How is the periodic table arranged, and how was it developed?
The periodic table; arrangement by atomic number into groups and periods; how Mendeleev arranged the early table; metals and non-metals; the development of the table once protons were discovered.
A focused answer to AQA GCSE Chemistry 4.1.2, covering how the periodic table is arranged by atomic number into groups and periods, how Mendeleev built the early table and left gaps, and how the modern table is organised around electronic structure.
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What this dot point is asking
AQA wants you to describe how the modern periodic table is arranged in order of atomic number into groups and periods, explain why elements in a group have similar properties, and describe how Mendeleev built the early table, left gaps and predicted undiscovered elements, and how the table changed once protons and electronic structure were understood. The historical development is examined as an example of how evidence improves a scientific model.
Arrangement of the modern table
Elements are placed in order of increasing atomic number (number of protons). The columns are groups and the rows are periods.
Going down a group, atoms gain extra occupied shells (so they get larger); going across a period, the number of outer electrons increases by one each time until the shell is full and a new period begins.
Mendeleev's table
In 1869 Mendeleev arranged the known elements mainly in order of atomic mass.
When the predicted elements (such as germanium, which he called eka-silicon) were later discovered and matched his predictions closely, his table gained acceptance, because successful predictions are strong evidence for a model.
How the table developed
- Early chemists tried to arrange elements by atomic mass alone, but this sometimes put elements with different properties together.
- Mendeleev's gaps and swaps solved much of this problem and let him predict undiscovered elements.
- The discovery of protons allowed elements to be ordered by atomic number, which removed the need for swaps and explained why some heavier elements come before lighter ones.
- Knowledge of electronic structure then explained why groups have similar properties: it is the outer electrons that control reactions.
Metals and non-metals
Most elements are metals, found on the left and centre of the table. Non-metals are on the right. The dividing line runs roughly diagonally down the right side, so an element's position immediately suggests whether it is a metal or a non-metal.
Try this
Q1. State the property that all elements in the same group share. [1 mark]
- Cue. The same number of electrons in their outer shell.
Q2. Give one reason Mendeleev left gaps in his early periodic table. [1 mark]
- Cue. For elements that had not yet been discovered, so similar elements stayed aligned.
Q3. State what property the modern periodic table is ordered by. [1 mark]
- Cue. Atomic number (the number of protons).
Exam-style practice questions
Practice questions written in the style of AQA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
AQA 20194 marksMendeleev arranged the elements in his 1869 periodic table. Explain how he ordered the elements, why he left gaps, and why he sometimes placed a heavier element before a lighter one. State how the modern table differs in its ordering.Show worked answer →
A 4-mark question on the development of the periodic table.
Ordering (1 mark): Mendeleev arranged the known elements mainly in order of increasing atomic mass. Gaps (1 mark): he left gaps for elements he believed had not yet been discovered, and predicted their properties so similar elements stayed aligned in groups. Swaps (1 mark): he sometimes placed a heavier element before a lighter one so that elements with similar chemical properties lined up in the same group. Modern table (1 mark): elements are now ordered by atomic number (number of protons), which explains his swaps.
Markers reward all four ideas, especially the link between the swaps and atomic number.
AQA 20213 marksExplain why all the elements in Group 1 of the periodic table have similar chemical properties, and describe how the number of occupied electron shells changes as you go down a group.Show worked answer →
A 3-mark question linking position to electronic structure.
Similar properties (2 marks): all Group 1 elements have the same number of electrons in their outer shell (one), and it is the outer electrons that determine how an element reacts, so they react in similar ways. Down a group (1 mark): each element down the group has one more occupied electron shell than the one above it, so the atoms get larger.
Markers want the same-outer-electrons reason and the increasing-number-of-shells observation.
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Sources & how we know this
- AQA GCSE Chemistry (8462) specification — AQA (2016)