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KS3 Science · Year 8Chapter 7 · Atomic structure and the periodic table
Year 8 · Chapter 7

Atomic structure and the periodic table

By the end of this chapter you will be able to

  • Name the three subatomic particles and give their relative mass and charge
  • Work out the protons, neutrons and electrons of an atom from its atomic and mass numbers
  • Draw or read electron shells for the first 20 elements
  • Use the periodic table: groups and periods, and the link between group and outer electrons
  • Describe the trends in reactivity and physical properties in groups 1 and 7

You should already know

  • Atoms, elements and the periodic table (Year 7 Chapter 6)
  • Chemical reactions and word equations (Year 7 Chapter 9)

Where this leads

  • Reactions of metals and the reactivity series (Year 8 Chapter 8)

National Curriculum for England, Key Stage 3 science, Year 8. Content: 8C1 atomic structure: protons, neutrons and electrons (relative mass and charge), atomic number, mass number, and electron shells for the first 20 elements, 8C2 the periodic table: groups and periods; trends in group 1 (alkali metals) and group 7 (halogens) and the reactivity of each group.

Exercise P  Do you remember?

1
FWhat is the symbol for sodium? Write the symbol.
2
FWhat is a substance made of only one type of atom called? Write the word.
3
FWhat is the name of the smallest part of an element that keeps its properties? Write the word.
4
FWhat is 9 plus 15 minus 4? Write the number.
5
FWhat is the name of the gas that is not a metal and makes up about a fifth of the air? Write the word.
7.1

Protons, neutrons and electrons

Year 8
8C1: protons, neutrons and electrons, with their relative mass and charge
Video lesson: Protons, neutrons and electronsComing soon
Story. Ali is told that everything around him is made of atoms. “If I cut a piece of copper smaller and smaller, what is left?” he asks. His teacher says, “An atom, made of smaller particles: protons and neutrons in the nucleus, and electrons moving around it.”

An atom has a nucleus made of protons and neutrons. Electrons move in shells around the nucleus. Protons have a positive charge, neutrons have no charge, and electrons have a negative charge. Atoms have no overall charge, because the number of protons equals the number of electrons.

The table gives the relative mass and charge of each particle.

ParticleRelative massRelative chargeWhere it is
proton1+1in the nucleus
neutron10in the nucleus
electronvery small (about 1/2000 of a proton)-1in shells around the nucleus
Naprotons 11, neutrons 12, electrons 11
An atom of sodium: 11 protons and 11 electrons, with 12 neutrons in the nucleus.

Key ideas

  • Protons are in the nucleus, with a relative mass of 1 and a charge of +1.
  • Neutrons are in the nucleus, with a relative mass of 1 and no charge.
  • Electrons are in shells, with a very small mass and a charge of -1.
  • An atom has no overall charge: protons equal electrons.
Worked example 7.1.1
What is the charge of a proton? Write the sign and number.

Method. Recall the charge.

  1. Recall. A proton carries a positive charge.
    positive
  2. Write. The relative charge of a proton is +1.
    +1
  3. Write the answer.
    +1

Check. Electrons have the opposite charge, -1. ✓

Worked example 7.1.2
An atom has 11 protons. How many electrons does it have, if it has no overall charge? Write the number.

Method. Use the rule that charges balance.

  1. Think. An atom with no overall charge has equal protons and electrons.
    equal
  2. Write. So the atom has 11 electrons.
    11
  3. Write the answer.
    11

Check. 11 protons and 11 electrons balance. ✓

Worked example 7.1.3
Which particle has no charge: proton, neutron or electron? Write the word.

Method. Recall the charges.

  1. Recall. Protons are +1 and electrons are -1.
    +1 and -1
  2. Decide. So the particle with no charge is the neutron.
    neutron
  3. Write the answer.
    neutron

Check. Neutrons are in the nucleus. ✓

Watch out

Watch out: the electron is not a large particle. Its mass is so small that it is taken as zero when working out the mass of an atom.

Exam tip

Exam tip. Give both the charge and the place: “proton, +1, in the nucleus”.

Try it (with your teacher, or a grown-up at home). Look at the atom diagram of sodium. Count the protons, neutrons and electrons, and check that the protons and electrons balance. Do not handle any chemical.

Exercise 7A  Protons, neutrons and electrons

1
FWhich particle is found in the nucleus and has a positive charge? Write the word.
2
FWhich particle has no charge? Write the word.
3
FWhich particle moves in shells around the nucleus? Write the word.
4
FWhat is the relative charge of an electron? Write the number with its sign.
5
FWhat is the relative mass of a proton? Write the number.
6
FWhat is the relative charge of a neutron? Write the number.
7
FWhich two particles have the same relative mass? Write the words.
8
FWhich particle has the smallest mass? Write the word.
9
FAn atom has 6 protons. How many electrons does it have if it is neutral? Write the number.
10
FAn atom has 8 protons and 8 electrons. Does it have an overall charge? Write yes or no.
11
PSRuby says that electrons are in the nucleus with the protons. Is Ruby right? Write yes or no.
12
PSWorking scientifically: a model shows an atom with 5 protons, 5 electrons and 6 neutrons. Is the model correct about the charge? Write yes or no.
13
RExplain why an atom has no overall charge, even though it has charged particles. Write one sentence.
14
RChallenge: explain why the mass of an atom is mostly in its nucleus. Write one sentence.
7.2

Atomic number and mass number

Year 8
8C1: atomic number (protons) and mass number (protons plus neutrons); working out the number of neutrons
Video lesson: Atomic number and mass numberComing soon
Story. Sam reads the symbol for chlorine in the periodic table: a small number at the bottom and a large one at the top. “What do these numbers tell me?” he asks. His teacher says, “The bottom number is the atomic number, which is the protons. The top is the mass number, which is protons plus neutrons.”

The atomic number of an element is the number of protons in each atom. It is the number that identifies the element. The mass number is the number of protons plus neutrons.

So the number of neutrons = mass number minus atomic number. Atoms with no overall charge also have the same number of electrons as protons.

The table gives the numbers for four elements. The mass numbers are for their most common forms.

ElementSymbolAtomic numberMass numberProtonsNeutronsElectrons
sodiumNa1123111211
chlorineCl1735171817
oxygenO816888
potassiumK1939192019

Key ideas

  • Atomic number = number of protons (and electrons in a neutral atom).
  • Mass number = protons + neutrons.
  • Neutrons = mass number minus atomic number.
  • The atomic number identifies the element.
Worked example 7.2.1
An atom of sodium has atomic number 11 and mass number 23. How many neutrons does it have? Write the number.

Method. Subtract the atomic number from the mass number.

  1. Subtract. Neutrons = mass number minus atomic number = 23 minus 11.
    23 - 11
  2. Calculate. So the number of neutrons is 12.
    23 - 11 = 12
  3. Write the answer.
    12

Check. 11 protons + 12 neutrons = 23. ✓

Worked example 7.2.2
An atom has 17 protons and a mass number of 35. How many neutrons does it have? Write the number.

Method. Subtract the protons from the mass number.

  1. Subtract. Neutrons = 35 minus 17.
    35 - 17
  2. Calculate. So the atom has 18 neutrons.
    35 - 17 = 18
  3. Write the answer.
    18

Check. 17 + 18 = 35. ✓

Worked example 7.2.3
What is the atomic number of oxygen? Write the number.

Method. Read the table.

  1. Read. Oxygen has 8 protons.
    8 protons
  2. Name it. So its atomic number is 8.
    8
  3. Write the answer.
    8

Check. Oxygen has 8 electrons when neutral. ✓

Watch out

Watch out: the atomic number is the number of protons, not the number of neutrons. Mass number is the protons and neutrons added together.

Exam tip

Exam tip. Neutrons = mass number minus atomic number. Check by adding the protons and neutrons back to the mass number.

Try it (with your teacher, or a grown-up at home). Look at the table of elements. For each one, work out the neutrons and check the answer by adding the protons and neutrons back together. Do not make any compound yourself.

Exercise 7B  Atomic number and mass number

1
FWhat is the atomic number of an element? Write the words.
2
FWhat is the mass number of an atom? Write the words.
3
FWhat is the atomic number of sodium? Write the number.
4
FWhat is the atomic number of chlorine? Write the number.
5
FWhat is the mass number of oxygen? Write the number.
6
FHow many neutrons does oxygen have? Write the number.
7
FHow many neutrons does potassium have? Write the number.
8
FHow many electrons does a neutral chlorine atom have? Write the number.
9
FAn atom has 12 protons and 12 neutrons. What is its mass number? Write the number.
10
PSAli says that the atomic number of an atom equals its number of neutrons. Is Ali right? Write yes or no.
11
PSWorking scientifically: an atom has a mass number of 31 and an atomic number of 15. Work out the neutrons. Write the number.
12
PSTwo atoms both have 17 protons, but one has 18 neutrons and one has 20 neutrons. Are they the same element? Write yes or no.
13
RExplain why the atomic number tells you the element, but the mass number does not. Write one sentence.
14
RChallenge: an atom has a mass number of 40 and an atomic number of 20. Name the element and say how many electrons a neutral atom has. Write the element name and the number.
7.3

Electron shells

Year 8
8C1: electron shells for the first 20 elements: the first shell holds 2, the next two shells hold 8, and the outer shell up to 8
Video lesson: Electron shellsComing soon
Story. Omar draws the atoms of the first 20 elements in his notebook. “Where do the electrons go?” he asks. His teacher says, “Fill the shells from the inside. The first shell holds 2, and the next two shells hold 8 each. The outer shell holds the rest.”

Electrons fill shells around the nucleus, starting with the shell closest to it. For the first 20 elements, the first shell holds up to 2 electrons, and the second and third shells hold up to 8 each. The fourth shell begins to fill after the third shell has 8 electrons; for these elements, the outer shell holds the rest, up to 2.

The number of electrons in the outer shell shows the group of the element, which you will see in the next section.

The table gives the shells for five of these elements.

ElementSymbolElectron shellsOuter shell electrons
lithiumLi2, 11
oxygenO2, 66
sodiumNa2, 8, 11
chlorineCl2, 8, 77
potassiumK2, 8, 8, 11

Key ideas

  • The first shell holds up to 2 electrons.
  • The second and third shells hold up to 8 each.
  • Fill the shells from the nucleus outwards.
  • The outer shell electrons decide the group of an element.
Worked example 7.3.1
How many electrons are in the first shell of a neutral lithium atom? Write the number.

Method. Use the first shell rule.

  1. Count. Lithium has 3 electrons.
    3
  2. Fill. The first shell takes the first 2 electrons.
    the first shell is full at 2
  3. Write. So the first shell holds 2 electrons, and the outer shell has 1.
    2
  4. Write the answer.
    2

Check. Lithium has shells 2, 1. ✓

Worked example 7.3.2
What is the electron arrangement of a neutral chlorine atom? Write the numbers in order.

Method. Use the atomic number as the number of electrons.

  1. Count. Chlorine has 17 electrons.
    17
  2. Fill. Fill 2 in the first shell, 8 in the second, then 7 in the third.
    2, 8, 7
  3. Write the answer.
    2, 8, 7

Check. 2 + 8 + 7 = 17. ✓

Worked example 7.3.3
How many outer shell electrons does a neutral oxygen atom have? Write the number.

Method. Fill the shells for oxygen.

  1. Count. Oxygen has 8 electrons.
    8
  2. Fill. The first shell has 2, and the outer shell has 6.
    2, 6
  3. Write. So the outer shell has 6 electrons.
    6
  4. Write the answer.
    6

Check. 2 + 6 = 8. ✓

Watch out

Watch out: the outer shell is the last shell that has electrons. Do not count the full inner shells as the outer shell.

Exam tip

Exam tip. Write the shells as numbers: 2, 8, 8, 2 for calcium. Check that they add up to the atomic number.

Try it (with your teacher, or a grown-up at home). Draw the electron shells of a neutral carbon atom on squared paper: a nucleus, a circle for shell one with 2 dots, and a larger circle for shell two with 4 dots. Check that the total is 6.

Exercise 7C  Electron shells

1
FHow many electrons can the first shell hold? Write the number.
2
FHow many electrons can the second shell hold, for the first 20 elements? Write the number.
3
FHow many electrons does a neutral carbon atom have? Write the number.
4
FWhat is the electron arrangement of neutral sodium? Write the numbers in order.
5
FWhat is the outer shell electron number of neutral sodium? Write the number.
6
FWhat is the electron arrangement of neutral oxygen? Write the numbers in order.
7
FHow many shells does a neutral potassium atom have? Write the number.
8
FWhat is the outer shell electron number of neutral calcium? Write the number.
9
FHow many electrons are in the third shell of neutral argon, arranged 2, 8, 8? Write the number.
10
PSRuby says the outer shell of a neutral chlorine atom has 8 electrons. Is Ruby right? Write yes or no.
11
PSWorking scientifically: draw a diagram that shows the electron arrangement of potassium. Write the numbers in order: 2, 8, 8, 1.
12
RExplain why neon is said to have a full outer shell. Write one sentence.
13
RChallenge: a neutral atom has the arrangement 2, 8, 2. Name the element. Write the name.
7.4

Groups and periods

Year 8
8C2: the periodic table: groups (columns) share the number of outer electrons; periods (rows) show the number of shells
Video lesson: Groups and periodsComing soon
Story. Sam notices that sodium, lithium and potassium all react in a similar way. “Why are they in the same column?” he asks. His teacher says, “Elements in one group have the same number of outer electrons. That is why they behave in similar ways.”

The periodic table is arranged in groups (columns) and periods (rows). Elements in the same group have the same number of outer shell electrons, so they react in similar ways. The period number shows the number of electron shells an atom has.

Group 1 elements have 1 outer electron, group 2 have 2, and so on up to group 7 (the halogens) with 7. Group 0 has a full outer shell.

12345670HHeLiBeBCNOFNeNaMgAlSiPSClArKCa1234Groups run down the columns; periods are the rows (period number = number of shells).
A simplified periodic table of the first 20 elements, with groups and periods.

Key ideas

  • Groups are the columns; periods are the rows.
  • Elements in one group have the same number of outer electrons.
  • The period number is the number of electron shells.
  • Group 0 elements have a full outer shell.
Worked example 7.4.1
Which group is sodium in? Write the number.

Method. Use the outer electron.

  1. Count. Sodium has 1 outer shell electron.
    1
  2. Name it. So sodium is in group 1.
    1
  3. Write the answer.
    1

Check. Sodium is in period 3. ✓

Worked example 7.4.2
Which period is potassium in? Write the number.

Method. Count the shells.

  1. Count. Potassium has four shells: 2, 8, 8, 1.
    4 shells
  2. Name it. So potassium is in period 4.
    4
  3. Write the answer.
    4

Check. Potassium has 1 outer electron, so group 1. ✓

Worked example 7.4.3
How many outer electrons does an element in group 7 have? Write the number.

Method. Use the group number.

  1. Think. The group number is the number of outer electrons, for groups 1 to 7.
    group number
  2. Name it. So a group 7 element has 7 outer electrons.
    7
  3. Write the answer.
    7

Check. Chlorine has 7 outer electrons. ✓

Watch out

Watch out: the group number is not the number of shells. The period number is the number of shells.

Exam tip

Exam tip. Use the table: the column gives the number of outer electrons; the row gives the number of shells.

Try it (with your teacher, or a grown-up at home). Look at a labelled periodic table, and find sodium, chlorine and calcium. Say the group and the period of each. Do not handle any chemical.

Exercise 7D  Groups and periods

1
FWhat are the vertical columns of the periodic table called? Write the word.
2
FWhat are the horizontal rows of the periodic table called? Write the word.
3
FWhich group has elements with 1 outer electron? Write the number.
4
FHow many outer electrons does an element in group 2 have? Write the number.
5
FWhich group is chlorine in? Write the number.
6
FWhich period is oxygen in? Write the number.
7
FWhich group is neon in? Write the number.
8
FWhich two elements, from sodium, chlorine, and calcium, are in the same period? Write the words.
9
PSAli says that elements in the same period have the same number of outer electrons. Is Ali right? Write yes or no.
10
PSWorking scientifically: name one thing that is the same for all elements in group 1. Write the word.
11
RExplain why sodium and potassium react in similar ways. Write one sentence.
12
RChallenge: an element is in period 3 and group 6. How many shells and outer electrons does it have? Write the two numbers.
7.5

Trends in groups 1 and 7

Year 8
8C2: trends in group 1 (alkali metals), where reactivity increases down the group; and group 7 (halogens), where reactivity decreases down the group
Video lesson: Trends in groups 1 and 7Coming soon
Story. Omar watches a teacher drop small pieces of metal into a trough of water. The sodium skates across the surface and fizzes. “Is the potassium different?” he asks. His teacher says, “Potassium reacts even more strongly. Group 1 metals get more reactive as you go down the group.”

The alkali metals are group 1. They are soft, and they react with water to make a metal hydroxide and hydrogen. Going down group 1, the metals become more reactive, and the melting point falls.

The halogens are group 7. Going down group 7, the halogens become less reactive, and the boiling point rises. A more reactive halogen can displace a less reactive one from a solution of its salt.

The table shows the melting points of group 1 metals. The reaction with water is a teacher demonstration only.

MetalSymbolMelting point (degrees C)Reaction with water
lithiumLi181reacts slowly with water
sodiumNa98reacts with water, fizzing and moving on the surface
potassiumK63reacts more vigorously than sodium with water
Melting points of group 1 metals (degrees C)181lithium (Li)98sodium (Na)63potassium (K)Melting point falls as you go down group 1.
Melting points of lithium, sodium and potassium: they fall down group 1.

The table shows the boiling points and appearance of the halogens. Halogens are toxic, so pupils never handle them.

HalogenSymbolBoiling point (degrees C)State and colour at room temperature
fluorineF-188pale yellow gas
chlorineCl-34green gas
bromineBr59red-brown liquid
iodineI184grey-black solid

Key ideas

  • Group 1 (alkali metals): reactivity increases down the group.
  • Group 1 melting points fall down the group.
  • Group 7 (halogens): reactivity decreases down the group.
  • Group 7 boiling points rise down the group.
Worked example 7.5.1
Which group 1 metal has the lowest melting point? Write the name.

Method. Read the table.

  1. Look. The melting points are 181, 98 and 63 degrees C.
    63
  2. Name it. So potassium has the lowest melting point.
    potassium
  3. Write the answer.
    potassium

Check. The melting point falls down the group. ✓

Worked example 7.5.2
Which halogen is a grey-black solid at room temperature? Write the name.

Method. Read the table.

  1. Look. The table gives grey-black as the colour and state of one halogen.
    grey-black solid
  2. Name it. So the halogen is iodine.
    iodine
  3. Write the answer.
    iodine

Check. Iodine has the highest boiling point in group 7. ✓

Worked example 7.5.3
In a displacement, does chlorine displace bromide from a solution? Write yes or no.

Method. Use the rule of reactivity.

  1. Think. Chlorine is more reactive than bromine, so it can displace bromide.
    more reactive
  2. Decide. So the answer is yes.
    yes
  3. Write the answer.
    yes

Check. Bromine is then produced from the solution. ✓

Watch out

Watch out: the group 1 trend is the opposite of the group 7 trend. Group 1 gets more reactive going down; group 7 gets less reactive going down.

Exam tip

Exam tip. State both the trend and the group: “in group 1, reactivity increases down the group”.

Try it (with your teacher, or a grown-up at home). Look at the table of group 1 melting points. Say in one sentence how the melting point changes down the group. Do not heat or touch any metal; the demonstration is for your teacher to show.

Exercise 7E  Trends in groups 1 and 7

1
FWhat is the group 1 family of metals called? Write the words.
2
FWhat is the group 7 family of elements called? Write the word.
3
FGoing down group 1, does reactivity increase or decrease? Write the word.
4
FGoing down group 7, does reactivity increase or decrease? Write the word.
5
FUsing the table, what is the melting point of sodium? Write the number of degrees C.
6
FWhich group 7 element is a green gas at room temperature? Write the name.
7
FWhich group 7 element has the highest boiling point? Write the name.
8
FWhich halogen is the most reactive, fluorine or iodine? Write the name.
9
FDoes iodine displace bromide from a solution? Write yes or no.
10
PSRuby says that potassium is less reactive than sodium because it is lower in group 1. Is Ruby right? Write yes or no.
11
PSWorking scientifically: a teacher shows a demonstration of a group 1 metal with water. Name one safety measure. Accept any sensible measure: goggles, a teacher-led demonstration, or a small piece only.
12
RExplain why reactivity increases going down group 1. Write one sentence.
13
RChallenge: explain why the halogens become less reactive going down group 7. Write one sentence.

Chapter summary

SkillMethod
AtomNucleus of protons (+1) and neutrons (0); electrons (-1) in shells; no overall charge.
NumbersAtomic number = protons. Mass number = protons plus neutrons. Neutrons = mass minus atomic.
ShellsFirst shell 2; next two shells 8 each; the rest in the outer shell.
Groups and periodsGroup = outer electrons; period = number of shells.
Group 1Alkali metals: reactivity increases down; melting point falls down.
Group 7Halogens: reactivity decreases down; boiling point rises down.

Can you…?

  • I can name the three subatomic particles and give their relative mass and charge
  • I can work out the protons, neutrons and electrons of an atom
  • I can draw or read electron shells for the first 20 elements
  • I can use groups and periods to find the outer electrons and the shells
  • I can describe the trends in reactivity and physical properties in groups 1 and 7

Key terms

proton
a particle in the nucleus with a relative charge of +1
neutron
a particle in the nucleus with no charge
electron
a particle with a relative charge of -1 that moves in shells
atomic number
the number of protons in an atom
mass number
the number of protons plus neutrons in an atom
electron shell
a level around the nucleus where electrons are found
group
a column of the periodic table; elements in a group share outer electrons
period
a row of the periodic table; the period number is the number of shells
alkali metal
a group 1 metal, which reacts with water
halogen
a group 7 element

Exercise 7Z  Check-up

1
Which particle has a negative charge? Write the word.
2
What is the mass number of an atom? Write the words.
3
An atom has an atomic number of 19 and a mass number of 39. How many neutrons does it have? Write the number.
4
What is the electron arrangement of neutral calcium? Write the numbers in order.
5
How many outer shell electrons does neutral sodium have? Write the number.
6
Which group is chlorine in? Write the number.
7
Which period is potassium in? Write the number.
8
What is the name of the group 1 metal with the highest melting point in the table? Write the name.
9
Going down group 7, does reactivity increase or decrease? Write the word.
10
Explain why sodium and potassium react in a similar way. Write one sentence.

Total 14 marks. Allow about 14 minutes.

Exercise 7X  Practice test questions

1
FWhat is the relative charge of a proton? Write the number with its sign.[1 mark]
2
FName the part of an atom that contains protons and neutrons. Write the word.[1 mark]
3
FAn atom has a mass number of 23 and 11 protons. How many neutrons does it have? Write the number.[2 marks]
4
FGive the electron arrangement of a neutral chlorine atom. Write the numbers in order.[2 marks]
5
FWhich period is an element with 3 shells in? Write the number.[1 mark]
6
FName the group 1 metal that reacts most vigorously with water: lithium or potassium. Write the name.[1 mark]
7
FWhat is the name of the group 7 element that is a pale yellow gas? Write the name.[1 mark]
8
RExplain why calcium is in group 2. Write one sentence.[2 marks]
9
RExplain why a group 1 metal melts at a lower temperature going down the group. Write one sentence.[2 marks]
10
FWhich halogen displaces bromide from a solution: chlorine or iodine? Write the name.[1 mark]
← Chapter 6: The nervous system and reflexesChapter 8: Reactions of metals →

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