How is the Periodic Table organised, and why is it useful?
The development and modern organisation of the Periodic Table by atomic number into periods and groups, the position of metals and non-metals, and how the table predicts properties.
A CCEA GCSE Chemistry answer on the Periodic Table, covering how Mendeleev's table developed into the modern one, how elements are arranged by atomic number into periods and groups, where metals and non-metals sit, and how the layout lets us predict the properties of an element.
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What this dot point is asking
CCEA wants you to outline how the Periodic Table developed from Mendeleev's work to the modern table, describe how the modern table is organised by atomic number into periods and groups, locate metals and non-metals, and explain how the layout lets chemists predict an element's properties.
How the table developed
Mendeleev even reversed a few pairs against the strict mass order so that elements landed in the right group by their properties. The modern table resolves these problems by ordering elements by atomic number (number of protons) rather than atomic mass, which was only possible once the structure of the atom was understood.
Periods and groups
So the position of an element tells you about its electrons: the group number gives the number of outer-shell electrons, and the period number gives the number of occupied shells. This is why the table is more than a list, it is organised by electronic structure, which is what controls chemistry.
Metals and non-metals
The table is divided by a stepped line. To the left and centre are the metals: they are shiny, good conductors of heat and electricity, malleable, and form positive ions. To the right are the non-metals: they are mostly poor conductors, brittle if solid, and form negative ions or share electrons. A few elements near the dividing line (such as silicon) show in-between behaviour.
Examples in context
Example 1. Predicting a new element's chemistry. When element 117 (tennessine) was made, chemists could predict it would behave like a halogen because it sits in Group 7, below astatine. The table lets scientists anticipate the chemistry of even the newest synthetic elements from their position alone.
Example 2. Sorting elements for industry. Engineers use the metal and non-metal divide to choose materials: metals on the left for wiring and structures because they conduct and bend, non-metals on the right for insulators. The everyday choice of copper for cables and sulfur for chemicals traces straight back to where elements sit in the table.
Try this
Q1. State what all elements in the same group have in common. [1 mark]
- Cue. The same number of electrons in their outer shell.
Q2. Explain why Mendeleev left gaps in his table. [2 marks]
- Cue. To keep elements with similar properties together; the gaps were for undiscovered elements whose properties he predicted.
Exam-style practice questions
Practice questions written in the style of CCEA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
CCEA 20183 marksMendeleev left gaps in his Periodic Table. Explain why he did this and why his table is regarded as a success.Show worked answer →
Markers want the reason for the gaps and the evidence that they were justified.
Mendeleev arranged the known elements by atomic mass and grouped those with similar properties together. He left gaps where no known element fitted the pattern of properties, because forcing a known element into the gap would have broken the trend.
His table is regarded as a success because he predicted the properties of the missing elements (such as germanium) from the trends around the gaps. When those elements were later discovered, their properties matched his predictions closely, which confirmed that his arrangement reflected something real.
Markers reward the idea that gaps preserved the property pattern, and that the accurate prediction of undiscovered elements proved the table worked.
CCEA 20212 marksState how the modern Periodic Table is ordered, and how this differs from Mendeleev's original ordering.Show worked answer →
The marks are for both ordering rules.
The modern Periodic Table is arranged in order of increasing atomic number (number of protons).
Mendeleev arranged his table in order of increasing atomic mass. Ordering by atomic number rather than atomic mass corrects the few places where the mass order put elements in the wrong group (for example it places the elements correctly without the pair reversals Mendeleev had to make by hand).
Markers reward modern table by atomic number and Mendeleev by atomic mass.
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Sources & how we know this
- CCEA GCSE Chemistry specification (1110) — CCEA (2017)