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Ionic bonding and structure (GCSE)

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KS4 National Curriculum Statement(s) covered

  • Types of chemical bonding: ionic, covalent, and metallic
  • Bulk properties of materials related to bonding and intermolecular forces
  • Changes of state of matter in terms of the relative strength of chemical bonds and intermolecular forces

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Forming ions
Ionic structure
Dot-and-cross diagrams
Bonding models

Ionic bonding involves the electrostatic attraction between oppositely charged ions. This type of bonding typically occurs between metals and non-metals. Metal atoms lose electrons to form cations, while non-metal atoms gain electrons to form anions. The electrostatic attraction between these ions forms a strong bond and creates a regular, repeating lattice structure.

Forming Ions

Ions are atoms (or groups of atoms) that have lost or gained electrons and thus carry a charge. Metal atoms lose electrons to form positively charged ions (cations), while non-metal atoms gain electrons to form negatively charged ions (anions). 

The Periodic Table is a handy helping guide as to which elements' atoms will gain or lose electrons, when forming ions:

group number example element outershell electrons electrons lost or gained ion formed
1 sodium (Na) 1 lose 1 1+
2 magnesium (Mg) 2 lose 2 2+
3 aluminium (Al) 3 lose 3 3+
4 silicon (Si) 4 N/A N/A
5 phosphorus (P) 5 gain 3 3-
6 sulfur (S) 6 gain 2 2-
7 chlorine (Cl) 7 gain 1 1-
0 argon (Ar) 8 already full/stable N/A

Examples:

  • Sodium (Na, Group 1) loses one electron to form an Na⁺ ion
  • Sulfur (S, Group 6) gains two electrons to form a S²⁻ ion
ion name ion symbol/formula example compound
oxide O²⁻ magnesium oxide (MgO)
hydroxide OH⁻ lithium hydroxide (LiOH)
halide F⁻, Cl⁻, Br⁻, I⁻ sodium chloride (NaCl)
nitrate NO₃⁻ potassium nitrate (KNO₃)
carbonate CO₃⁻ calcium carbonate (CaCO₃)
sulfate SO₄²⁻ barium sulfate (BaSO₄)

Ionic Structure

3D ionic lattice (space-filling model)
Figure 1: 3D ionic lattice (space-filling model)

Ionic substances are considered "giant" substances because they form extensive 3D lattice structures composed of repeating units of positive and negative ions. The strong electrostatic forces of attraction between these oppositely charged ions extend throughout the entire substance.

In a giant ionic lattice, each ion is surrounded by ions of the opposite charge, and the electrostatic forces of attraction act in all directions. This uniform attraction results in strong, stable structures.

Properties of ionic compounds:

  • They have high melting and boiling points because the strong electrostatic forces require a lot of energy to break
  • They conduct electricity when molten or dissolved in water because the ions are free to move and carry charge
  • They do not conduct electricity when solid because the ions are fixed in place and cannot move
  • Some ionic substances are water soluble, depending on the attraction between the ions and water molecules

In this animated gif, a water-soluble salt is added to water. As the salt dissolves, its ions separate and disperse throughout the water. This happens because the water molecules surround and interact with the ions, pulling them apart and overcoming the forces of attraction that hold the ions together in the solid state. This process, called dissociation, allows the salt to dissolve completely in the water. 

3D ionic lattice (space-filling model)
Figure 2: GIF from a simulation by PhET Interactive Simulations, University of Colorado Boulder, licensed under CC-BY-4.0.

However, not all ionic substances are soluble in water. If the forces of attraction between the ions in the solid are too strong, water molecules cannot pull them apart, and the substance will not dissolve.

Dot-and-Cross Diagrams

Dot-and-cross diagrams are a useful way to represent the transfer or sharing of electrons in chemical bonding. They show the outer shell electrons of atoms as crosses (×) or dots (•). This visual representation helps in understanding how atoms end up with full outer shells through bonding.

Only the outer shell electrons are involved in chemical bonding, so inner shells aren't often drawn for these diagrams. Ionic dot-and-cross diagrams, however, do require you to draw the new, 'revealed' outer shell for metal ions (after they have given their outer shell electrons to the non-metal atom).

How sodium and fluorine form sodium ions and fluoride ions (ionic bonding).
Figure 3: How sodium and fluorine form sodium ions and fluoride ions (ionic bonding). Gif licensed under CC BY-SA 3.0

Steps to draw dot-and-cross diagrams

  1. Determine the number of electrons in the outer shell of each atom.
  2. Decide whether the atoms will transfer or share electrons to achieve full outer shells.
  3. Decide which outer shell electrons of one atom will be represented with dots and which one with crosses.
  4. Draw the dot-and-cross diagram
  5. Double check each atom has access to 8 electrons (or 2 if it is hydrogen)
  6. For ionic compounds, include the charges on the resulting ions.

Worked Example - sodium chloride (NaCl)

sodium atom showing outer shell electrons (1) with the previous full shell showingchlorine atom showing outer shell electrons (7)
  1. Sodium (Na) atoms have one electron in their outer shell.
    Chlorine (Cl) atoms have seven electrons in their outer shell.
  2. This is an ionic substance, so:
    Sodium will lose one electron to achieve a full outer shell, becoming Na⁺.
    Chlorine will gain one electron to achieve a full outer shell, becoming Cl⁻.
  3. Represent sodium's new, relealed outer shell electrons with dots.
    Represent chlorine's outer shell electrons with crosses, plus one dot for the gained electron.
  4. Draw the ionic dot-and-cross:carbon nanotube ball-and-stick model
  5. The sodium atom 'revealed' a full shell of electrons (8) when it became a sodium ion.
    The chloride ion has access to its original 7 electrons, and now one more from sodium (totalling 8).

    Worked Example - aluminium oxide (Al₂O₃)

    aluminium atom showing outer shell electrons (3) with the previous full shell showingoxygen atom showing outer shell electrons (6)
    1. Aluminium (Al) atoms have three electrons in their outer shell.
      Oxygen (O) atoms have six electrons in their outer shell.
    2. This is an ionic substance, so:
      Two aluminium atoms will each lose three electrons to form two Al³⁺ ions.
      Three oxygen atoms will each gain two electrons to form three O²⁻ ions.
    3. Represent aluminium's new, revealed outer shell electrons with dots.
      Represent oxygen's outer shell electrons with crosses, plus two dots for the gained electrons.
    4. Draw the ionic dot-and-cross:carbon nanotube ball-and-stick model
    5. Each aluminium atom 'revealed' a full shell of electrons (8) when they became aluminium ions.
      The oxide ion has access to its original 6 electrons, and now two more from aluminium (totalling 8).

      Bonding Models

      model example positives limitations
      chemical formula NaCl shows how many atoms (either discrete - covalent; or ratio - ionic) does not show which type of bonding, or the shape of molecule/lattice structure
      dot-and-cross diagram ionic dot-and-cross model shows the transfer of electrons and the charges on ions does not show the 3D shape or relative sizes of ions, or the uniform nature of ionic bonding
      space-filling model ionic space-filling model shows the 3D shape and relative sizes of atoms does not show how ions were formed or the charges on ions, or which element is which (unless a key is given)
      ball-and-stick model ionic ball-and-stick model shows the 3D shape
      • shows bonds as physical 'sticks' rather than forces of attraction (ionic bonds)
      • shows bonds between specific ions in ionic bonding (when bonding is actually uniform)
      • shows spaces between ions
      • does not show how ions were formed or the charges on ions, or which element is which (unless a key is given)

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      Did you know?

      • Sea shells are made of calcium carbonate, which is an example of a compound formed through ionic bonding.
      • Table salt (sodium chloride) is not just used for seasoning food; it’s also used to de-ice roads in winter.
      • Ionic compounds are typically brittle; they shatter when a force is applied due to the repulsion between like-charged ions.

      Why do we care?

      • Understanding how ions form explains why substances like table salt dissolve in water, which is crucial for processes like cooking and maintaining body fluids.
      • Knowing the structure of ionic compounds, such as the strong lattice structure in sodium chloride, helps us understand why these materials are used in road gritting and food preservation.
      • Dot-and-cross diagrams make it easier to visualise ionic bonding, helping students grasp concepts needed for further studies in chemistry and practical applications like creating new materials.
      • Bonding models of ionic substances are essential for predicting their properties, aiding in the development of products like durable ceramics, effective medications, and efficient batteries.

      Key information

      • Ionic substances form through the transfer of electrons from a metal to a non-metal, creating positive and negative ions.
      • Ionic compounds form giant lattice structures, resulting in high melting and boiling points.
      • Ionic substances conduct electricity when molten or dissolved in water because ions are free to move.