Edexcel A Level Chemistry:复习笔记1.4.3 Ionic Trends

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