Ionic Bond Strength (Edexcel International A Level Chemistry)

• Elements in the periodic table are arranged in order of increasing atomic number and placed in vertical columns (groups) and horizontal rows (periods)
• The elements across the periods show repeating patterns in chemical and physical properties
• This is called periodicity

All elements are arranged in the order of increasing atomic number from left to right

Atomic radius

• The atomic radius is the distance between the nucleus and the outermost electron of an atom
• The atomic radius is measured by taking two atoms of the same element, measuring the distance between their nuclei and then halving this distance
• In metals this is also called the metallic radius and in non-metals, the covalent radius

Atomic radii of Period 3 elements

• You can see a clear trend across the period:

The graph shows a decrease in atomic radii of Period 3 elements across the period

• Across the period, the atomic radii decrease
• This is because the number of protons (the nuclear charge) and the number of electrons increases by one every time you go an element to the right
• The elements in a period all have the same number of shells (so the shielding effect is the same)
• This means that as you go across the period the nucleus attracts the electrons more strongly pulling them closer to the nucleus
• Because of this, the atomic radius (and thus the size of the atoms) decreases across the period

The diagram shows that across Period 3, the elements gain extra electrons in the same principal quantum shell

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