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 higher 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

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?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 as seen in the diagram below:

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

Orbitals

• 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 them
  • Each atomic orbital can be occupied by a maximum of two electrons
• This means that the number of orbitals in each subshell is as follows:
  • 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)
• The orbitals have specific 3-D shapes

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 at AS Level

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)
• The order of the subshells in terms of increasing energy does?not?follow a regular pattern at n= 3 and higher

The ground state of an atom is achieved by filling the lowest energy subshells first