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GCSE Chemistry · GCSEChapter 1 · Atoms, elements, compounds and isotopes
GCSE · Chapter 1

Atoms, elements, compounds and isotopes

By the end of this chapter you will be able to

  • Name the three particles in an atom and give their charges and positions
  • Use atomic number and mass number to find protons, neutrons and electrons
  • Write chemical symbols and formulae, and count the atoms in a formula
  • Describe how the atomic model changed, and what evidence changed it
  • Explain what isotopes are, and [H] calculate a relative atomic mass
  • Convert nanometres into metres, using standard form

You should already know

  • Elements and compounds at KS3 (what a substance is)
  • Standard form and powers of ten (KS3 Maths)

Where this leads

  • Electron arrangement and the periodic table (Chapter 2)
  • Relative formula mass and balancing equations (Chapter 7)
  • Ionic and covalent bonding (Chapters 4 and 5)

GCSE Chemistry. Content follows the GCSE (9-1) subject content for England (Edexcel and AQA). Content references: Describe the atom: protons, neutrons and electrons, their relative charge and mass, Use atomic number and mass number to work out the numbers of particles in an atom, Distinguish elements, compounds and mixtures, and use chemical symbols and formulae, Describe how the model of the atom developed (Dalton, plum pudding, nuclear model), Explain isotopes and their use, [H] Calculate relative atomic mass from isotope abundance, Know the size of atoms and the nanoscale, and use standard form for very small lengths.

Exercise P  Do you remember?

1
FWhat is 12 + 9? Write the number.
2
FWrite 0.5 as a fraction in its simplest form.
3
FWhat is 100 divided by 4? Write the number.
4
FWrite 3 x 102 as an ordinary number. Write the number.
5
FWhich is smaller, 1 x 10-3 or 1 x 10-6? Write the number.
1.1

Atoms and subatomic particles

GCSE
Describe the atom: protons, neutrons and electrons, their relative charge and mass, Use atomic number and mass number to work out the numbers of particles in an atom
Video lesson: Atoms and subatomic particlesComing soon
Context. Sam is building a model of a sodium atom for a science lesson. The card says sodium has atomic number 11 and mass number 23. Sam asks, “How many protons, neutrons and electrons should I draw?” The numbers on the card are enough to answer.

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

ParticleRelative massRelative chargeWhere it is
proton1+1in the nucleus
neutron10 (no charge)in the nucleus
electronvery small (about 1/1840)-1outside the nucleus, in shells
The three subatomic particles
11p12nnucleus: protons and neutronselectrons on shellsatom 23Na
A sodium atom: 11 protons, 12 neutrons and 11 electrons (not to scale)

Key ideas

  • The atomic number is the number of protons. Every atom of an element has the same atomic number.
  • The mass number is the number of protons plus neutrons.
  • In a neutral atom, the number of electrons equals the number of protons.
  • Number of neutrons = mass number - atomic number.
Worked example 1.1.1
A sodium atom has atomic number 11 and mass number 23. How many neutrons does it have? Write the number.

Method. Use the rule: neutrons = mass number - atomic number.

  1. Take the numbers. The mass number is 23 and the atomic number is 11.
    mass 23, atomic 11
  2. Subtract. Neutrons = 23 - 11.
    23 - 11 = 12
  3. Write the answer.
    12

Check. 11 protons + 12 neutrons = 23, the mass number. ✓

Worked example 1.1.2
An atom has 8 protons and 8 neutrons. How many electrons does a neutral atom of it have? Write the number.

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

  1. Count the protons. The number of protons is 8.
    8 protons
  2. Use the neutral rule. A neutral atom has equal numbers of protons and electrons.
    8 = 8
  3. Write the answer. So there are 8 electrons.
    8 electrons
  4. Write the answer.
    8

Check. 8 electrons cancel the 8 protons, so the atom has no overall charge. ✓

Worked example 1.1.3
An atom has mass number 7 and atomic number 3. Find the number of protons, neutrons and electrons. Write three numbers, in this order.

Method. Read the atomic number and mass number, then work out the rest.

  1. Protons. The atomic number is the number of protons, so there are 3.
    protons = 3
  2. Neutrons. Neutrons = mass number - atomic number = 7 - 3.
    7 - 3 = 4
  3. Electrons. A neutral atom has as many electrons as protons, so 3.
    electrons = 3
  4. Write the answer.
    3, 4, 3

Check. Protons 3 + neutrons 4 = 7, the mass number. ✓

Watch out

Watch out: the atomic number is the smaller number on the periodic table, and it gives the protons. Do not use it as the mass. Subtract it from the mass number to find the neutrons.

Exam tip

Exam tip. When you are asked for “the number of neutrons”, write the number, not the calculation. Show the subtraction on the way if you have room.

Exercise 1A  Protons, neutrons, electrons and mass numbers

1
FWhich particle in an atom has a relative charge of +1? Write the word.
2
FWhich particle has no charge? Write the word.
3
FWhich particles are found in shells outside the nucleus? Write the word.
4
FAn atom has 6 protons. How many electrons does a neutral atom of this element have? Write the number.
5
FFluorine has atomic number 9 and mass number 19. How many neutrons does a fluorine atom have? Write the number.
6
FComplete the table for an atom of lithium-7, which has atomic number 3 and mass number 7.
(a)Number of protons
(b)Number of neutrons
7
FOxygen-16 has atomic number 8 and mass number 16. Find the number of protons and the number of neutrons. Write two numbers.
8
PSAn atom has 15 protons and 16 neutrons. What is its mass number? Write the number.
9
PSA neutral atom has mass number 40 and 20 neutrons. Find its atomic number. Write the number.
10
RA pupil says, “The nucleus has most of the mass of the atom but has no charge.” Explain why this is wrong, in one sentence. Accept any sentence that says the nucleus contains positively charged protons.
11
PSExplain why an atom has no overall charge, in one sentence. Accept any sentence that says the positive protons are balanced by the negative electrons.
12
PSAn atom has atomic number 19 and mass number 39. Work out the number of neutrons, then the number of electrons in the neutral atom. Write two numbers, in this order.
13
PSChlorine-35 has atomic number 17. How many neutrons does it have? Write the number.
14
RExplain why the atomic number tells you the element, in one sentence. Accept any sentence that says the number of protons is different for each element.
1.2

Elements, compounds and mixtures

GCSE
Distinguish elements, compounds and mixtures, Use chemical symbols and formulae to count atoms
Video lesson: Elements, compounds and mixturesComing soon
Context. Jo is cleaning a kitchen. She has salt, water, a bottle of vinegar and a box of baking soda. “Which of these are elements?” she asks. “And which are compounds?” Chemists use the same ideas to sort every substance.

An element is a substance made of only one type of atom. Each element has its own symbol, of one or two letters, and the first letter is always a capital. A compound contains two or more elements chemically joined in a fixed ratio. A mixture contains two or more substances that are not chemically joined, so each keeps its own properties.

ElementSymbolElementSymbol
sodiumNapotassiumK
ironFecopperCu
silverAggoldAu
leadPbtinSn
chlorineClmagnesiumMg
Some common element symbols to learn
FormulaNameAtoms in one unit
H2Owater2 H and 1 O: 3 atoms
CO2carbon dioxide1 C and 2 O: 3 atoms
NH3ammonia1 N and 3 H: 4 atoms
CH4methane1 C and 4 H: 5 atoms
H2SO4sulfuric acid2 H, 1 S and 4 O: 7 atoms
Formulae: the numbers below the symbols show how many atoms of each element are in one unit

Key ideas

  • A formula tells you which elements are present and how many atoms of each.
  • Use the small number after a symbol for the number of atoms of that element; if there is no number, there is one atom.
  • The compound has a fixed composition; a mixture can have any proportions.
  • Elements and compounds are pure substances; mixtures are not.
Worked example 1.2.1
How many atoms are there in one molecule of sulfuric acid, H2SO4? Write the number.

Method. Add up the numbers in the formula.

  1. Count hydrogen. There are 2 hydrogen atoms.
    H: 2
  2. Count sulfur and oxygen. There is 1 sulfur atom and 4 oxygen atoms.
    S: 1, O: 4
  3. Add. Total = 2 + 1 + 4.
    2 + 1 + 4 = 7
  4. Write the answer.
    7

Check. Seven atoms: 2 H, 1 S and 4 O. ✓

Worked example 1.2.2
Is carbon dioxide an element, a compound or a mixture? Write the word.

Method. Carbon dioxide has two different elements chemically joined.

  1. Look at the formula. CO2 has the symbols C and O.
    C and O
  2. Decide. Two different elements in a fixed ratio form a compound.
    two elements, fixed ratio
  3. Write the answer.
    compound

Check. A compound has a fixed formula, so it is not a mixture. ✓

Worked example 1.2.3
A sample of air is a mixture of nitrogen, oxygen and other gases. Is air a compound? Write yes or no.

Method. Check whether the parts are chemically joined.

  1. Look. Air contains several gases that are not chemically joined.
    not chemically joined
  2. Decide. A compound must have its elements chemically joined in a fixed ratio.
    fixed ratio
  3. Write the answer.
    no

Check. Air can have different amounts of each gas, so it is a mixture. ✓

Watch out

Watch out: a number in front of a formula (for example 3 in 3H2O) multiplies every atom in it. The number after a symbol applies only to that symbol. Do not mix these up when you count.

Exam tip

Exam tip. Write the symbol with a capital letter and a small letter if there is one: Co is cobalt, but CO is carbon monoxide. Only the first letter is a capital for elements.

Exercise 1B  Elements, compounds, mixtures and formulae

1
FWrite the chemical symbol for sodium. Write the symbol.
2
FWrite the name of the element with the symbol Fe. Write the word.
3
FWrite the chemical symbol for copper. Write the symbol.
4
FHow many atoms are there in one molecule of water, H2O? Write the number.
5
FHow many hydrogen atoms are there in one molecule of ammonia, NH3? Write the number.
6
FHow many oxygen atoms are there in one molecule of carbon dioxide, CO2? Write the number.
7
FIs sodium chloride an element, a compound or a mixture? Write the word.
8
FIs a mixture held together by chemical bonds? Write yes or no.
9
PSHow many atoms are there in 3 molecules of chlorine gas, Cl2? Write the number.
10
PSHow many atoms are there in one molecule of methane, CH4? Write the number.
11
PSWrite the formula of a compound made of one carbon atom and four hydrogen atoms. Write the formula.
12
RA pupil says, “Air is a compound because it contains oxygen.” Explain why this is wrong, in one sentence. Accept any sentence that says air is a mixture because its gases are not chemically joined.
13
PSExplain the difference between a mixture and a compound, in one sentence. Accept any sentence that says a compound has elements chemically joined in a fixed ratio, and a mixture is not chemically joined.
1.3

The development of the atomic model

GCSE
Describe how the model of the atom developed (Dalton, plum pudding, nuclear model), Explain how evidence from the gold foil experiment changed the model
Video lesson: The development of the atomic modelComing soon
Context. In a science museum, a visitor sees three drawings of the atom, one after another. “Which one is right?” she asks. Each drawing was the best idea of its time, and each was changed when new evidence arrived.

Dalton (early 1800s) said atoms were tiny solid spheres that could not be split. Later, experiments showed that atoms contain smaller particles. In the plum pudding model, the electrons were spread through a positive sphere, like fruit in a pudding.

In the gold foil experiment, Rutherford and his team fired positively charged alpha particles at a very thin sheet of gold. Most went straight through. A few were deflected, and very few bounced back. This showed that the atom has a tiny, dense, positive nucleus, and that most of the atom is empty space.

Dalton (early 1800s)solid spherePlum pudding (about 1897)electrons spread in a positive sphereNuclear model (about 1911)tiny dense nucleus, mostly empty space
Three models of the atom, from left to right
thin gold foilalpha particlesmost pass straight througha few are deflected; very few bounce back
The gold foil experiment: the paths of alpha particles (not to scale)

Key ideas

  • Dalton: atoms are solid spheres.
  • Plum pudding: a positive sphere with electrons spread through it.
  • Nuclear model: a tiny, dense, positive nucleus; electrons move around it; most of the atom is empty space.
  • Models change when new evidence does not fit the old model.
Worked example 1.3.1
In the gold foil experiment, most alpha particles went straight through the foil. What does this tell us about the atom? Write the words.

Method. Think about what the alpha particles passed through.

  1. Look. The alpha particles passed through the foil with no change of direction.
    straight through
  2. Conclude. So most of the atom is empty space.
    empty space
  3. Write the answer.
    most of the atom is empty space

Check. Only a few alpha particles were deflected, so the nucleus is very small. ✓

Worked example 1.3.2
A few alpha particles were deflected by large angles. What does this tell us about the nucleus? Write the words.

Method. A large deflection needs a strong positive charge in a small place.

  1. Look. Alpha particles are positively charged, and they were pushed away by something.
    pushed away
  2. Conclude. The nucleus is positive, small and dense.
    small and dense
  3. Write the answer.
    the nucleus is small, dense and positive

Check. Like charges repel, so a positive nucleus pushes back an alpha particle. ✓

Worked example 1.3.3
Which particle, found in the nucleus, has no charge and was discovered after the plum pudding model? Write the word.

Method. Find the particle with no charge.

  1. Recall. Neutrons were found after the plum pudding model, by James Chadwick.
    neutron
  2. Check. A neutron has no charge, so it is in the nucleus without affecting its charge.
    no charge
  3. Write the answer.
    neutron

Check. Protons and neutrons are both found in the nucleus. ✓

Watch out

Watch out: a model is not a photograph. The nuclear model is still a model; electrons are not solid dots on fixed orbits. Use the model to explain evidence, and say when it changed.

Exam tip

Exam tip. When a question asks why a model changed, name the new evidence (for example, the deflected alpha particles) and the change it caused.

Try it (with your teacher). Use a sheet of paper and some small beads. Put the beads in a box and, without looking, roll small marbles at it. Use the paths of the marbles to draw a model of the box contents. Then compare your model with the gold foil results.

Exercise 1C  The development of the atomic model

1
FWho proposed that atoms are tiny solid spheres in the early 1800s? Write the name.
2
FIn the plum pudding model, what particles were spread through a positive sphere? Write the word.
3
FWho led the gold foil experiment? Write the surname.
4
FWhat type of particle was fired at the gold foil? Write the words.
5
FWhich model describes a tiny, dense, positive nucleus with most of the atom as empty space? Write the name of the model.
6
FWhich particle was discovered by James Chadwick, and has no charge? Write the word.
7
FIn the gold foil experiment, what does it mean if an alpha particle bounces straight back? Write the words.
8
PSExplain 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
PSExplain why a few alpha particles were deflected, in one sentence. Accept any sentence that says they came close to a small, positive nucleus that repelled them.
10
PSPredict what would happen to the results if the nucleus were much larger. Write one sentence. Accept any sentence that says more alpha particles would be deflected or bounced back.
11
RA scientist says, “The model of the atom will never change.” Explain why this is wrong. Accept any sentence that says models are changed when new evidence does not fit them.
1.4

Isotopes and relative atomic mass [H]

GCSE
Explain isotopes and their use, [H] Calculate relative atomic mass from isotope abundance
Video lesson: Isotopes and relative atomic mass [H]Coming soon
Context. Carbon-14 is used by archaeologists to estimate the age of some ancient wood. The carbon in the wood is not all the same kind of carbon. Some carbon atoms have more neutrons than others, and this is why the method works.

Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons. Isotopes have the same atomic number but different mass numbers. Isotopes of an element have the same chemical properties, because they have the same number of electrons.

IsotopeProtonsNeutronsElectrons (neutral atom)
hydrogen-1101
hydrogen-2 (deuterium)111
hydrogen-3 (tritium)121
carbon-12666
carbon-14686
Some isotopes and their particles

Key ideas

  • Isotopes have the same number of protons (atomic number) and different numbers of neutrons (mass number).
  • Isotopes of an element react in the same way.
  • Some isotopes are radioactive and can be used for dating and medical tests.

[H] Higher tier only. The relative atomic mass (Ar) of an element is the weighted mean of the mass numbers of its isotopes. Use the percentage abundance of each isotope: Ar = (mass number x % abundance + mass number x % abundance) / 100. The answer is often not a whole number.

Worked example 1.4.1
An element has two isotopes, mass 35 (75%) and mass 37 (25%). Calculate its relative atomic mass. Write the number.

Method. Multiply each mass by its percentage, add the results, then divide by 100.

  1. Multiply. Multiply each mass number by its percentage.
    35 x 75 = 2625; 37 x 25 = 925
  2. Add. Add the two results.
    2625 + 925 = 3550
  3. Divide. Divide by 100 for the average.
    3550 / 100 = 35.5
  4. Write the answer.
    35.5

Check. The answer is between 35 and 37, which it must be. ✓

Worked example 1.4.2
Neon has two isotopes: neon-20 (90%) and neon-22 (10%). Calculate its relative atomic mass. Write the number.

Method. Use the same method with the two mass numbers.

  1. Multiply. Multiply each mass by its percentage.
    20 x 90 = 1800; 22 x 10 = 220
  2. Add. Add the results.
    1800 + 220 = 2020
  3. Divide. Divide by 100.
    2020 / 100 = 20.2
  4. Write the answer.
    20.2

Check. The answer is closer to 20, because most atoms are neon-20. ✓

Worked example 1.4.3
Name the isotope of carbon that has 8 neutrons. Write the name with its mass number.

Method. Add the protons and neutrons, using carbon’s 6 protons.

  1. Protons. Carbon has 6 protons.
    6 protons
  2. Mass number. Mass number = 6 + 8 = 14.
    6 + 8 = 14
  3. Name. Carbon with mass number 14 is carbon-14.
    carbon-14
  4. Write the answer.
    carbon-14

Check. Carbon-12 has 6 neutrons, so carbon-14 is the isotope with 8. ✓

Watch out

Watch out: the mass number of an element on the periodic table is an average, so it is usually not a whole number. It is the relative atomic mass, not the mass number of one isotope.

Exam tip

Exam tip. In an abundance question, check the percentages add to 100 before you start. If they do not, read the question again.

Exercise 1D  Isotopes and relative atomic mass

1
FWhat do all isotopes of one element have in common? Write the word.
2
FHow many neutrons does carbon-14 have? Write the number.
3
FWhich isotope of hydrogen has 2 neutrons? Write the name.
4
FDo isotopes of the same element have the same chemical properties? Write yes or no.
5
FWhat is the difference between two isotopes of the same element? Write the word.
6
FChlorine-35 and chlorine-37 are isotopes. How many more neutrons does chlorine-37 have? Write the number.
7
PSName the element with 6 protons and 8 neutrons in its nucleus, giving its mass number. Write the name with mass number.
8
PS[H] An element has isotopes with mass numbers 63 (70%) and 65 (30%). Calculate its relative atomic mass. Write the number to one decimal place.
9
PS[H] Boron has two isotopes: boron-10 (20%) and boron-11 (80%). Calculate its relative atomic mass. Write the number.
10
PS[H] An element has isotopes with mass numbers 64 and 66, in equal amounts (50% each). Calculate its relative atomic mass. Write the number.
11
R[H] Explain why the relative atomic mass of chlorine is not a whole number, in one sentence. Accept any sentence that says it is a weighted mean of isotopes with different mass numbers.
12
RExplain why isotopes are useful for dating ancient objects, in one sentence. Accept any sentence that says a radioactive isotope decays at a known rate, so its amount shows the age.
1.5

The size of atoms, the nanoscale and standard form

GCSE
Know the size of atoms and the nanoscale, Use standard form for very small lengths
Video lesson: The size of atoms, the nanoscale and standard formComing soon
Context. A scientist measures a nanoparticle in a sunscreen. The label says the particles are about 50 nanometres across. “How does that compare with an atom?” asks a student. To answer, we need to convert nanometres into metres.

Atoms are about 0.1 nm across, which is about 1 x 10-10 m. A nanometre (nm) is one billionth of a metre. The nanoscale is from about 1 nm to 100 nm. Very small lengths are easier to write in standard form.

UnitIn metresStandard form
1 nanometre (nm)0.000000001 m1 x 10-9 m
1 atom (about)0.0000000001 m1 x 10-10 m
1 micrometre (um)0.000001 m1 x 10-6 m
Very small units and their values

Key ideas

  • 1 nm = 1 x 10-9 m.
  • Atoms are about 0.1 nm wide.
  • Nanoparticles are between about 1 and 100 nm.
  • To convert nm to m, multiply by 10-9. To convert m to nm, divide by 10-9, or multiply by 109.
Worked example 1.5.1
Write 50 nm in metres in standard form. Write the answer.

Method. Multiply by 10-9.

  1. Convert. Multiply 50 by 10-9.
    50 x 10-9 m
  2. Standard form. Write 50 as 5 x 10, so the number is 5 x 101 x 10-9.
    5 x 101 x 10-9
  3. Combine powers. Add the powers: 1 + (-9) = -8.
    5 x 10-8
  4. Write the answer.
    5 x 10^-8 m

Check. 5 x 10-8 m = 0.00000005 m, which is 50 nanometres. ✓

Worked example 1.5.2
An atom is about 0.1 nm across. Write this in metres in standard form. Write the answer.

Method. Use 1 nm = 1 x 10-9 m.

  1. Convert. 0.1 nm = 0.1 x 10-9 m.
    0.1 x 10-9
  2. Standard form. Write 0.1 as 1 x 10-1, so the power is -1 + (-9) = -10.
    1 x 10-10
  3. Write the answer.
    1 x 10^-10 m

Check. Check: 0.1 nm = 0.1 x 10-9 m = 10-10 m. ✓

Worked example 1.5.3
How many times wider is a 40 nm nanoparticle than an atom that is 0.1 nm across? Write the number.

Method. Divide the two widths in the same unit.

  1. Same unit. Both are in nm: 40 nm and 0.1 nm.
    40 nm and 0.1 nm
  2. Divide. The ratio is 40 / 0.1.
    40 / 0.1 = 400
  3. Write the answer.
    400

Check. 0.1 x 400 = 40. ✓

Watch out

Watch out: 0.1 nm is not 0.1 m. The prefix nano means 10-9, so the number of metres is always much smaller than the number of nanometres.

Exam tip

Exam tip. Write standard form with exactly one digit before the decimal point. For example, 0.0005 m is 5 x 10-4 m, not 0.5 x 10-3 m.

Exercise 1E  The size of atoms and nanoscale

1
FHow many nanometres are in one metre? Write the power of ten, as 10 to a power.
2
FWrite 1 nanometre in metres in standard form. Write the answer.
3
FWrite 2 x 10-9 m in nanometres. Write the number.
4
FWrite 0.25 nm in metres in standard form. Write the answer.
5
FA nanoparticle is 50 nm across. Write this in metres in standard form. Write the answer.
6
FWhat is the size range of nanoparticles, in nanometres? Write the range.
7
FWrite the chemical symbol for potassium. Write the symbol.
8
FWrite the chemical symbol for silver. Write the symbol.
9
FWrite the formula of water. Write the formula.
10
PSAn atom is about 0.1 nm across, and a nanoparticle is 40 nm across. How many times wider is the nanoparticle? Write the number.
11
PSWrite the size of a 0.0000000001 m atom in standard form. Write the answer.
12
RA scientist says, “An atom is 0.1 m across.” Explain why this is wrong, in one sentence. Accept any sentence that says an atom is about 0.1 nm, which is much smaller than 0.1 m.
13
RExplain why standard form is useful for writing the size of an atom, in one sentence. Accept any sentence that says it avoids writing many zeros.

Chapter summary

SkillMethod
AtomNucleus (protons and neutrons) with electrons in shells around it.
Atomic numberNumber of protons; identifies the element; equals electrons in a neutral atom.
Mass numberProtons plus neutrons. Neutrons = mass number - atomic number.
Element, compound, mixtureOne type of atom; two or more elements chemically joined in a fixed ratio; not chemically joined.
Atomic modelsDalton (solid sphere), plum pudding (electrons in a positive sphere), nuclear model (small dense nucleus).
IsotopesSame protons, different neutrons; same chemical properties.
[H] Relative atomic massWeighted mean: (mass x %) added over 100.
Nanoscale1 nm = 1 x 10-9 m; atoms are about 0.1 nm across.

Can you…?

  • I can name the three particles and give their charges and positions
  • I can use atomic number and mass number to find protons, neutrons and electrons
  • I can tell an element, a compound and a mixture apart
  • I can count atoms in a formula and write simple formulae
  • I can describe the atomic models and the evidence that changed them
  • I can explain isotopes and convert nanometres to metres in standard form
  • [H] I can calculate a relative atomic mass from isotope percentages

Key terms

atomic number
the number of protons in an atom; it identifies the element
mass number
the number of protons plus neutrons in an atom
isotope
atoms of the same element with the same protons but a different number of neutrons
compound
two or more elements chemically joined in a fixed ratio
mixture
two or more substances not chemically joined; each keeps its own properties
alpha particle
a positively charged particle fired in the gold foil experiment
nucleus
the small, dense, positive centre of an atom
nanometre
one billionth of a metre (1 x 10-9 m)
relative atomic mass
[H] the weighted mean mass of the isotopes of an element

Exercise 1Z  Check-up

1
Name the part of an atom that has no charge. Write the word.
2
An atom has atomic number 12 and mass number 24. How many neutrons does it have? Write the number.
3
Write the symbol for magnesium. Write the symbol.
4
How many atoms are there in one molecule of CO2? Write the number.
5
Which model of the atom has a tiny, dense, positive nucleus? Write the name of the model.
6
Write 0.5 nm in metres in standard form. Write the answer.
7
An element has isotopes with mass numbers 35 (75%) and 37 (25%). Calculate its relative atomic mass. Write the number.
8
Name the particle that carries a charge of +1 and is found in the nucleus. Write the word.
9
Explain why isotopes of an element have the same chemical properties, in one sentence. Accept any sentence that says they have the same number of electrons.
10
Write the formula for a compound with one sodium atom and one chlorine atom. Write the formula.

Total 26 marks. Allow about 26 minutes.

Exercise 1X  Exam-style questions

1
FName the part of an atom that is found in the nucleus and has no charge. Write the word.[1 mark]
2
FAn atom has atomic number 9 and mass number 19. How many electrons does a neutral atom of it have? Write the number.[1 mark]
3
FWrite the chemical symbol for iron. Write the symbol.[1 mark]
4
FName the scientist who discovered the neutron. Write the surname.[1 mark]
5
PSExplain why most alpha particles passed straight through the gold foil. Write one sentence. Accept any sentence that says most of the atom is empty space.[2 marks]
6
RAn element has a relative atomic mass of 35.5. Its two isotopes are chlorine-35 and chlorine-37. Which isotope is more common? Write the mass number, and give a reason in one sentence. Accept chlorine-35 with a reason that the answer is closer to 35.[3 marks]
7
PS[H] Calculate the relative atomic mass of an element with isotopes of mass 20 (90%) and 22 (10%). Write the number.[3 marks]
8
PSWrite 40 nm in metres in standard form. Write the answer.[2 marks]
9
RExplain the difference between an element and a compound, using one example of each. Accept any answer that gives an element such as oxygen and a compound such as water.[3 marks]
10
RExplain why the nuclear model replaced the plum pudding model, in one sentence. Accept any sentence that says the gold foil results showed a small, dense nucleus that the plum pudding model could not explain.[3 marks]
11
PSCalculate the number of atoms in one molecule of H2SO4, and the number of atoms in 2 molecules. Write two numbers, in this order.[2 marks]
12
PSWrite 0.25 nm in metres in standard form, then say which is larger: 0.25 nm or 1 x 10-10 m. Accept 2.5 x 10^-10 m, and say 0.25 nm is larger because 0.25 nm = 2.5 x 10-10 m, which is larger than 1 x 10-10 m.[4 marks]
Chapter 2: Electron arrangement and the periodic table →

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