Shells & Orbitals (AQA AS Chemistry): Revision Note

Electron Shells

Shells

• Our knowledge and understanding of atomic structure have evolved over time

• The arrangement of electrons in an atom is called the electron configuration

• Electrons are arranged around the nucleus in principal energy levels or principal quantum shells

• Principal quantum numbers (n) are used to number the energy levels or quantum shells

  • The lower the principal quantum number, the closer the shell is to the nucleus

    • So, the first shell, which is the closest to the nucleus, is n = 1

  • The higher the principal quantum number, the greater the energy of the shell and the further away from the nucleus

• Each principal quantum number has a fixed number of electrons it can hold

  • n = 1: up to 2 electrons

  • n = 2: up to 8 electrons

  • n = 3: up to 18 electrons

  • n = 4: up to 32 electrons

Subshells

• The principal quantum shells are split into sub-shells, which are given the letters s, p, and d

  • Elements with more than 58 electrons also have an f shell

  • The energy of the electrons in the sub-shells increases in the order s < p < d

• The order of sub-shells appears to overlap for the higher principal quantum shells, as seen in the diagram below:

Orbitals

• Sub-shells contain one or more atomic orbitals

• Orbitals exist at specific energy levels, and electrons can only be found at these specific levels, not in between them

  • Each atomic orbital can be occupied by a maximum of two electrons

• This means that the number of orbitals in each sub-shell is as follows:

  • s : one orbital (1 x 2 = total of 2 electrons)

  • p : three orbitals ( 3 x 2 = total of 6 electrons)

  • d : five orbitals (5 x 2 = total of 10 electrons)

  • f : seven orbitals (7 x 2 = total of 14 electrons)

• The orbitals have specific 3-D shapes

s orbital shape

• The s orbitals are spherical

• The size of the s orbitals increases with increasing shell number

  • E.g., the s orbital of the third quantum shell (n = 3) is bigger than the s orbital of the first quantum shell (n = 1)

p orbital shape

• The p orbitals have a dumbbell shape

• Every shell has three p orbitals except for the first one (n = 1)

• The p orbitals occupy the x, y, and z axes and point at right angles to each other, so are oriented perpendicular to one another

• The lobes of the p orbitals become larger and longer with increasing shell number

• Note that the shape of the d orbitals is not required

Ground state

• The ground state is the most stable electronic configuration of an atom, which has the lowest amount of energy

• This is achieved by filling the sub-shells of energy with the lowest energy first (1s)

• The order of the sub-shells in terms of increasing energy does not follow a regular pattern at n = 3 and higher

Related topics

•  Electron Configuration

• Ionisation Energy

Electron Arrangement Summary

Summary of the Arrangement of Electrons in Atoms Table