CIE AS Chemistry (9701) exams from 2022

Revision Notes

1.1.7 Electron Subshells & Orbitals

Electron Orbitals

  • Each shell can be divided further into subshells, labelled s, p, d and f
  • Each subshell can hold a specific number of orbitals:
    • s subshell : 1 orbital
    • p subshell : 3 orbitals labelled px, py and pz
    • d subshell : 5 orbitals
    • f subshell : 7 orbitals
  • Each orbital can hold a maximum number of 2 electrons so the maximum number of electrons in each subshell are as follows:
    • s : 1 x 2 = total of 2 electrons
    • p : 3 x 2 = total of 6 electrons
    • d : 5 x 2 = total of 10 electrons
    • f : 7 x 2 = total of 14 electrons
  • In the ground state, orbitals in the same subshell have the same energy and are said to be degenerate, so the energy of a px orbital is the same as a py orbital

Atomic Structure Electrons in Atoms, downloadable AS & A Level Chemistry revision notes

Shells are divided into subshells which are further divided into orbitals

Summary of the arrangement of electrons in atoms table

Atomic Structure Electrons in Atoms Table, downloadable AS & A Level Chemistry revision notes

Subshells & Energy

  • The principal quantum shells increase in energy with increasing principal quantum number
    • Eg. n = 4 is higher in energy than n = 2
  • The subshells increase in energy as follows: s < p < d < f
    • The only exception to these rules is the 3d orbital which has slightly higher energy than the 4s orbital, so the 3d orbital is filled before the 4s orbital
  • All the orbitals in the same subshell have the same energy and are said to be degenerate
    • Eg. px, py and pz are all equal in energy

Atomic Structure Energy Levels, downloadable AS & A Level Chemistry revision notes

Relative energies of the shells and subshells

The s & p Orbitals

s orbitals

  • The s orbitals are spherical in shape
  • The size of the s orbitals increases with increasing shell number
    • Eg. the s orbital of the third quantum shell (n = 3) is bigger than the s orbital of the first quantum shell (n = 1)

Atomic Structure The s Orbitals, downloadable AS & A Level Chemistry revision notes

The s orbitals become larger with increasing principal quantum number

p orbitals

  • The p orbitals are dumbbell-shaped
  • Every shell has three p orbitals except for the first one (n = 1)
  • The p orbitals occupy the x, y and z-axis 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

Atomic Structure The p Orbitals, downloadable AS & A Level Chemistry revision notes

The p orbitals become larger and longer with increasing principal quantum number

Author: Francesca

Fran has taught A level Chemistry in the UK for over 10 years. As head of science, she used her passion for education to drive improvement for staff and students, supporting them to achieve their full potential. Fran has also co-written science textbooks and worked as an examiner for UK exam boards.
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