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Sub-shells & Orbitals (CIE AS Chemistry)

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Electron Orbitals & Sub-shells

  • 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

Detailed summary of electron shells

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

Principal quantum number, n
(shell)
Sub-shells possible
(s, p, d, f)
Orbitals per sub-shell Orbitals per principal quantum number Electrons per sub-shells Electrons per shell
1 s 1 1 2 2
2 s 1 4 2 8
p 3 6
3 s 1 9 2 18
p 3 6
d 5 10
4 s 1 16 2 32
p 3 6
d 5 10
f 7 14

Energy of the Sub-shells

  • The principal quantum shells increase in energy with increasing principal quantum number
    • E.g. n = 4 is higher in energy than n = 2

  • The sub-shells 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
    • Because of this, the 4s orbital is filled before the 3d orbital

  • All the orbitals in the same sub-shell have the same energy and are said to be degenerate
    • E.g. px, py and pz are all equal in energy

Relative energies of the shells and sub-shells

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

The 4s orbital is lower in energy than the 3d orbital

Shape of s & p Orbitals

s orbitals

  • The s orbitals are spherical in shape
  • 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)

s orbital diagram

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

p orbital diagram

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

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

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Richard

Author: Richard

Expertise: Chemistry

Richard has taught Chemistry for over 15 years as well as working as a science tutor, examiner, content creator and author. He wasn’t the greatest at exams and only discovered how to revise in his final year at university. That knowledge made him want to help students learn how to revise, challenge them to think about what they actually know and hopefully succeed; so here he is, happily, at SME.