where the mass sits.
- 💡Eliminate options that contradict the idea that the atom is mostly empty space.
- 💡Learn 1 × 10⁻¹⁰ m together with its equivalent 0.1 nm so either form is recognised.
- 💡Check the power of ten first: an atom's size must be a very small fraction of a metre.
- 💡Compare any quoted value with 10⁻¹⁰ m and reject answers that are thousands of times too big.
- 💡Learn the phrase 'mass per unit volume' word for word as the definition.
- 💡Recall the equation density = mass ÷ volume so you can rearrange it if needed.
- 💡Check units in each option: density must combine a mass unit with a volume unit.
- 💡For each option, check both the spacing and the arrangement described before deciding.
- 💡Remember the order of density: generally solid highest, liquid next, gas lowest.
- 💡Link any statement about density back to how closely the particles are packed.
- 💡Write the equation, then list the known mass and both volumes before touching the options.
- 💡Convert every volume to m³ in one step and note the conversion factor beside your working.
- 💡Use ratio reasoning as a check: quarter the volume and density should rise fourfold at constant mass.
Common Mistakes
- Multiplies mass by volume instead of dividing; divide mass by volume to obtain density.
- Uses grams or cubic centimetres without converting; convert to kg and m³ so the answer is in kg/m³.
- Assumes a bigger sample must be denser; keep density fixed for a given material and let mass rise with volume.
- Writes the unit as kg/m² or kg m³; record density in kg/m³.
- Credits Thomson with the nuclear model; keep Thomson as electrons in a positive spread and give the nucleus to Rutherford.
- Says all alpha particles were deflected; most passed through and only a few were deflected strongly.
- Places electrons anywhere in the atom in the Bohr model; put them in fixed shells or energy levels.
- Treats the models as guesses with no evidence; link each change to experimental evidence that did not fit.
- Places most of the atom's mass in the electron cloud; correct this by stating that almost all mass is in the nucleus.
- Treats the nucleus as roughly the same size as the atom; correct this by stating that the nuclear radius is much smaller.
- Reverses the charges by calling the nucleus negative; correct this by stating that the nucleus is positive and electrons are negative.
- Confuses the order of magnitude with an exact atomic diameter; treat 1 × 10⁻¹⁰ m as a typical scale.
- Writes the size with a positive exponent; use the negative exponent 10⁻¹⁰ m for a very small length.
- Assumes molecules are far larger than atoms; keep small molecules at a similar order of magnitude.
- Defines density as volume per unit mass; state it as mass per unit volume.
- Gives mass alone as the definition; include the volume the mass occupies.
- Omits units from the definition; quote kg/m³ or g/cm³ as examples.
- Claims gas particles are heavier or lighter than solid particles; correct this by explaining that the difference in density comes from spacing, not particle mass.
- Says liquid particles are far apart like a gas; correct this by stating that liquid particles remain close together but are irregularly arranged.
- Describes solid particles as free to move around; correct this by stating that they vibrate about fixed positions in a regular arrangement.
- Uses a changed mass value after the volume change; keep the mass identical and alter only the volume.
- Forgets to convert cm³ to m³ and divides kilograms by cubic centimetres; divide the volume by 1 000 000 first.
- Assumes density stays the same when a gas is compressed; recognise that reduced volume at constant mass raises density.
- Inverts the relationship and multiplies mass by volume; divide mass by volume instead.