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KS3 Science · Year 7Chapter 6 · Atoms, elements and the periodic table
Year 7 · Chapter 6

Atoms, elements and the periodic table

By the end of this chapter you will be able to

  • Name the parts of an atom and their charges, and say why an atom has no overall charge
  • Say what an element is, and write the symbols of common elements
  • Use atomic number and mass number to find the protons, neutrons and electrons of an atom
  • Read the periodic table for the group and period of an element
  • Sort elements into metals and non-metals using their properties

You should already know

  • Materials and their properties, from Year 6 Science
  • Cells and particles (Chapters 2 and 4 of this book)

Where this leads

  • Compounds, which are made from elements chemically bonded together (Chapter 7)

National Curriculum for England, Key Stage 3 science, Year 7. Content: C4 atoms and elements: an element has only one type of atom; each element has a symbol; atoms are the building blocks of matter, C4 the periodic table: groups, periods, metals and non-metals (basic).

Exercise P  Do you remember?

1
FWhat do we call the change when a solid turns into a liquid? Write the word.
2
FHow many grams are in 1 kilogram? Write the number.
3
FWhat is 4 times 5? Write the number.
4
FName the process in which living things release energy from food. Write the word.
5
FIn the particle model, what state has particles that are far apart and move quickly in all directions? Write the word.
6.1

Inside the atom

Year 7
C4: atoms are the building blocks of matter; an atom has a nucleus of protons and neutrons, with electrons in shells around it
Video lesson: Inside the atomComing soon
Story. Sam is building a model of an atom out of craft beads. “The centre has the heavy part,” he says, “and the light parts go round the outside.” His teacher says, “You have it right. Let us name each part and its charge.”

Every atom has a small, dense nucleus at its centre. The nucleus contains protons, which have a positive charge, and neutrons, which have no charge.

Electrons have a negative charge. They move around the nucleus in shells. The first shell holds up to 2 electrons. The next shells hold more.

An atom has the same number of protons and electrons. The positive and negative charges cancel, so the atom has no overall charge. It is neutral.

Nasodium atom
A sodium atom: 11 electrons in shells of 2, 8 and 1.

The table gives the electrons in the shells of some atoms.

AtomElectrons in shells (inner to outer)
Na2, 8, 1
Mg2, 8, 2
Al2, 8, 3
S2, 8, 6
Cl2, 8, 7

Key ideas

  • The nucleus contains protons (positive) and neutrons (no charge).
  • Electrons (negative) are in shells around the nucleus.
  • A neutral atom has the same number of protons and electrons.
  • The first shell holds up to 2 electrons.
Worked example 6.1.1
A sodium atom has 11 electrons. How many protons does it have? Write the number.

Method. Use the rule for a neutral atom.

  1. Recall the rule. A neutral atom has the same number of protons and electrons.
    same number
  2. Apply it. So sodium has 11 protons.
    11
  3. Write the answer.
    11

Check. The charges cancel out, so the atom is neutral. ✓

Worked example 6.1.2
Which particle in the nucleus has no charge? Write the word.

Method. Recall the particles.

  1. Recall. Protons are positive and electrons are negative.
    positive and negative
  2. Decide. The particle with no charge, in the nucleus, is the neutron.
    neutron
  3. Write the answer.
    neutron

Check. Neutrons and protons are both in the nucleus. ✓

Worked example 6.1.3
How many electrons are in the outer shell of a sodium atom? Write the number.

Method. Read the shells of sodium.

  1. Read the shells. Sodium has shells of 2, 8 and 1 electrons.
    2, 8, 1
  2. Name the outer shell. The outer shell is the last one, with 1 electron.
    1
  3. Write the answer.
    1

Check. 2 + 8 + 1 = 11, the total number of electrons. ✓

Watch out

Watch out: electrons do not sit inside the nucleus. They are outside it, in shells. The nucleus contains protons and neutrons only.

Exam tip

Exam tip. For a charge question, give the charge: positive, negative or no charge.

Try it (with your teacher, or a grown-up at home). Make a simple model of an atom with a little ball for the nucleus, and paper rings for the shells. Put 11 counters on the rings for sodium: 2 on the first ring, 8 on the second, and 1 on the third. Check that the total is the same as the number of protons.

Exercise 6A  Inside the atom

1
FWhich two particles are found in the nucleus? Write the two words.
2
FWhat is the charge of an electron? Write one word: positive or negative.
3
FWhat is the charge of a proton? Write one word: positive or negative.
4
FWhat is the charge of a neutron? Write the words.
5
FDo electrons move around the nucleus in shells? Write yes or no.
6
FDoes a neutral atom have an overall charge? Write yes or no.
7
PSHow many electrons does a neutral sodium atom have? Write the number.
8
PSHow many electrons does the outer shell of potassium hold? Write the number. Potassium has shells of 2, 8, 8 and 1.
9
PSA helium atom has 2 electrons. How many protons does it have? Write the number.
10
PSAn atom has 6 protons. How many electrons does it have, if the atom is neutral? Write the number.
11
PSBen says an atom has no protons because it is neutral. Is Ben right? Write yes or no.
12
RExplain why an atom has no overall charge. Write one sentence.
13
RChallenge: explain why the nucleus is positively charged, even though a neutron has no charge. Write one sentence.
6.2

Elements and symbols

Year 7
C4: an element is a substance made of only one type of atom; each element has a chemical symbol
Video lesson: Elements and symbolsComing soon
Story. Priya finds a bottle labelled “Na”. “What does that mean?” she asks. Her teacher says, “It is the symbol for sodium. Every element has a symbol, which is a short way of writing its name.”

An element is a substance that is made of only one type of atom. Oxygen, carbon and sodium are all elements.

Each element has a chemical symbol of one or two letters. The first letter is always a capital, and the second letter, if there is one, is always lower case. For example, Na is sodium, and Cl is chlorine. The letters C and Cl are different elements.

The table gives the symbols and types of the elements in this chapter.

ElementSymbolAtomic numberMass numberType
heliumHe24non-metal
carbonC612non-metal
nitrogenN714non-metal
oxygenO816non-metal
sodiumNa1123metal
magnesiumMg1224metal
aluminiumAl1327metal
sulphurS1632non-metal
chlorineCl1735non-metal
potassiumK1939metal
calciumCa2040metal

Key ideas

  • An element is made of only one type of atom.
  • Symbols have one or two letters.
  • The first letter is a capital; the second letter, if there is one, is lower case.
  • Each element has a different symbol.
Worked example 6.2.1
What is the symbol for oxygen? Write the symbol.

Method. Read the table.

  1. Find oxygen. In the table, oxygen has the symbol O.
    O
  2. Check the form. It has one letter, which is a capital.
    capital
  3. Write the answer.
    O

Check. Oxygen is written with one capital letter. ✓

Worked example 6.2.2
What is the symbol for calcium? Write the symbol.

Method. Read the table.

  1. Find calcium. In the table, calcium has the symbol Ca.
    Ca
  2. Check the form. The first letter is a capital, and the second is lower case.
    capital then lower case
  3. Write the answer.
    Ca

Check. Ca is not the same as C, which is carbon. ✓

Worked example 6.2.3
Is the symbol Na for sodium or for nitrogen? Write the word.

Method. Read the table.

  1. Read the table. Na is the symbol for sodium.
    sodium
  2. Decide. Nitrogen is N, so Na is for sodium.
    sodium
  3. Write the answer.
    sodium

Check. Check each symbol against the table. ✓

Watch out

Watch out: the symbol has a capital first letter. Write Na, not NA or na. Also, C is carbon, and Cl is chlorine.

Exam tip

Exam tip. Learn the symbols with a capital first letter, and check the second letter is lower case.

Try it (with your teacher, or a grown-up at home). Look at a labelled periodic table, or the table in this section. Choose three elements from everyday life, such as iron or copper, and write down their symbols. Check the capital letter and the lower-case letter for each one.

Exercise 6B  Elements and symbols

1
FWhat is an element made of? Write the words.
2
FWhat is the symbol for sodium? Write the symbol.
3
FWhat is the symbol for oxygen? Write the symbol.
4
FWhat is the symbol for chlorine? Write the symbol.
5
FWhat is the symbol for magnesium? Write the symbol.
6
FWhich element has the symbol Ca? Write the name.
7
FWhat is the symbol for sulphur? Write the symbol.
8
FIs chlorine an element? Write yes or no.
9
PSHow many elements are listed in the table in this section? Write the number.
10
PSWhich element has the symbol K? Write the name.
11
PSAli writes NA for sodium. Is Ali correct? Write yes or no.
12
PSRuby writes N for nitrogen, and Na for sodium. Is Ruby correct? Write yes or no.
13
RExplain why the symbol for sodium is Na and not S. Write one sentence.
14
RChallenge: a pupil writes C for chlorine. Explain the mistake, and give the correct symbol. Write one sentence.
6.3

Atomic number and mass number

Year 7
C4: atoms of each element have a fixed number of protons (the atomic number); the mass number is protons plus neutrons
Video lesson: Atomic number and mass numberComing soon
Story. Ali looks at the periodic table and sees two numbers for each element. “One is 11 for sodium, and the other is 23,” he says. “What do they mean?” His teacher says, “The smaller number is the atomic number, and the bigger one is the mass number.”

The atomic number is the number of protons in an atom. Every atom of an element has the same atomic number, so the atomic number identifies the element.

The mass number is the number of protons plus neutrons. So the number of neutrons is the mass number minus the atomic number.

In a neutral atom, the number of electrons is the same as the number of protons.

neutrons = mass number - atomic number

The table gives the protons, neutrons and electrons of five atoms.

AtomProtonsNeutronsElectrons
Na111211
Mg121212
Al131413
S161616
Cl171817

Key ideas

  • Atomic number = number of protons.
  • Mass number = protons + neutrons.
  • Neutrons = mass number - atomic number.
  • In a neutral atom, electrons = protons.
Worked example 6.3.1
Chlorine has a mass number of 35 and an atomic number of 17. How many neutrons does it have? Write the number.

Method. Use neutrons = mass number - atomic number.

  1. Read the numbers. The mass number is 35 and the atomic number is 17.
    35 and 17
  2. Subtract. Neutrons = 35 minus 17.
    35 - 17 = 18
  3. Write the answer.
    18

Check. 17 + 18 = 35. ✓

Worked example 6.3.2
Oxygen has an atomic number of 8 and a mass number of 16. How many electrons does a neutral oxygen atom have? Write the number.

Method. Use electrons = protons in a neutral atom.

  1. Find the protons. The atomic number is the number of protons, which is 8.
    8 protons
  2. Use the rule. A neutral atom has as many electrons as protons, so it has 8 electrons.
    8
  3. Write the answer.
    8

Check. 8 protons and 8 electrons make a neutral atom. ✓

Worked example 6.3.3
An atom has 12 protons and 12 neutrons. What is its mass number? Write the number.

Method. Add the protons and neutrons.

  1. Add. Mass number = protons + neutrons = 12 + 12.
    12 + 12
  2. Work it out. The mass number is 24.
    24
  3. Write the answer.
    24

Check. 24 - 12 = 12 neutrons. ✓

Watch out

Watch out: the atomic number is the number of protons, not the mass number. Subtract the atomic number from the mass number to find the neutrons.

Exam tip

Exam tip. Write the two numbers, then say which one is the atomic number and which is the mass number.

Try it (with your teacher, or a grown-up at home). Look at a periodic table with atomic numbers. Choose sodium and magnesium. For each one, write the atomic number, the mass number and the number of neutrons. Check that the number of neutrons is the mass number minus the atomic number.

Exercise 6C  Atomic number and mass number

1
FWhat is the atomic number of sodium? Write the number.
2
FWhat does the atomic number tell us? Write the word: protons, neutrons or electrons.
3
FWhat is the mass number of oxygen? Write the number.
4
FHow many protons does a chlorine atom have? Write the number.
5
FHow many neutrons does an oxygen atom have? Write the number.
6
FHow many neutrons does a sodium atom have? Write the number.
7
FHow many electrons does a neutral magnesium atom have? Write the number.
8
PSHow many neutrons does a chlorine atom have? Write the number.
9
PSHow many neutrons does a calcium atom have, if its mass number is 40 and its atomic number is 20? Write the number.
10
PSAn atom has a mass number of 24 and 12 neutrons. What is its atomic number? Write the number.
11
PSTwo atoms have the same atomic number. Are they the same element? Write yes or no.
12
RExplain why two atoms of the same element can have different mass numbers. Write one sentence.
13
RChallenge: an atom has a mass number of 23 and 12 neutrons. Which element is it, and what is its atomic number? Write the element name.
6.4

The periodic table: groups and periods

Year 7
C4: the periodic table has columns called groups and rows called periods; the group and period of an element can be read from it
Video lesson: The periodic table: groups and periodsComing soon
Story. Ruby is looking at the periodic table on the classroom wall. “Why are the elements in columns?” she asks. Her teacher says, “Elements in the same column are in the same group, and they often behave in similar ways.”

The periodic table arranges the elements in order of their atomic number. The vertical columns are called groups. The horizontal rows are called periods.

Elements in the same group have similar chemical properties, because they have the same number of electrons in their outer shell (for groups 1 to 7). The group 0 elements are the noble gases, which have full outer shells.

123456701He2CNO3NaMgAlSCl4KCablue = metal, orange = non-metal; columns are groups, rows are periods
A simplified periodic table: groups are the columns, periods are the rows.

Key ideas

  • Groups are the vertical columns, and periods are the horizontal rows.
  • Group number = number of outer-shell electrons, for groups 1 to 7.
  • Group 0 holds the noble gases, such as helium.
  • The period number = the number of shells the atom has.
Worked example 6.4.1
In which group and period is sodium? Write the group number, then the period number.

Method. Find sodium on the grid.

  1. Find the column. Sodium is in the first column, so it is in group 1.
    group 1
  2. Find the row. Sodium is in the third row, so it is in period 3.
    period 3
  3. Write the answer.
    group 1, period 3

Check. Sodium has 3 shells, which matches period 3. ✓

Worked example 6.4.2
Which element is in group 7 and period 3? Write the symbol.

Method. Find group 7 in row 3.

  1. Find the column. Group 7 is the seventh column from the left.
    group 7
  2. Find the row. In period 3, the element in group 7 is chlorine.
    chlorine
  3. Write the symbol. The symbol for chlorine is Cl.
    Cl
  4. Write the answer.
    Cl

Check. Chlorine has 7 electrons in its outer shell. ✓

Worked example 6.4.3
Helium is in group 0. What does the group 0 number tell us about its outer shell? Write the word: full or empty.

Method. Think about the noble gases.

  1. Think about the noble gases. Group 0 elements have full outer shells.
    full outer shells
  2. Decide. So the answer is full.
    full
  3. Write the answer.
    full

Check. Full outer shells make these gases unreactive. ✓

Watch out

Watch out: count the group column from the left, and count the period row from the top. Helium is in period 1, not period 2.

Exam tip

Exam tip. Give the group and the period in that order: “group 1, period 3”.

Try it (with your teacher, or a grown-up at home). Look at a periodic table on the classroom wall, or the grid in this section. Find sodium and potassium. Say which group they are in, and which period. Explain why they are in the same column.

Exercise 6D  The periodic table: 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 is sodium in? Write the number.
4
FWhich group is magnesium in? Write the number.
5
FWhich group is chlorine in? Write the number.
6
FWhich period is sodium in? Write the number.
7
FWhich period is helium in? Write the number.
8
FWhich period is potassium in? Write the number.
9
PSWhich element is in group 6 and period 3? Write the name.
10
PSWhich element is in group 4 and period 2? Write the symbol.
11
PSSodium and potassium are both in group 1. Are they in the same column? Write yes or no.
12
PSWorking scientifically: a pupil says helium is in group 2 because it has two electrons. Is the pupil right? Write yes or no.
13
RExplain why sodium and potassium behave in a similar way. Write one sentence.
14
RChallenge: explain why the group number tells you the number of outer electrons for groups 1 to 7. Write one sentence.
6.5

Metals and non-metals

Year 7
C4: elements are sorted into metals and non-metals; metals are on the left and in the middle of the periodic table, non-metals on the right
Video lesson: Metals and non-metalsComing soon
Story. Omar has a box of samples. “Is this one a metal?” he asks, holding up a shiny piece. His teacher says, “Test it. Metals usually conduct electricity and heat, and they can be hammered into shape. Compare it with a non-metal sample.”

Most elements are metals. Metals are found on the left and in the middle of the periodic table. Non-metals are found on the right.

Metals usually conduct electricity and heat, they are shiny when freshly cut, and they are malleable, which means they can be hammered into shape. Solid non-metals are usually brittle, which means they break rather than bend.

The table compares the properties of metals and non-metals.

PropertyMetalsNon-metals
conducts electricitymetal: yes (most)non-metal: no (except graphite, which is carbon)
conducts heatmetal: yesnon-metal: no
shiny when freshly cutmetal: yesnon-metal: usually no
malleable (can be hammered into shape)metal: yesnon-metal: no, solid non-metals are brittle

Key ideas

  • Metals are on the left and in the middle of the periodic table.
  • Metals usually conduct electricity and heat, and are malleable.
  • Non-metals are usually poor conductors and brittle when solid.
  • Carbon, as graphite, is a non-metal that conducts electricity, so there are exceptions.
Worked example 6.5.1
Is sodium a metal or a non-metal? Write the word.

Method. Use the periodic table.

  1. Find sodium. Sodium is in group 1, on the left of the table.
    left
  2. Decide. Elements on the left are metals, so sodium is a metal.
    metal
  3. Write the answer.
    metal

Check. Sodium is in the metals in the table. ✓

Worked example 6.5.2
Is oxygen a metal? Write yes or no.

Method. Use the periodic table.

  1. Find oxygen. Oxygen is on the right side of the table.
    right
  2. Decide. So oxygen is a non-metal, and the answer is no.
    no
  3. Write the answer.
    no

Check. Oxygen is a non-metal. ✓

Worked example 6.5.3
Which property is shown by a metal and not usually by a non-metal: shiny when freshly cut, or brittle? Write the words.

Method. Compare the two properties.

  1. Compare. Metals are shiny when freshly cut.
    shiny
  2. Decide. Non-metals are usually brittle, not shiny.
    shiny when freshly cut
  3. Write the answer.
    shiny when freshly cut

Check. Brittle is a property of solid non-metals. ✓

Watch out

Watch out: a property can have exceptions. Graphite, a form of carbon, is a non-metal that conducts electricity. Use several properties, not one, to decide.

Exam tip

Exam tip. Name the position in the table, and at least one property, when you say whether an element is a metal.

Try it (with your teacher, or a grown-up at home). With your teacher, look at a sample of a metal, such as a piece of copper wire, and a non-metal, such as a piece of sulphur or a piece of carbon in pencil lead. Write down one property of each. Do not heat or burn any sample.

Exercise 6E  Metals and non-metals

1
FAre most elements metals or non-metals? Write the word.
2
FWhere are the metals found in the periodic table? Write the word: left or right.
3
FIs sodium a metal? Write yes or no.
4
FIs chlorine a metal? Write yes or no.
5
FDo metals usually conduct electricity? Write yes or no.
6
FAre solid non-metals usually brittle? Write yes or no.
7
FWhat is the word for a material that can be hammered into shape? Write the word.
8
PSHow many metals are in the table in section 6.5? Write the number, using the elements of this chapter.
9
PSHow many non-metals are in the table in section 6.2? Write the number, using the elements of this chapter.
10
PSWhich element is malleable: sulphur, or aluminium? Write the word.
11
PSAli says all non-metals are poor conductors of electricity. Is Ali right? Write yes or no.
12
RExplain why a metal is a better choice than a non-metal for a cooking pan. Write one sentence.
13
RChallenge: explain why colour alone is not a reliable way to tell a metal from a non-metal. Write one sentence.

Chapter summary

SkillMethod
AtomNucleus of protons (positive) and neutrons (no charge); electrons (negative) in shells.
ElementOnly one type of atom; each has a symbol, capital first letter.
NumbersAtomic number = protons. Mass number = protons + neutrons. Neutral: electrons = protons.
Periodic tableGroups are columns; periods are rows. Group 0 holds the noble gases.
Metals and non-metalsMetals are on the left and in the middle; they conduct and are malleable. Non-metals are on the right.

Can you…?

  • I can name the particles in an atom and their charges, and explain why an atom is neutral
  • I can say what an element is, and write the symbols of common elements
  • I can use atomic number and mass number to find the protons, neutrons and electrons
  • I can read the group and period of an element from the periodic table
  • I can sort elements into metals and non-metals, using their properties

Key terms

atom
the smallest part of an element that keeps its properties
nucleus
the centre of an atom, containing protons and neutrons
proton
a particle in the nucleus with a positive charge
neutron
a particle in the nucleus with no charge
electron
a particle with a negative charge, found in shells around the nucleus
element
a substance made of only one type of atom
symbol
a short form of an element's name, with one or two letters
atomic number
the number of protons in an atom
mass number
the number of protons plus neutrons in an atom
periodic table
the table of elements arranged in order of atomic number
group
a vertical column of the periodic table
period
a horizontal row of the periodic table
malleable
can be hammered into shape without breaking

Exercise 6Z  Check-up

1
Which two particles are found in the nucleus? Write the two words.
2
What is the charge of an electron? Write one word.
3
What is an element? Write the words.
4
What is the symbol for calcium? Write the symbol.
5
What is the atomic number of chlorine? Write the number.
6
An atom has a mass number of 23 and an atomic number of 11. How many neutrons does it have? Write the number.
7
Which group is oxygen in? Write the number.
8
Which period is sodium in? Write the number.
9
Is magnesium a metal or a non-metal? Write the word.
10
Explain why sodium and potassium are in the same group. Write one sentence.

Total 17 marks. Allow about 17 minutes.

Exercise 6X  Practice test questions

1
FWhat is the charge of a proton? Write one word.[1 mark]
2
RExplain why a neutral atom has no overall charge. Write one sentence.[2 marks]
3
FWhat is the symbol for oxygen? Write the symbol.[1 mark]
4
PSAn atom has an atomic number of 16 and a mass number of 32. How many neutrons does it have? Write the number.[2 marks]
5
FWhich group is chlorine in? Write the number.[1 mark]
6
FWrite the group and period of sodium, in that order. Write the two numbers.[2 marks]
7
FIs calcium a metal or a non-metal? Write the word.[1 mark]
8
RExplain why helium is a non-metal. Write one sentence.[2 marks]
9
PSAn atom has 12 protons and 12 neutrons. What is its mass number? Write the number.[1 mark]
10
PSWrite the symbol for the element with 11 protons. Write the symbol.[2 marks]
11
PSBen says a copper wire is a non-metal because it is shiny. Is Ben right? Write yes or no.[2 marks]
← Chapter 5: Diffusion and densityChapter 7: Compounds →

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