Ionic Bonding & Structures (Edexcel IAL Chemistry)

Ionic Dot & Cross Diagrams

Spec point — 3.3

Ionic Dot-and-Cross Diagrams

• As a general rule, metals are on the left of the periodic table and nonmetals are on the right-hand side

• Ionic bonding involves the transfer of electrons from a metallic element to a non-metallic element

• Transferring electrons usually leaves the metal and the non-metal with a full outer shell

• Metals lose electrons from their valence shell forming positively charged cations

• Non-metal atoms gain electrons forming negatively charged anions

• Once the atoms become ions, their electronic configurations are the same as a noble gas

  • A potassium ion (K⁺) has the same electronic configuration as argon: [2,8,8]⁺
  • A chloride ion (Cl^-) also has the same electronic configuration as argon: [2,8,8]^-

Forming cations by the removal of electrons from metals

Forming anions by the addition of electrons to nonmetals

• Cations and anions are oppositely charged and therefore attracted to each other

• Electrostatic attractions are formed between the oppositely charged ions to form ionic compounds

• The ionic bond is the electrostatic attraction formed between the oppositely charged ions, which occurs in all directions ( this called non-directional bonding)

• This form of attraction is very strong and requires a lot of energy to overcome

  • This causes high melting points in ionic compounds

Cations and anions bond together using strong electrostatic forces, which require a lot of energy to overcome

• The ions form a lattice structure which is an evenly distributed crystalline structure

• Ions in a lattice are arranged in a regular repeating pattern so that positive charges cancel out negative charges

• The attraction between the cations and anions is occurring in all directions

  • Each ion is attracted to all of the oppositely charged ions around it

• Therefore the final lattice is overall electrically neutral

Ionic solids are arranged in lattice structures

Dot and cross diagrams

• These 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

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 (N^3-)
  • 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

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 (O^2-)
  • 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

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