OCR AS Chemistry F332: Electronic Structure

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Electronic Structure
Electrons have fixed energy levels. The electrons move around the nucleus in certain
regions of the atom called shells or energy levels.
Each shell is given a number called the principal quantum number. The further a shell is
from the nucleus, the higher its energy and the larger its principal quantum number.
However, not all electrons in a shell have exactly the same energy. Therefore shells are
divided up into sub-shells that have slightly different energies.
The sub-shells consist of different numbers of orbitals, where an orbital is just the bit of
space which an electron moves in.
Each orbital can hold up to 2 electrons. If there are 2 electrons in an orbital, they spin in
opposite directions ­ this is called spin pairing.
Different types of sub-shell are labelled using the letters s, p, d and f.
The first shell contains an s-orbital, and so can hold just two electrons.
The second shell contains an s-orbital and 3 p-orbitals, and so can hold up to 8 electrons
altogether.
This table shows the sub-shells and electrons in the first four energy levels
Shell Sub-shells Total number of electrons
1st 1s 2
2nd 2s 2p 2 + (3 x 2) = 8
3rd 3s 3p 3d 2 + (3 x 2) + (5 x 2) = 18
4th 4s 4p 4d 4f 2 + (3 x 2) + (5 x 2) + (7 x 2) = 32
This table shows the number of electrons that fit in each type of sub shell
Sub-shell Number of Orbitals Maximum electrons
s 1 1x2=2
p 3 3x2=6
d 5 5 x 2 = 10
f 7 7 x 2 =14

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Filling up Atomic Orbitals
The arrangement of electrons in shells and orbitals is called the electronic configuration of
an atom.
The orbitals are filled in a definite order to produce the lowest energy arrangement
possible.
The orbitals are filled up in order of increasing energy.
Where there is more than one orbital with the same energy, these orbitals are first occupied
singly by electrons.
This keeps the electrons in an atom as far apart as possible.…read more

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Electron Shells Overlap
The larger the value of principal quantum number n, the higher the energy level and the
further the shell is from the nucleus.
As we move from one shell to the next, a new type of sub-shell is added.
Within a shell, the sub-shell energies increase in the order s, p, d and f.
You need to take care when you go beyond the 3p sub-shell.…read more

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Electronic Configurations and the Periodic Table
The periodic table is structured in blocks of 2, 6, 10 and 14 elements, linked to sub-shells.
The pattern mirrors the sub-shells that are being filled.
The s-block elements have an outer shell configuration of s1 or s2.…read more

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