2.2.1 Electron structure

Definitions

Term	Definition
Atomic orbitals	A region around the nucleus that can hold up to two electrons with opposite spins.
Sub-shells	A group of orbitals of the same type within a shell.
Electron configuration	A shorthand method for showing how electrons occupy sub-shells in an atom.

Atomic orbitals

Number of electrons that can fit in each shell

Shell number	Number of electrons ( \(= 2n^{2}\) )
1	2
2	8
3	18
4	32

Types of orbitals

• s-orbitals
  • Spherical shape
  • Each shell from \(n=1\) contains 1 s-orbital
  • Greater shell number \(n\) = greater radius of its s-orbital
  • 
• p-orbitals
  • Dumb-bell shape
  • Three separate p-orbitals at right angles to one another: \(p_x, p_y, p_z\)
  • Each shell from \(n=2\) contains 3 p-orbitals
  • Greater shell number \(n\) = its p-orbital is further from the nucleus
  • 
• d-orbitals
  • Each shell from \(n=3\) contains 5 d-orbitals
• f-orbitals
  • Each shell from \(n=4\) contains 7 f-orbitals

Filling of sub shells and orbitals

• Sub shells fill in the order of increasing energy (\(1s \rightarrow 2s \rightarrow 2p \dots\))
  • 
  • \(4s\) is filled before \(3d\)
• Inside each sub-shell
  • The orbitals all have the same energy within a sub-shell
  • One electron occupies each orbital before pairing starts to prevent any repulsion between paired electrons
  • Opposite spins within each orbital (one \(\uparrow\) and one \(\downarrow\)) as both electrons are negatively charged and repel one another so this minimises repulsion
  • The paired electrons are easier to remove than the unpaired ones
  • 
• More than one orbital within a sub-shell = the orbitals are degenerate (all the same)

Electron configurations

Writing electron configuration of atoms

• e.g. \(Li = 1s^2 2s^1\), \(F = 1s^2 2s^2 2p^5\)
• Shorthand notation: in terms of the previous noble gas + outer electron sub-shells
  • e.g. Krypton = \(1s^2 2s^2 2p^6 3s^2 3p^6 3d^{10} 4s^2 4p^6\) === \([Ar] 3d^{10} 4s^2 4p^6\)
  • Always show outer shell electrons
• (4s can be written before 3d)

Exceptions of atom electron configurations

• Chromium: \([Ar] 3d^5 4s^1\)
• Copper: \([Ar] 3d^{10} 4s^1\)
• Chromium and copper do not follow the expected pattern
  • Half-filled / fully filled d sub-shell gives additional stability

Electron configuration of ions

• The highest energy subshell gain or lose electrons
• d-block elements
  • \(4s\) is at a lower energy level than \(3d\) sub-shell so it is filled first
  • Once filled the energy level of \(3d\) falls below \(4s\) so \(4s\) empties before \(3d\)