Edexcel A-level Chemistry Electronic structure
This page covers the following topics:
The electronic structure is the way in which electrons are arranged in an atom. Electrons in atom are arranged in shells (or energy levels) surrounding the nucleus. For many atoms, the electrons in an atom occupy the shell nearest the nucleus first, until it is full.
The first shell can contain 2 electrons. For elements up to and including calcium, the second and third shells can contain 8 electrons. Their electronic structure can be written as a numbers x, y, z, where x represents the number of electrons in the first shell, y in the second and z in the third.
Each electron shell corresponds to an energy level. For example, electrons that are in the second electron shell are in the second energy level.
Dot and cross diagrams are a simple way of visualising electrons in the orbitals of atoms and compounds. The electrons in the outer shell are known as valence electrons, they usually participate in reactions. Information about how many electrons are in each shell can be taken directly from their electronic structure.
Electron shells in an atom are made up of atomic orbitals, which are regions around the nucleus that can hold up to two electrons, with opposite spins. There are s, p and d orbitals, each with a different shape. Here are all available orbitals for the first 4 energy levels:
1st → 1s
2nd → 2s 2p
3rd → 3s 3p 3d
4th → 4s 4p 4d 4f
The other 3 energy levels have a similar pattern to the 4th energy level. The fact that particular shells exist in a level doesn’t mean that they will be filled for an element in a corresponding period. For example, d-orbitals that are found in 3rd level will only start to be filled for 4th period elements as there are no d-block elements in period 3. s orbital exits alone, p orbitals in groups of 3, d in groups of 5 and f in groups of 7. Each orbital can fit 2 electrons.
Shells fill up in order s → p → d → f by going through each element from hydrogen to the element of interest and assigning them to corresponding orbitals. An electron configuration is written by using the aforementioned templates for the energy levels and by including superscript letters representing the number of electrons in each orbital. For example, calcium would have an electronic configuration of 1s² 2s² 2p⁶ 3s² 3p⁶ 4s².
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