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^2?
  • 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:

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

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

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

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

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

Relative energies of the shells and subshells