2.2

• Subshells

  • The principal quantum shells are split into subshells which are given the letters s, p, and d
  • Elements with more than 57 electrons also have an f shell
  • The energy of the electrons in the subshells increases in the order s < p < d
  • The order of subshells appear to overlap for the higher principal quantum shells
• Number of orbitals in each subshell

  • 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)
• Principal quantum numbers

  • The lower the principal quantum number, the closer the shell is to the nucleus
  • 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
• Electrons can only be found at specific energy levels in orbitals, not in between them
• Shape of d orbitals is not required
• p orbital shape

  • Dumbbell shape
  • Three p orbitals in every shell except the first one
  • Point at right angles to each other and occupy the x, y, and z axes
  • Lobes become larger and longer with increasing shell number
• Orbitals contain one or more atomic orbitals
• s orbital shape

  • Spherical shape
  • Size increases with increasing shell number
• Each atomic orbital can be occupied by a maximum of two electrons
• Number of electrons each principal quantum number 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
• Arrangement of electrons in an atom

  1. Called the electron configuration

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

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

  4. Each principal quantum number has a fixed number of electrons it can hold
• Representation of orbitals showing spherical s orbitals and p orbitals with lobes along the x, y, and z axis
• Electrons will occupy separate orbitals in the same subshell first to minimise this repulsion and have their spin in the same direction
• Electrons can be imagined as small spinning charges which rotate around their own axis in either a clockwise or anticlockwise direction
• The spin creates a tiny magnetic field with N-S pole pointing up or down
• The energy required to jump to a higher empty orbital is greater than the inter-electron repulsion, so they pair up and occupy the lower energy levels first
• The principal quantum number indicates the energy level of a particular shell but also indicates the energy of the electrons in that shell
• Even though there is repulsion between negatively charged electrons, they occupy the same region of space in orbitals
• This is known as Hund's Rule
• This is achieved by filling the subshells of energy with the lowest energy first (1s)
• The ground state of an atom is achieved by filling the lowest energy subshells first
• This can be done using the full electron configuration or the shorthand version
• The electrons are represented by opposite arrows to show the spin of the electrons
• Each box represents an atomic orbital
• Electrons with the same spin repel each other which is also called spin-pair repulsion
• A 2p electron is in the second shell and therefore has an energy corresponding to n = 2
• The spin of the electron is represented by its direction
• Electrons will then pair up, with a second electron being added to the first p orbital, with its spin in the opposite direction
• If there are three electrons in a p subshell, one electron will go into each p, p, and p orbital
• An orbital can only hold two electrons and they must have opposite spin - this is known as the Pauli Exclusion Principle
• Shapes of orbitals

  • x
  • y
  • z
• The electron configuration can also be represented using the orbital spin diagrams
• The ground state is the most stable electronic configuration of an atom which has the lowest amount of energy
• The order of the subshells in terms of increasing energy does not follow a regular pattern at n = 3 and higher
• Writing out the electronic configuration tells us how the electrons in an atom or ion are arranged in their shells, subshells and orbitals
• The boxes are arranged in order of increasing energy from lower to higher (i.e. starting from closest to the nucleus)
• Blocks in the Periodic Table based on electronic configuration

  • s block elements (valence electron(s) in s orbital)
  • p block elements (valence electron(s) in p orbital)
  • d block elements (valence electron(s) in d orbital)
  • f block elements (valence electron(s) in f orbital)
• Electron Configuration


  Describes how the electrons in an atom or ion are arranged in their shells, subshells, and orbitals
• Ions are formed when atoms lose or gain electrons
• Positive ions are formed by removing electrons from the outer subshell