2.2 - Ionic Bonding

• how are ions formed 


  formed when atoms gain or lose electrons
• cations


  when an atom loses an electron becoming a positive ion
• anion


  a negative ion formed when an atom gains an electron
• ionic bonding


  strong electrostatic force of attraction between 2 oppositely charged ions in a lattice
• lattice


  3D arrangement of ions or atoms in a lattice
• ionic compounds 


  when oppositely charged ions are held together by electrostatic attractions (form an ionic bond)
• structure of ionic compounds 


  giant ionic lattice structures because each ion is electrostatically attracted in all directions to ions of the opposite charge
• factors affecting strength of ionic bonds


  ionic charge

  ionic radii
• how ionic charge affects the strength of the ionic bond 


  the greater the charge on an ion the stronger the electrostatic force of attraction so the stronger the ionic bond

  so more energy needed to overcome bond so the higher the melting / boiling point
• how ionic radii affects the strength of the ionic bond


  the smaller the ione radii the closer the ions are so the stronger the electrostatic forces of attraction so a stronger ionic bond so more energy is required to overcome forces of attraction so a higher melting & boiling point
• high charge density


  ionic compound with a big charge and a small ion
• Ionic radii down a group


  increases because the number of shells increases due to an increasing atomic number
• isoelectronic ions 


  ions of different atoms with the same number of electrons
• ionic radii across a period (or of isoelectric ions)


  decreases because number of protons increases but number of electrons stays the same so nuclear charge increases so electrons are more strongly attracted to the nucleus so are pulled closer to it
• working out radii of ions


  use x-ray diffraction methods and use diffraction patterns to calculate radii of individual ions
• radii of positive ions of elements


  smaller than atomic radius because atom loses an electron from its outer shell turning it into an ion
• radii of negative ions of elements


  larger than atomic radius because electrons are added to the outer shell
• properties of ionic compounds 


  hard, brittle, crystalline substances

  high melting and boiling points

  soluble in uwater and other polar solvents

  insoluble in non-polar solvents

  only conduct electricity when molten or dissolved in water
• why ionic compounds only dissolve in water and other polar solvents


  water is polar so molecules can attract positive and negative ions and break up the structure
• why ionic compounds only conduct electricity when molten or dissolved in water


  as a solid ions are in fixed positions due to strong ionic bond

  when molten or aqueous charged ions are free to move so conduct electricity
• why ionic compounds have high melting & boiling points


  ions are held together by strong electrostatic forces of attraction between positive and negative ions so lots of energy is needed to overcome them
• why ionic compounds are brittle


  when struck with a hammer the layers slide and like charges align so they repel and structure breaks apart
• what dot-and-cross diagrams show


  the arrangement of electrons in an atom or ion

  which atom the bonding electrons originally came from
• why dot-and- cross diagrams are useful


  they provide a balance sheet for keeping track of the electrons when ionic compounds form
• evidence for charged particles


  Can be observed during electrolysis of coloured compounds

  example: electrolysis of Copper (ll) chromate (VI)
• electrolysis of copper (ll) chromate(VI) on net filter paper


  green solution

  blue Cu2+ cations are attracted to negative cathode and migrate towards it so solution around it turns blue

  yellow Cr4 2- anions are attracted by positive anode and migrate towards it so solution around anode turns yellow