IB Chemistry SL

Revision Notes

2.1.6 Energy Levels & Sublevels

Electron Energy Levels

Shells

  • The arrangement of electrons in an atom is called the electronic 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
    • The higher the principal quantum number, the lesser the energy of the shell
  • 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
  • There is a pattern here – the mathematical relationship between the number of electrons and the principal energy level is 2n
    • So for example, in the third shell n = 3 and the number of electrons is 2 x (32 ) = 18

 

Electrons are arranged in principal quantum shells, which are numbered by principal quantum numbers

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 subshell
    • The energy of the electrons in the subshells increases in the order s < p < d
  • The order of subshells overlap for the higher principal quantum shells as seen in the diagram below:

Atomic Structure Principal-Quantum-Subshells, downloadable AS & A Level Chemistry revision notes

Electrons are arranged in principal quantum shells, which are numbered by principal quantum numbers

Orbitals

  • The subshells 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
    • Each atomic orbital can be occupied by a maximum of two electrons
  • The orbitals have specific 3D shapes

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

Representation of orbitals (the dot represents the nucleus of the atom) showing spherical s orbitals (a), p orbitals containing ‘lobes’ along the x, y and z axis

 

  • Note that the shape of the d orbitals is not required for IB Chemistry

Atomic Structure Summary, downloadable AS & A Level Chemistry revision notes

An overview of the shells, subshells and orbitals in an atom

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 subshells of energy with the lowest energy first (1s) – this is called the Aufbau Principle
  • The order of the subshells in terms of increasing energy does not follow a regular pattern at n= 3 and higher

Atomic Structure Ground-State, downloadable AS & A Level Chemistry revision notes

The Aufbau Principle – following the arrows gives you the filling order

Sublevels & 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 4s orbital is filled before the 3d 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

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