How are electrons arranged in atoms, and how does this explain the Periodic Table?
Electron arrangement in shells for the first 20 elements, writing electron configurations, and the link between outer-shell electrons, the group number and chemical reactivity.
A CCEA GCSE Chemistry answer on electron arrangement, covering how electrons fill shells for the first 20 elements, how to write electron configurations, and how the number of outer-shell electrons links to the group, the period and the chemical reactivity of an element.
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What this dot point is asking
CCEA wants you to write the electron arrangement of the first 20 elements, using the rule that shells fill in order, and to use that arrangement to read off an element's group and period and to explain why elements in the same group behave alike.
How electrons fill shells
To write an electron arrangement, find the atomic number (which equals the number of electrons in a neutral atom), then fill the shells in order until they are used up. For chlorine, atomic number 17, the arrangement is 2,8,7: two in the first shell, eight in the second, and the remaining seven in the third.
Reading the Periodic Table from the configuration
The arrangement tells you exactly where an element sits.
- Group = number of electrons in the outer shell. Sodium (2,8,1) is in Group 1; oxygen (2,6) is in Group 6.
- Period = number of occupied shells. Sodium (2,8,1) has three shells, so it is in Period 3.
This works because the Periodic Table is built up by filling shells: each new period starts a new shell, and moving across a period adds one electron to the outer shell each time.
Outer electrons and reactivity
The outer-shell electrons are the ones involved in bonding, so they control chemistry. Atoms react to gain a full outer shell, either by losing, gaining or sharing electrons.
- Group 1 metals have one outer electron, which they lose easily to form a +1 ion.
- Group 7 non-metals have seven outer electrons, so they gain one to form a -1 ion.
- Elements in the same group have the same number of outer electrons, so they react in the same way and have similar properties.
Examples in context
Example 1. Predicting an unknown element. Given only the configuration 2,8,1, a chemist can predict that the element is a soft, very reactive metal that forms a +1 ion, stored under oil, simply because one outer electron places it in Group 1 with sodium and lithium. This shows how electron arrangement predicts behaviour before any experiment.
Example 2. Why neon lights are inert. Neon (2,8) has a full outer shell, so it does not react with the gases around it or the glass of the tube; it just glows when electrons are excited. Its stability, and therefore its safe use in lighting, follows directly from its full outer shell.
Try this
Q1. Write the electron arrangement of an atom with atomic number 16. [1 mark]
- Cue. 2,8,6.
Q2. State how you find the group of an element from its electron arrangement. [1 mark]
- Cue. The group equals the number of electrons in the outer shell.
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 20193 marksAn atom of an element has the electron arrangement 2,8,7. State its group, its period, and whether it is a metal or a non-metal.Show worked answer →
Markers want each answer tied to the configuration.
The number of electrons in the outer shell gives the group. Here the outer shell has 7 electrons, so the element is in Group 7 (the halogens).
The number of occupied shells gives the period. There are three shells (2,8,7), so it is in Period 3.
Elements on the right of the Periodic Table with nearly full outer shells are non-metals, so this element (chlorine) is a non-metal.
Markers reward Group 7 from 7 outer electrons, Period 3 from three shells, and non-metal.
CCEA 20212 marksExplain, in terms of electron arrangement, why sodium and lithium have similar chemical properties.Show worked answer →
The marks are for linking similar chemistry to the outer shell.
Sodium has the electron arrangement 2,8,1 and lithium has 2,1. Both have one electron in their outer shell.
Chemical properties are decided by the outer-shell electrons, because these are the electrons involved in bonding and reactions. Since sodium and lithium both have one outer electron, they react in the same way (both are Group 1 metals that lose one electron to form a +1 ion), so they have similar chemical properties.
Markers reward identifying that both have one outer-shell electron and that the outer electrons determine chemical behaviour.
Related dot points
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A CCEA GCSE Chemistry answer on atomic structure, covering the masses and charges of protons, neutrons and electrons, atomic number and mass number, how to read symbols, isotopes, and how to calculate relative atomic mass from isotopic abundances.
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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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A CCEA GCSE Chemistry answer on group trends, covering the properties and reactivity of the Group 1 alkali metals, the Group 7 halogens with their displacement reactions, and the unreactive Group 0 noble gases, and explaining each trend in terms of electron arrangement.
- Ionic bonding as the transfer of electrons to form charged ions, drawing dot-and-cross diagrams, the giant ionic lattice, and how the structure explains the properties of ionic compounds.
A CCEA GCSE Chemistry answer on ionic bonding, covering how electrons transfer from metals to non-metals to form ions, dot-and-cross diagrams, the giant ionic lattice, and how this structure explains the high melting points, conductivity and solubility of ionic compounds.
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A CCEA GCSE Chemistry answer on covalent bonding, covering how non-metal atoms share pairs of electrons to fill their outer shells, dot-and-cross diagrams for molecules such as hydrogen, water, ammonia and methane, and why simple molecular substances have low melting points and do not conduct.
Sources & how we know this
- CCEA GCSE Chemistry specification (1110) — CCEA (2017)