Edexcel A Level Chemistry:复习笔记1.4.2 Representing Ionic Bonding

Ionic Dot-and-Cross Diagrams

 

  • Dot and cross diagrams are diagrams that show the arrangement of the outer-shell electrons in an ionic or covalent compound or element
    • The electrons are shown as dots and crosses

     

  • In a dot and cross diagram:
    • Only the outer electrons are shown
    • The charge of the ion is spread evenly which is shown by using brackets
    • The charge on each ion is written at the top right-hand corner

     

Ionic compounds

  • Ionic bonds are formed when metal atoms transfer electrons to a non-metal to form a positively charged and negatively charged ion
  • The atoms achieve a noble gas configuration

1.3-Chemical-Bonding-Ionic-Dot-Cross

Dot-and-cross diagrams of ionic compounds in which one of the atoms transfers their valence electrons to the other

 

Calcium fluoride

  • Calcium is a Group 2 metal
    • It loses its 2 outer electrons to form a calcium ion with a +2 charge (Ca2+)

     

  • Fluorine is a Group 7 non-metal
    • It gains 1 electron to form a fluoride ion with a -1 charge (F-)

     

  • As before, the positive and negative ions are attracted to each other via an ionic bond
  • However, to cancel out the 2+ charge of the calcium ion, 2 fluorine atoms are needed
    • Each fluorine atom can only accept 1 electron from the calcium atom
    • 2 fluoride ions will be formed

     

  • Calcium fluoride is made when 1 calcium ion and 2 fluoride ions form ionic bonds, CaF2
  • The final ionic solid of CaF2 is neutral in charge

Worked Example

Draw a dot cross diagram for lithium nitride

Answer

    • Lithium is a Group 1 metal
    • It loses its outer electron to form a lithium ion with a +1 charge (Li+)
    • Nitrogen is a Group 5 non-metal
    • It gains 3 electrons to form a nitride ion with a -3 charge (N3-)
    • To cancel out the -3 charge of the nitride ion, 3 lithium atoms are needed and 3 lithium ions will be formed
    • Lithium nitride is made when 1 nitride ion and 3 lithium ions form ionic bonds
    • The final ionic solid of Li3N is neutral in charge

1.3-Chemical-Bonding-Lithium-Nitride-Cross-Diagram

Dot and cross diagram to show the ionic bonding in lithium nitride

 

Worked Example

Draw a dot cross diagram for aluminium oxide

Answer

    • Aluminium is a Group 3 metal
    • It loses its outer electrons to form an aluminium ion with a +3 charge (Al3+)
    • Oxygen is a Group 6 non-metal
    • It gains 2 electrons to form an oxide ion with a -2 charge (O2-)
    • To cancel out the negative and positive charges, 2 aluminium and 3 oxygen atoms are needed
    • Aluminium oxide is made when 2 aluminium ions and 3 oxygen ions form ionic bonds
    • The final ionic solid of Al2O3 is neutral in charge

1.3-Chemical-Bonding-Aluminium-Oxide-Cross-Diagram

Dot and cross diagram to show the ionic bonding in aluminium oxide

 

 

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