The periodic table — AQA GCSE Chemistry
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The periodic table explained
The periodic table covers 4 specification points, including The elements in the periodic table are… and The table is called a periodic table because….
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The elements in the periodic table are arranged in order of atomic (proton) number and so that elements with similar properties are in columns, known as groups.
The periodic table orders elements by increasing atomic (proton) number, which is the number of protons in the nucleus and equals the number of electrons in a neutral atom. Reading left to right across a period, one proton and one electron are added each time, so nuclear charge and outer-shell occupancy change gradually. Reading down a group, atoms gain filled inner shells but keep the same outer-shell electron count, so chemical behaviour repeats. For example, sodium (atomic number 11) sits in Group 1 with lithium (3) and potassium (19) because each has one outer electron, while chlorine (17) sits in Group 7 with fluorine (9) because each has seven. Columns are called groups; rows are periods.
The table is called a periodic table because similar properties occur at regular intervals.
The word periodic means happening at regular intervals. In the periodic table, chemical and physical properties repeat as you move through the elements in order of atomic number. After a set number of elements, a new period begins and the pattern of outer-shell electron arrangements repeats, so elements with one outer electron appear again, then those with two, and so on. For example, lithium (3), sodium (11) and potassium (19) all react vigorously with water and form 1⁺ ions, and they are separated by eight elements in the first two cases. This repeating pattern is why Mendeleev's arrangement could predict unknown elements and why the modern table is described as periodic rather than merely sequential.
Elements in the same group in the periodic table have the same number of electrons in their outer shell (outer electrons) and this gives them similar chemical properties.
Chemical behaviour is controlled mainly by the outer-shell electrons, because these are the electrons involved in bonding and reactions. Elements in the same group have equal numbers of outer electrons, so they form similar ions and show similar reactivity trends. Group 1 elements each have one outer electron, lose it to form 1⁺ ions and react vigorously with water. Group 7 elements each have seven outer electrons, gain one to form 1⁻ ions and react with metals to form salts. Group 0 elements have full outer shells, so they are unreactive. The total number of electrons differs within a group, but the outer count stays the same.
Students should be able to: explain how the position of an element in the periodic table is related to the arrangement of electrons in its atoms and hence to its atomic number predict possible reactions and probable reactivity of elements from their positions in the periodic table. AQA GCSE Chemistry 8462. GCSE exams Jun
An element's position follows directly from its atomic number and electron arrangement. The atomic number gives the number of protons and, in a neutral atom, the number of electrons. Electrons fill shells in a regular way, so the outer-shell count determines the group and the number of occupied shells determines the period. For example, magnesium has atomic number 12, with two outer electrons in its third shell, placing it in Group 2 and Period 3. This position lets you predict behaviour: Group 1 metals lose one electron and react vigorously with water, Group 7 non-metals gain one electron and displace less reactive halogens, and Group 0 elements with full outer shells are unreactive.
Your focus
- Describe the periodic table as arranged in order of increasing atomic (proton) number.
- Identify groups as columns containing elements with similar properties.
- Relate an element's group to its number of outer-shell electrons using a named example.
Show all 12 objectives
- Define the term periodic in the context of the periodic table.
- Describe how similar properties recur at regular intervals.
- Illustrate periodicity with a named group of elements.
- State the number of outer-shell electrons for a given main-group element.
- Explain how outer-shell electrons lead to similar chemical properties within a group.
- Relate outer-electron count to the charge on the ion formed.
- Deduce an element's group and period from its atomic number and electron arrangement.
- Explain how electron arrangement determines position in the periodic table.
- Predict probable reactions and reactivity trends from group position using named examples.
The periodic table exam tips
Marking Points
- State that elements are placed in order of increasing atomic (proton) number, not relative atomic mass.
- Identify that the atomic number equals the number of protons and, in a neutral atom, the number of electrons.
- Explain that vertical columns are called groups and that elements within a group show similar chemical properties.
- Link similar properties to the same number of outer-shell electrons rather than to equal total electron numbers.
- Use a named example, such as lithium, sodium and potassium in Group 1, to show the pattern.
- Recognise that horizontal rows are periods and that across a period the outer-shell electron count increases by one per element.
- Define periodic as recurring at regular intervals.
- Explain that similar properties reappear after a fixed number of elements in atomic number order.
- Link the repetition to repeating patterns of outer-shell electron arrangement.
- Use a group example, such as Group 1 metals, to show properties recurring.
- Contrast the periodic pattern with a simple list that shows no repetition.
- State that elements in the same group have the same number of outer-shell electrons.
- Explain that outer-shell electrons determine bonding and chemical reactions.
- Link a group's outer-electron count to the charge on the ion it forms, such as one outer electron giving a 1⁺ ion.
- Use a named group to show similar properties, for example Group 1 metals reacting with water.
- Recognise that total electron number changes down a group while outer-electron number does not.
- Link atomic number to proton number and, for a neutral atom, electron number.
- Explain that the number of outer-shell electrons determines the group.
- Explain that the number of occupied electron shells determines the period.
- Use a named element to show the link between electron arrangement and position.
- Predict reactions or reactivity from group position, such as Group 1 reacting with water or Group 7 displacement.
- Recognise that reactivity trends within a group depend on distance of the outer electron from the nucleus and shielding.
Examiner Tips
- 💡When asked why elements are ordered as they are, name atomic (proton) number explicitly and define it as proton count.
- 💡Support any statement about similar properties with a group example and the shared outer-electron count.
- 💡Check whether a question asks about position, order or properties, and answer each part separately to avoid losing credit.
- 💡Define the key term periodic in the first sentence of your answer.
- 💡Give one concrete group example to show the repeat, naming the elements and their shared behaviour.
- 💡If asked why the table is called periodic, mention both regular intervals and repeating properties.
- 💡Always write outer-shell electrons, not just electrons, when explaining group similarity.
- 💡Name the group and its outer-electron count before describing the shared reaction.
- 💡Use ion charges with correct superscript signs, for example Na⁺ and Cl⁻, to show the link between outer electrons and bonding.
- 💡Write the electron configuration first, then read group from outer electrons and period from shell count.
- 💡When predicting a reaction, name the group, state the outer-electron behaviour and give the expected product or observation.
- 💡Use correct ion notation with superscript signs, for example Mg²⁺ and O²⁻, when explaining electron loss or gain.
Common Mistakes
- Saying elements are arranged in order of relative atomic mass; correct this by stating the order is increasing atomic (proton) number, which resolves anomalies such as argon before potassium.
- Confusing groups with periods; correct this by defining groups as vertical columns with similar properties and periods as horizontal rows.
- Claiming all atoms in a group have identical electron arrangements; correct this by saying they share the same number of outer-shell electrons while inner shells differ.
- Thinking periodic means the table is arranged in rows; correct this by explaining that periodic refers to repeating property patterns.
- Assuming every element has unique properties with no repetition; correct this by citing a group where similar reactions recur.
- Confusing the interval with a fixed mass difference; correct this by linking the repeat to atomic number and outer-electron patterns.
- Saying group members have the same total number of electrons; correct this by specifying the same number of outer-shell electrons only.
- Believing reactivity is identical within a group; correct this by noting similar properties but trends in reactivity, such as Group 1 becoming more reactive down the group.
- Ignoring the full outer shell of Group 0; correct this by explaining that a full outer shell makes noble gases unreactive.
- Using mass number instead of atomic number to find position; correct this by using proton number, which is the atomic number.
- Placing an element in the wrong period by counting outer electrons rather than occupied shells; correct this by counting the shells that contain electrons.
- Predicting identical reactivity for all group members; correct this by describing a trend, such as increasing reactivity down Group 1.