How are electrons arranged in atoms, and how does this link to the periodic table?
The arrangement of electrons in shells (energy levels), writing electronic configurations for the first 20 elements, and the link between the number of outer-shell electrons and an element's group and chemical behaviour.
A focused answer to OCR Gateway GCSE Chemistry A topic C1.3, covering how electrons fill shells (energy levels), writing electronic configurations for the first 20 elements, and how the number of outer electrons determines an element's group and reactivity.
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What this topic is asking
OCR wants you to describe how electrons are arranged in shells (energy levels) around the nucleus, write the electronic configuration of the first 20 elements, and explain the crucial link between the number of outer-shell electrons and an element's group and chemical behaviour. This connects the structure of the atom directly to the periodic table and to bonding.
How electrons fill shells
The maximum number of electrons in each shell at GCSE is:
- First shell (lowest energy, nearest the nucleus): up to 2 electrons.
- Second shell: up to 8 electrons.
- Third shell: up to 8 electrons (for the first 20 elements).
So an atom with electrons fills the first shell with , the second with , leaving in the third shell, giving the configuration .
Writing electronic configurations
To write the configuration for any of the first 20 elements:
- Find the atomic number, which equals the number of electrons in a neutral atom.
- Fill the shells from the inside out, respecting the maximum of each shell.
- Write the numbers in order, separated by commas or dots (for example ).
Linking configuration to the periodic table
The electronic configuration explains the structure of the periodic table:
- The number of the outer (highest) shell that contains electrons equals the period (row) the element is in.
- The number of electrons in the outer shell equals the group number (for the main groups). For example, an element with the configuration has outer electrons and is in Group 7.
This is why elements in the same group have similar chemical properties: they have the same number of outer electrons, and it is the outer electrons that take part in reactions.
Why the outer shell controls reactivity
For example, sodium () loses its single outer electron to reach (the same as neon), forming . Chlorine () gains one electron to reach (the same as argon), forming . This is the basis of ionic bonding.
The periodic table
The modern periodic table arranges elements in order of increasing atomic number (proton number) so that elements with similar properties fall in the same vertical group. The electronic configuration is the reason this works: elements line up in groups because they share the same number of outer electrons.
Exam-style practice questions
Practice questions written in the style of OCR exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
OCR 20183 marksAn atom of an element has 17 electrons. Write its electronic configuration, state which group of the periodic table it is in, and explain how you know from its configuration.Show worked answer →
A C1.3 structured question. Reward: the electronic configuration is filled shell by shell, (2 in the first shell, 8 in the second, 7 in the third), which uses all 17 electrons. The element is in Group 7, because the number of electrons in the outer shell (7) equals the group number. Markers credit the correct configuration of 2.8.7, the answer Group 7, and the reasoning that the outer-shell electron count gives the group number. (The element is chlorine.) A common error is putting more than 8 in the second shell.
OCR 20224 marksSodium has the electronic configuration 2.8.1. Explain why sodium is very reactive and why it is in Group 1, and predict the charge on a sodium ion.Show worked answer →
A Higher tier question linking configuration to reactivity. Reward: sodium is in Group 1 because it has 1 electron in its outer shell (the group number equals the number of outer electrons). It is very reactive because it has only 1 outer electron, which it readily loses to gain a full, stable outer shell like a noble gas; losing one electron is easy. When sodium loses this one electron it forms an ion with a charge of (written ), because it now has one more proton than electrons. Markers credit the link between 1 outer electron and Group 1, the explanation that losing the outer electron gives a stable full shell, and the predicted charge of 1+.
Related dot points
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