Edexcel A Level Chemistry:复习笔记1.4.3 Ionic Trends

Ionic Radius


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 which 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


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
    • N3- (7 protons)
    • O2- (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
    • N3- has an ionic radius of 0.171 nm and Al3+ has an ionic radius of 0.054 nm