• 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 p_x, p_y and p_z
  • 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 p_x orbital is the same as a p_y orbital

Shells are divided into subshells which are further divided into orbitals

Summary of the arrangement of electrons in atoms table

• 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. p_x, p_y and p_z are all equal in energy

Relative energies of the shells and subshells

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)

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

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