1.6.3 Ionic Bond Strength

Ionic radius

• The ionic radius of an element is a measure of the size of an ion
• Ionic radii show predictable patterns
  • Ionic radii increase with increasing negative charge
  • Ionic radii decrease with increasing positive charge
• For negative ions
  • They are formed by atoms gaining electrons 
  • The outermost electrons are further away from the positively charged nucleus and are therefore held only weakly to the nucleus which increases the ionic radius
  • The greater the negative charge, the larger the ionic radius
• For positive ions
  • Positively charged ions are formed by atoms losing electrons
  • The nuclear charge remains the same but there are now fewer electrons that undergo a greater electrostatic force of attraction to the nucleus which decreases the ionic radius
  • The greater the positive charger, the smaller the ionic radius

Trends in the ionic radii across a period and down a group

• The strength of ionic bonding, therefore, depends on both the charges and the sizes of the two ions
• Higher charges and smaller sizes produce stronger bonds
• This is because:
  • As the ionic charge increases, or the ionic radius decreases, the charge density increases

Isoelectronic Ions

• These are ions that have the same electronic configuration 
• For example the following ions all have the electronic configuration of 1s² 2s² 2p⁶
• However, they all have different numbers of protons, therefore will have a different ionic radius
  
  • N^3- (7 protons)
  • O^2- (8 protons)
  • F^- (9 protons)
  • Na⁺ (11 protons)
  • Mg²⁺ (12 protons)
  • Al³⁺ (13 protons)
• As the number of protons in the nucleus of the ion increases, the electrons get pulled in more closely to the nucleus
• The radii of the isoelectronic ions therefore fall across this series of ions 
  • N^3- has an ionic radius of 0.171 nm and Al³⁺ has an ionic radius of 0.054 nm