AQA GCSE Combined Science (8464), Higher tier · Chemistry › Atomic structure and the periodic table › The periodic table
Practise The periodic table. 11 exam-style questions on this subtopic, at up to four difficulty levels, with full mark schemes and a progress tracker. Free, no account needed.
How the modern periodic table is arranged (by atomic number, in groups and periods), and why elements in the same group have similar chemical properties. You also need to predict an element's reactions from its position.
Grade by grade
What you need to be able to do, from the first marks up to the top grade.
3
Identify groups and periodsGroups are the vertical columns; periods are the horizontal rows.
4
State how the elements are orderedIn order of atomic (proton) number.
5
Explain why group members react similarlyThey have the same number of electrons in their outer shell.
5
Link position to electronic structureGroup number = outer electrons (Groups 1 to 7); period = number of occupied shells.
6
Explain why it is called 'periodic'Similar properties occur at regular intervals as the atomic number increases.
7
Predict reactions from an element's positionUse the group's typical reactions and trends, e.g. caesium reacts with water like sodium, but more vigorously.
Notes
How the table is arranged
The elements are arranged in order of atomic (proton) number.
Elements with similar properties are in the same vertical column, called a group.
The horizontal rows are called periods.
It is called the periodic table because similar properties occur at regular intervals as the atomic number increases.
Groups and electrons
Elements in the same group have the same number of electrons in their outer shell.
Chemical reactions depend on the outer electrons, so elements in the same group have similar chemical properties.
For Groups 1 to 7, the group number = the number of outer electrons. Group 0 elements have a full outer shell.
The period number = the number of occupied shells.
So an element's position depends on the arrangement of its electrons, which depends on its atomic number.
Making predictions
Use the group to predict the type of reaction: rubidium is in Group 1, so, like sodium, it reacts with water to form an alkaline hydroxide and hydrogen.
Use the trend down the group to predict reactivity: Group 1 elements get more reactive down the group; Group 7 elements get less reactive.
Metals are on the left and towards the bottom of the table; non-metals are on the right and towards the top.
Cheatsheet
Order: increasing atomic (proton) number
Group: vertical column; same number of outer electrons; similar chemical properties
Period: horizontal row; same number of occupied shells
Group number = number of outer electrons (Groups 1 to 7)
Group 0: full outer shell
'Periodic': similar properties repeat at regular intervals
How to answer each type of question
Recall how the periodic table is arranged
1 mark each4
Ordered by atomic number (proton number), not by mass.
Columns are groups; rows are periods.
Example. (a) Complete the sentence: the elements in the modern periodic table are arranged in order of ...... (b) What name is given to a vertical column of the periodic table?
Show the model answer
(a) atomic number / proton number (1) (not 'atomic mass') (b) group (1)
Explain why elements in a group have similar properties
2 marks5
State the number of outer electrons they all have.
Say that chemical properties depend on the outer electrons.
Example. Lithium, sodium and potassium react with water in similar ways. Explain why, in terms of electrons.
Show the model answer
They all have one electron in their outer shell (1). The reactions of an element depend on its outer electrons, so they react in similar ways (1).
Explain an element's position from its atomic number
3 marks6
Use the atomic number to write the electronic structure.
Outer electrons give the group.
Number of occupied shells gives the period.
Example. Magnesium has atomic number 12. Explain why magnesium is in Group 2 and period 3 of the periodic table.
Show the model answer
Magnesium has 12 electrons, arranged 2,8,2 (1). It has 2 electrons in its outer shell, so it is in Group 2 (1). It has 3 occupied shells, so it is in period 3 (1).
Predict the properties of an element from its position
2 to 4 marks7
Find the group and recall the trend down it.
Compare with the element next to it that you know about.
Give the prediction, then the reason (the trend or similarity).
Example. Astatine is below iodine in Group 7. (a) Predict the state of astatine at room temperature. Give a reason. (b) Predict whether astatine would displace iodine from potassium iodide solution. Give a reason.
Show the model answer
(a) Solid (1). Melting and boiling points increase down Group 7, and iodine is already a solid (1). (b) No (1). Reactivity decreases down Group 7, so astatine is less reactive than iodine (1).
Shortcuts and memory tricks
Periods run across, like lines of writing on a page; groups run down, like columns in a newspaper.
Group = number of outer electrons; period = number of shells.
Same group → same number of outer electrons → same kind of reactions.
Where marks are lost
Saying the modern table is arranged by atomic mass or atomic weight. That was how the early tables were arranged.
Saying elements in a group have the same number of electrons. They have the same number of outer electrons.
Mixing up groups (columns) and periods (rows).
Saying elements in a group have the same properties. Their properties are similar and change gradually down the group.
Exam technique
In 'explain' questions, the key phrase is 'the same number of electrons in the outer shell'.
When predicting, give the prediction and the pattern it is based on.
Use the periodic table on the data sheet: it gives the atomic number and relative atomic mass of every element.
Quick recall
Cover the answers and test yourself. The app has these as flashcards that come back just before you'd forget them.
The formula of hydrogen sulfide is H2S. Predict the formula of hydrogen selenide.
H2Se
Give the number of occupied energy levels (shells) and the number of outer-shell electrons in a calcium atom.
4 occupied shells; 2 outer-shell electrons
Sample questions
Written for this site in the style of AQA exam questions. They are not taken from real past papers.
Question 1Easy4 marks
The modern periodic table contains more than 100 elements.
(a) Complete the sentence. Tick (✓) one box. The elements in the periodic table are arranged in order of ................[1]
atomic number
mass number
number of neutrons
relative atomic mass
(b) What name is given to a vertical column of elements in the periodic table?[1]
(c) Why do elements in the same vertical column of the periodic table have similar chemical properties?[1]
(d) Why is the table called a periodic table?[1]
Show the answer and mark scheme
(a)Answer: atomic number
(b)Answer: a group
group
(c)Answer: Their atoms have the same number of electrons in the outer shell.
(atoms have the) same number of electrons in the outer shell / energy level
(d)Answer: Similar properties occur at regular intervals.
similar properties occur at regular intervals
Question 2Medium4 marks
Calcium is in Period 4 and Group 2 of the periodic table.
(a) Give the number of occupied energy levels (shells) and the number of outer-shell electrons in a calcium atom.[2]
(b) Use this information to give the electronic structure of a calcium atom.[1]
The position of an element in the periodic table is related to the arrangement of electrons in its atoms.
(a) Silicon has the atomic number 14. Explain how the position of silicon in the periodic table is related to the arrangement of electrons in its atoms.[3]
(b) Selenium, Se, is in Group 6, directly below sulfur. Sulfur burns in oxygen to form sulfur dioxide, SO2. Predict the formula of the compound formed when selenium burns in oxygen.[1]
(c) The formula of hydrogen sulfide is H2S. Predict the formula of hydrogen selenide.[1]
(d) Gallium is in Group 3 and Period 4. Give the number of electrons in the outer shell of a gallium atom and the number of occupied energy levels.[2]
Show the answer and mark scheme
(a)Answer: Silicon is 2,8,4: three occupied energy levels so Period 3; four outer electrons so Group 4.
electronic structure is 2,8,4
three occupied energy levels / shells so it is in Period 3
four electrons in the outer shell so it is in Group 4
(b)Answer: SeO2
SeO2
(c)Answer: H2Se
H2Se
(d)Answer: 3 outer electrons; 4 occupied energy levels