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GCSE Physics · GCSEChapter 10 · Atomic structure and isotopes
GCSE · Chapter 10

Atomic structure and isotopes

By the end of this chapter you will be able to

  • Name the parts of an atom and their relative masses and charges
  • Read nuclide notation and find the numbers of particles in an atom
  • Explain how the alpha scattering experiment changed the model of the atom
  • Explain what isotopes are, and why they have the same chemical behaviour
  • [H] Explain how energy levels account for the light given out by atoms

You should already know

  • Particles in an atom: protons, neutrons and electrons (KS3 Physics or Chemistry)
  • Powers of ten and standard form (KS3 Maths)

Where this leads

  • Radioactivity, nuclear equations and half-life (Chapter 11)
  • Nuclear fission and fusion (Chapter 12)
  • The electromagnetic spectrum (Chapter 19)

GCSE Physics. Content follows the GCSE (9-1) subject content for England (Edexcel and AQA). Content references: Describe the nuclear model: a nucleus of protons and neutrons, with electrons in shells outside it, Describe how the model of the atom developed: Dalton, plum pudding, Rutherford and the alpha scattering experiment, and Bohr, Use mass number and atomic number, and nuclide notation, to find the numbers of protons, neutrons and electrons, Explain isotopes, and that they have the same number of protons but different numbers of neutrons, [H] Describe electron energy levels, and how an electron gives out a photon when it moves down a level.

Exercise P  Do you remember?

1
FWhat is 10 squared? Write the number.
2
FWhat is 1 x 10 to the power -3, written as a decimal? Write the number.
3
FWhich particle in an atom has a positive charge? Write the word.
4
FWhich particle has no charge? Write the word.
5
FWhat is 23 minus 11? Write the number.
10.1

The nuclear model of the atom

GCSE
Describe the nuclear model: a nucleus of protons and neutrons, with electrons in shells outside it
Video lesson: The nuclear model of the atomComing soon
Context. Leo asks his teacher why an atom is mostly empty space if it looks solid. “Most of the mass is in a tiny nucleus at the centre,” says the teacher. The nuclear model of the atom explains this.

An atom has a small, dense nucleus at its centre, made of protons and neutrons. Electrons move around the outside of the nucleus, in shells. Almost all the mass of the atom is in the nucleus, and the atom is mostly empty space.

ParticleRelative massRelative chargeWhere it is
proton1+1nucleus
neutron10nucleus
electronabout 1/1840-1shells around the nucleus
The three particles of an atom
11p12nsodium atom (not to scale)electrons in shells 2, 8, 1
A sodium atom (not to scale): the nucleus is tiny compared with the shells

Key ideas

  • Protons and neutrons are in the nucleus; electrons are in shells outside it.
  • Protons have a charge of +1, electrons -1, and neutrons 0.
  • The electron has a very small mass (about 1/1840 of a proton).
  • The nucleus is about 10 000 times narrower than the whole atom (example).
Worked example 10.1.1
An atom has 11 protons and 12 neutrons. Calculate its mass number. Write the number.

Method. The mass number is the number of protons plus neutrons.

  1. Take the numbers. Protons = 11 and neutrons = 12.
    11 and 12
  2. Add. Mass number = 11 + 12 = 23.
    11 + 12 = 23
  3. Write the answer.
    23

Check. 23 - 12 = 11 protons. ✓

Worked example 10.1.2
An atom has 11 protons. How many electrons does a neutral atom have? Write the number.

Method. A neutral atom has as many electrons as protons.

  1. Compare. The atom is neutral, so the electrons balance the protons.
    balance
  2. Count. Electrons = protons = 11.
    11
  3. Write the answer.
    11

Check. 11 protons (+11) and 11 electrons (-11) give zero charge. ✓

Worked example 10.1.3
Which particles are found in the nucleus, and which are outside it? Write the two groups of names, nucleus first.

Method. Recall where each particle is.

  1. Nucleus. The nucleus contains protons and neutrons.
    protons, neutrons
  2. Outside. Electrons are in shells outside the nucleus.
    electrons
  3. Write the answer.
    protons and neutrons; electrons

Check. ✓

Watch out

Watch out: the nucleus holds almost all the mass but only the positive charge. The electrons carry the negative charge and the mass is very small.

Exam tip

Exam tip. Say “protons and neutrons are in the nucleus” and “electrons are in shells” exactly, as the examiners use these words.

Exercise 10A  Parts of the atom

1
FWhich particle is found in the nucleus and has no charge? Write the word.
2
FWhich particle is found in shells outside the nucleus? Write the word.
3
FWhat is the relative charge of a proton? Write the number, with its sign.
4
FWhat is the relative charge of an electron? Write the number, with its sign.
5
FAn atom has 6 protons and 6 neutrons. Calculate its mass number. Write the number.
6
FAn atom has mass number 16 and 8 protons. How many neutrons does it have? Write the number.
7
FA neutral atom has 9 protons. How many electrons does it have? Write the number.
8
FWhere is most of the mass of an atom? Write the word: nucleus or shells.
9
PSAn atom has mass number 39 and atomic number 19. How many neutrons does it have? Write the number.
10
PSA neutral atom has 15 protons and 16 neutrons. Calculate the mass number. Write the number.
11
RExplain why the nucleus has a positive charge but the atom as a whole has no charge, in one sentence. Accept any sentence that says the protons are positive and the electrons balance them.
12
RA pupil says “the electrons are in the nucleus, with the protons.” Explain why this is wrong. Accept any answer that says electrons are in shells outside the nucleus.
13
FCalculate the atomic number of an atom with 20 protons. Write the number.
14
PSAn atom of mass number 7 has 3 protons. Find the number of neutrons and electrons, separated by a comma (neutrons first).
10.2

How the model of the atom developed

GCSE
Describe how the model of the atom developed: Dalton, plum pudding, Rutherford and the alpha scattering experiment, and Bohr
Video lesson: How the model of the atom developedComing soon
Context. Ahmed reads that scientists once thought the atom was a solid ball. Later, a famous experiment with a thin gold foil changed the picture. Models change when new evidence does not fit the old one. This is how science works.

Dalton (early 1800s) suggested that atoms were tiny solid spheres. Thomson (about 1897) discovered electrons and suggested the plum pudding model: negative electrons spread in a positive sphere. In 1911 Rutherford and his team fired alpha particles at a thin gold foil. Most went straight through, a few were deflected, and very few bounced back. This showed that the atom has a small, dense, positive nucleus and is mostly empty space. Later, Bohr suggested that electrons move in fixed shells, or energy levels.

Dalton (early 1800s)solid sphereplum pudding (1897)electrons in a positive spherenuclear model (1911)small dense nucleus
Dalton, plum pudding and the nuclear model
thin gold foilalpha particlesmost go straight througha few deflected; very few bounce back
The alpha scattering experiment: most alpha particles pass through, a few are deflected, and very few bounce back

Key ideas

  • Dalton: a solid sphere (no internal structure).
  • Thomson: electrons in a positive sphere (plum pudding).
  • Rutherford: a small, dense, positive nucleus; most of the atom is empty space.
  • Bohr: electrons in fixed shells or energy levels.
Worked example 10.2.1
In the alpha scattering experiment, most alpha particles went straight through the foil. What does this tell us about the atom? Write one sentence.

Method. Think about what the particles passed through.

  1. Look. The particles passed through the foil without being deflected.
    straight through
  2. Decide. So most of the atom is empty space.
    empty space
  3. Write the answer.
    Most of the atom is empty space.

Check. ✓

Worked example 10.2.2
Why did a few alpha particles bounce back? Write one reason. Accept any sentence that says the particles hit a small, dense, positive nucleus.

Method. Think about what the alpha particle hit.

  1. Look. A few alpha particles came very close to a small, positive region.
    positive region
  2. Decide. The like charges repelled them, and the small dense nucleus caused the bounce.
    repelled
  3. Write the answer.
    Accept any sentence that says the particles hit a small, dense, positive nucleus.

Check. ✓

Worked example 10.2.3
Which scientist discovered the electron, and which model did he suggest? Write the name of the model.

Method. Recall the model that had electrons in a positive sphere.

  1. Who. Thomson discovered the electron in 1897.
    Thomson
  2. Recall. Thomson discovered the electron and suggested the plum pudding model.
    plum pudding
  3. Write the answer.
    plum pudding

Check. ✓

Watch out

Watch out: the nuclear model did not come from one experiment alone. Rutherford’s result led to the nuclear model, and later evidence refined it.

Exam tip

Exam tip. When asked how a model changed, say what the new evidence was, and then what the new model says.

Exercise 10B  The development of the atomic model

1
FWhich model says atoms are small solid spheres? Write the scientist: Dalton or Thomson.
2
FWhich model has negative electrons spread in a positive sphere? Write the name of the model.
3
FWhat particles were fired at the gold foil in the Rutherford experiment? Write the name.
4
FWhat is the name of the material used as the target in the experiment? Write the name.
5
FIn the alpha scattering experiment, what fraction of the particles were deflected or bounced back? Write the word: most, a few or very few, for those deflected.
6
FWhat did Rutherford conclude about the nucleus? Write one word: small or large.
7
FWhich scientist suggested that electrons move in fixed shells? Write the name.
8
RExplain why most alpha particles went straight through the foil, in one sentence. Accept any sentence that says most of the atom is empty space.
9
RA pupil says “the plum pudding model was wrong because electrons do not exist.” Explain why this is wrong. Accept any answer that says electrons were discovered, so the model was partly right but was replaced by the nuclear model.
10
RExplain how the alpha scattering experiment changed the plum pudding model, in two sentences. Accept any answer that says the atom has a small, dense, positive nucleus and is mostly empty space, not a spread-out sphere.
11
PSPut these in the order they were proposed, earliest first: nuclear model, Dalton, plum pudding. Write the three words separated by commas.
12
RWhy did the scientists change the model of the atom? Write one reason in one sentence. Accept any sentence that says new evidence, such as the alpha scattering results, did not fit the old model.
10.3

Isotopes

GCSE
Explain isotopes, and that they have the same number of protons but different numbers of neutrons
Video lesson: IsotopesComing soon
Context. Carbon-12 and carbon-14 are both carbon. Carbon-14 is used to date old wood and bones. “How can two atoms of carbon be different?” asks Mei. The answer is that they are isotopes: they have the same number of protons, but different numbers of neutrons.

Isotopes are atoms of the same element, with the same number of protons (the same atomic number), but a different number of neutrons (a different mass number). Isotopes have the same chemical properties because they have the same electrons, but they can have different nuclear properties. The mass number is shown as the top number in nuclide notation: AZX, where A is the mass number, Z the atomic number and X the symbol.

NuclideMass number (top)Atomic number (bottom)ProtonsNeutronsElectrons (neutral)
23Na2311111211
35Cl3517171817
14C146686
4He42222
Nuclide notation and the numbers of particles
IsotopeProtonsNeutronsSame element?
carbon-1266yes
carbon-1367yes
carbon-1468yes (radioactive)
Isotopes of carbon: the same protons, different neutrons

Key ideas

  • Isotopes: same protons (same element), different neutrons.
  • Mass number (A) = protons + neutrons.
  • Atomic number (Z) = protons.
  • Neutrons = A - Z.
Worked example 10.3.1
Chlorine-35 and chlorine-37 are isotopes. How many more neutrons does chlorine-37 have? Write the number.

Method. Find the neutrons in each isotope.

  1. Neutrons. Chlorine-35 has 35 - 17 = 18 neutrons; chlorine-37 has 37 - 17 = 20.
    18 and 20
  2. Difference. 20 - 18 = 2 more neutrons.
    20 - 18 = 2
  3. Write the answer.
    2

Check. Both have 17 protons, so they are the same element. ✓

Worked example 10.3.2
Carbon-14 has the same atomic number as carbon-12. Write the number of protons in carbon-14.

Method. Use the atomic number of carbon.

  1. Recall. Carbon has atomic number 6.
    6
  2. Decide. So carbon-14 has 6 protons, the same as carbon-12.
    6 protons
  3. Write the answer.
    6

Check. Carbon-14 has 14 - 6 = 8 neutrons. ✓

Worked example 10.3.3
A nuclide is written as 14C. How many neutrons does it have? Write the number.

Method. Use neutrons = mass number - atomic number.

  1. Take the numbers. Mass number = 14, and atomic number of carbon = 6.
    14 and 6
  2. Subtract. Neutrons = 14 - 6 = 8.
    14 - 6 = 8
  3. Write the answer.
    8

Check. 6 protons + 8 neutrons = 14. ✓

Watch out

Watch out: the isotope has the same atomic number, not the same mass number. The mass number is different for each isotope.

Exam tip

Exam tip. When asked what isotopes have in common, say “the same number of protons”, not “the same mass”.

Exercise 10C  Isotopes and nuclide notation

1
FWhat do isotopes of the same element have the same number of? Write the word.
2
FWhat do isotopes of the same element have different numbers of? Write the word.
3
FWhat is the mass number of sodium-23? Write the number.
4
FHow many neutrons does carbon-12 have? Write the number.
5
FHow many neutrons does carbon-13 have? Write the number.
6
PSAn isotope of hydrogen has mass number 2 and atomic number 1. How many neutrons does it have? Write the number.
7
PSTwo atoms have 17 protons. One has 18 neutrons and the other has 20 neutrons. Are they isotopes of the same element? Write yes or no.
8
PSWrite the number of neutrons in the nuclide 39K, which has atomic number 19. Write the number.
9
PSWhich two isotopes of hydrogen are named deuterium and tritium? Write the mass numbers, separated by a comma, deuterium first.
10
RExplain why isotopes of an element react in the same way chemically, in one sentence. Accept any sentence that says they have the same number of electrons and so the same chemical properties.
11
RA pupil says “isotopes have different numbers of protons.” Explain why this is wrong. Accept any answer that says isotopes have the same number of protons, which is what makes them the same element.
12
PSCarbon-12 and carbon-14 are isotopes. Write the difference in their numbers of neutrons. Write the number.
13
FAn atom has mass number 12 and atomic number 6. Write its nuclide notation as the mass number on top and the atomic number below, using the symbol C. Write the notation with the numbers only, as 12, 6.
10.4

The size of atoms and nuclei

GCSE
Explain the size of an atom and its nucleus, and that the nucleus is much smaller than the atom
Video lesson: The size of atoms and nucleiComing soon
Context. Mia is comparing sizes: a human hair, a bacterium and an atom. “How many atoms fit across a hair?” she asks. Atoms are very small, so we use standard form.

An atom is about 0.1 nanometres across, which is 1 x 10-10 m. The nucleus is much smaller: about 10 000 to 100 000 times narrower than the atom. Almost all the mass of an atom is in its nucleus, but almost all the volume is empty space.

Key ideas

  • An atom is about 0.1 nm across: 1 x 10-10 m.
  • 1 nm = 1 x 10-9 m.
  • The nucleus is about 10 000 to 100 000 times narrower than the atom.
  • Standard form: a number between 1 and 10 times a power of 10.
Worked example 10.4.1
An atom is 1 x 10-10 m across. How many metres is that written as a decimal? Write the number.

Method. Move the decimal point 10 places to the left.

  1. Power. 10-10 = 1 / 10 000 000 000.
    1 / 10^10
  2. Write. So the length is 0.0000000001 m.
    0.0000000001
  3. Write the answer.
    0.0000000001 m

Check. ✓

Worked example 10.4.2
A nucleus is 1 x 10-14 m across and an atom is 1 x 10-10 m across. How many times wider is the atom? Write the number.

Method. Divide the atom width by the nucleus width.

  1. Divide. (1 x 10-10) / (1 x 10-14) = 1 x 104.
    10^4
  2. Write. 104 = 10 000 times.
    10 000
  3. Write the answer.
    10000

Check. Example values: an atom is 10 000 times wider than this nucleus. ✓

Worked example 10.4.3
Write 0.5 nanometres in metres, in standard form. Write the number.

Method. 1 nm = 1 x 10-9 m, so multiply.

  1. Convert. 0.5 nm = 0.5 x 1 x 10-9 m.
    0.5 x 10^-9
  2. Standard form. 0.5 x 10-9 = 5 x 10-10 m.
    5 x 10^-10
  3. Write the answer.
    5 x 10^-10 m

Check. 5 x 10^-10 = 0.0000000005 m. ✓

Watch out

Watch out: a smaller number with a larger negative power is smaller in size. 10-10 is smaller than 10-9.

Exam tip

Exam tip. When converting nanometres, multiply by 10-9. Check that your standard form number is between 1 and 10.

Exercise 10D  Sizes of atoms and nuclei

1
FWhat is 1 nanometre in metres? Write the power of ten.
2
FAn atom is about 0.1 nm across. Write this in standard form in metres. Write the number.
3
FWhich is larger: a nucleus or an atom? Write the word.
4
PSHow many times wider is an atom of width 1 x 10-10 m than a nucleus of width 1 x 10-14 m? Write the number.
5
PSConvert 2 nm into metres in standard form. Write the number.
6
RExplain why the atom looks solid to us but is mostly empty space, in one sentence. Accept any sentence that says the atoms are very small and the particles are much smaller than the atom.
7
PSA nucleus is 100 000 times narrower than an atom of width 1 x 10-10 m. Find the width of the nucleus in standard form. Write the number.
8
FWrite 1 x 10-3 as a decimal. Write the number.
9
PSWhich is smaller: 1 x 10-9 m or 1 x 10-10 m? Write the value, for example 1 x 10^-10.
10
RA pupil says a nucleus is the same size as an atom. Explain why this is wrong, in one sentence. Accept any sentence that says the nucleus is much smaller than the whole atom.
10.5

Electron energy levels [H]

GCSE
[H] Describe electron energy levels, and how an electron gives out a photon when it moves down a level
Video lesson: Electron energy levels [H]Coming soon

[H] Higher tier only. Electrons can only have certain amounts of energy. These fixed amounts are called energy levels. An electron can move up a level by absorbing energy (excitation), and when it falls back down it gives out a photon of light or other electromagnetic radiation. The energy of the photon equals the difference between the two levels.

Energy levels of an electron (arbitrary units)n = 10n = 22n = 35n = 49photon outenergy levels: an electron jumps down and releases a photon
Energy levels: a fall from level 4 to level 2 releases a photon

Key ideas

  • [H] Electrons occupy fixed energy levels.
  • [H] An electron that moves up a level must absorb energy (excitation).
  • [H] An electron that falls down a level gives out a photon.
  • [H] Energy of photon = difference between the two levels.
Worked example 10.5.1
[H] In the diagram, the electron falls from level 4 (9 units) to level 2 (2 units). How much energy is given out? Write the number of units.

Method. The energy released is the difference between the levels.

  1. Subtract. Energy released = 9 - 2 = 7 units.
    9 - 2 = 7
  2. Decide. So the photon has 7 units of energy.
    7 units
  3. Write the answer.
    7

Check. ✓

Worked example 10.5.2
[H] An electron moves from level 2 (2 units) up to level 3 (5 units). Does it absorb or give out energy? Write absorb or give out.

Method. Moving up needs energy.

  1. Look. The electron moves to a higher energy level.
    up
  2. Decide. Moving up means it absorbs energy.
    absorb
  3. Write the answer.
    absorb

Check. ✓

Worked example 10.5.3
[H] An electron jumps from level 1 (0 units) to level 3 (5 units). Does it absorb or give out energy, and how much? Write the answer as: absorb, 5.

Method. Moving up a level needs energy.

  1. Look. The electron moves to a higher level, so it gains energy.
    up
  2. Calculate. The energy gained is 5 - 0 = 5 units, which is absorbed.
    5 - 0 = 5
  3. Write the answer.
    absorb, 5

Check. ✓

Watch out

Watch out: [H] an electron does not move gradually between levels; it jumps. The energy of the photon depends only on the two levels, not on the path.

Exam tip

Exam tip. [H] Write the energy difference first, then state whether energy is absorbed or given out.

Exercise 10E  [H] Energy levels and photons

1
F[H] An electron falls from level 3 (5 units) to level 2 (2 units). How much energy is given out? Write the number of units.
2
F[H] An electron falls from level 4 (9 units) to level 3 (5 units). How much energy does the photon carry? Write the number of units.
3
F[H] An electron moves from level 1 (0 units) up to level 2 (2 units). Does it absorb or give out energy? Write absorb or give out.
4
F[H] What is emitted when an electron falls to a lower energy level? Write the word.
5
PS[H] Which is the bigger photon energy: a fall from level 4 to 3 (4 units) or a fall from level 4 to 1 (9 units)? Write the number of units of the bigger one.
6
R[H] Explain why the light from an atom has only certain colours, in one sentence. Accept any sentence that says electrons can only fall between fixed energy levels, so only certain photon energies are given out.
7
PS[H] An electron falls from level 4 (9 units) to level 2 (2 units), then from level 2 down to level 1 (0 units). Find the total energy given out in both falls. Write the number of units.
8
F[H] An electron falls from level 2 (2 units) to level 1 (0 units). How much energy is given out? Write the number of units.
9
PS[H] Which transition gives the bigger photon energy: a fall from level 4 to level 1, or a fall from level 2 to level 1? Write 4 to 1 or 2 to 1.
10
PS[H] An electron in level 3 (5 units) falls to level 4 (9 units) after absorbing a photon. Is the photon energy absorbed or given out, and how much? Write the answer as: absorbed, 4 units.

Chapter summary

SkillMethod
Nuclear modelProtons and neutrons in the nucleus; electrons in shells outside; mostly empty space
Development of the modelDalton (sphere), Thomson (plum pudding), Rutherford (nucleus), Bohr (shells)
Numbers of particlesProtons = atomic number; neutrons = mass number - atomic number; electrons = protons when neutral
IsotopesSame protons, different neutrons; same element and chemistry
SizesAtom about 0.1 nm (1 x 10^-10 m); nucleus much smaller
[H] Energy levelsElectrons jump between fixed levels; photon energy = difference

Can you…?

  • I can describe the nuclear model of the atom
  • I can describe how the model changed, and the evidence for each change
  • I can use mass number and atomic number to find the particles
  • I can explain what isotopes are
  • I can convert nanometres to metres in standard form
  • [H] I can explain energy levels and photons

Key terms

nucleus
the small, dense centre of an atom, containing protons and neutrons
mass number
the number of protons plus neutrons in a nucleus
atomic number
the number of protons in a nucleus
isotope
atoms of the same element with the same protons and different neutrons
alpha particle
a positively charged particle made of two protons and two neutrons, emitted in some decays
[H] energy level
a fixed amount of energy an electron can have in an atom
[H] photon
a packet of electromagnetic radiation

Exercise 10Z  Check-up

1
Name the particles in the nucleus of an atom. Write the two names.
2
An atom has mass number 23 and atomic number 11. How many neutrons does it have? Write the number.
3
Which scientist showed that the atom has a small, dense, positive nucleus? Write the name.
4
What did most alpha particles do in the gold foil experiment? Write the word: passed, bounced or stopped.
5
What are isotopes? Write the word: same protons or same neutrons.
6
Write the number of neutrons in chlorine-37, which has atomic number 17. Write the number.
7
Write 1 x 10-10 m as a decimal. Write the number.
8
Name the particle with a relative charge of -1 that moves in shells. Write the word.
9
Explain how the plum pudding model was changed by the nuclear model, in one sentence. Accept any sentence that says the positive charge is in a small nucleus, not spread through the atom.
10
Calculate the number of electrons in a neutral atom with atomic number 8. Write the number.

Total 31 marks. Allow about 31 minutes.

Exercise 10X  Exam-style questions

1
PSAn atom has mass number 35 and atomic number 17. Write the number of protons, neutrons and electrons in a neutral atom, in that order, separated by commas.[2 marks]
2
RDescribe the observation in the alpha scattering experiment that showed the nucleus is small, and say what it led to. Accept any answer that says a few alpha particles were deflected or bounced back, which showed a small, dense, positive nucleus.[3 marks]
3
RExplain what isotopes are, and give the difference between carbon-12 and carbon-14 in terms of particles. Accept any answer that says isotopes have the same protons, and carbon-14 has two more neutrons than carbon-12.[3 marks]
4
PSAn atom is 0.1 nm across and its nucleus is 1 x 10-14 m across. Show that the atom is about 10 000 times wider than the nucleus, using standard form. Write the factor as a power of ten.[3 marks]
5
RExplain how the nuclear model of the atom explains why most alpha particles pass through a gold foil, but a few are deflected. Accept any answer that says most of the atom is empty space, and a few alpha particles come close to the small, positive nucleus and are repelled.[4 marks]
6
PSAn atom has mass number 39 and 20 neutrons. Identify the element using the atomic number, and write the number of electrons in a neutral atom. Write two numbers, the atomic number then the electrons, separated by a comma.[3 marks]
7
R[H] An electron in an atom falls from a higher energy level to a lower one, and gives out a photon. Explain why the photon has a definite energy. Accept any answer that says the energy of the photon is the difference between the two fixed energy levels.[3 marks]
8
RDescribe the models of the atom in order: Dalton, Thomson and Rutherford. For each, give one feature. Accept any answer that gives a solid sphere for Dalton, electrons in a positive sphere for Thomson, and a small positive nucleus for Rutherford.[4 marks]
9
PSA radioactive atom has an atomic number of 6 and 8 neutrons. Identify the element, the mass number, and whether it is an isotope of carbon-12. Write three values separated by commas: the element symbol, the mass number, then yes or no.[4 marks]
10
RDescribe the nucleus of an atom in terms of its charge and where most of the mass is, in two sentences. Accept any answer that says the nucleus is positive and holds most of the mass of the atom.[2 marks]
← Chapter 9: The particle model, density and internal energyChapter 11: Radioactivity, nuclear equations and half-life →

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