CIE AS Chemistry (9701) 2019-2021

Revision Notes

2.1.2 Structure & Bonding of Period 3 Elements

Period 3: Structure & Bonding

Melting point

Melting points of the elements across Period 3 table

The Periodic Table - Table 3_Properties of the Elements in Period 3, downloadable AS & A Level Chemistry revision notes

The Periodic Table - Melting Point Graph, downloadable AS & A Level Chemistry revision notes

Ions of Period 3 elements with increasing positive charge (metals) and increasing of outer electrons across the period

  • The above trends can be explained by looking at the bonding and structure of the elements

Bonding & structure of the elements table

The Periodic Table - Table 4_Properties of the Elements in Period 3, downloadable AS & A Level Chemistry revision notes

  • The table shows that Na, Mg and Al are metallic elements which form positive ions arranged in a giant lattice in which the ions are held together by a ‘sea’ of delocalised electrons around them.

 

The Periodic Table - Metallic Lattice, downloadable AS & A Level Chemistry revision notes

Metal cations form a giant lattice held together by electrons that can freely move around

  • The electrons in the ‘sea’ of delocalised electrons are those from the valence shell of the atoms
  • Na will donate one electron into the ‘sea’ of delocalised electrons, Mg will donate two and Al three electrons
  • As a result of this, the metallic bonding in Al is stronger than in Na
  • This is because the electrostatic forces between a 3+ ion and the larger number of negatively charged delocalised electrons is much larger compared to a 1+ ion and the smaller number of delocalised electrons in Na
  • Because of this, the melting points increase going from Na to Al
  • Si has the highest melting point due to its giant molecular structure in which each Si atom is held to its neighbouring Si atoms by strong covalent bonds
  • P, S, Cl and Ar are non-metallic elements and exist as simple molecules (P4, S8, Cl2 and Ar as a single atom)
  • The covalent bonds within the molecules are strong, however, between the molecules, there are only weak instantaneous dipole-induced dipole forces
  • It doesn’t take much energy to break these intermolecular forces
  • Therefore, the melting points decrease going from P to Ar (note that the melting point of S is higher than that of P as sulphur exists as larger S8 molecules compared to the smaller P4 molecule)

Electrical conductivity

  • The electrical conductivity decreases going across the Period 3 elements

Electrical conductivity decreases Period 3 elements table

The Periodic Table - Table 5_Properties of the Elements in Period 3, downloadable AS & A Level Chemistry revision notes

  • Going from Na to Al, there is an increase in the number of valence electrons that are donated to the ‘sea’ of delocalised electrons
  • Because of this, in Al there are more electrons available to move around through the structure when it conducts electricity, making Al a better electrical conductor than Na
  • Due to the giant molecular structure of Si, there are no delocalised electrons that can freely move around within the structure
  • Si is therefore not a good electrical conductor and is classified as a semimetal (metalloid)
  • The lack of delocalised electrons is also why P and S cannot conduct electricity

Variation in First Ionisation Energy

  • The first ionisation energy (IE1) is the energy required to remove one mole of electrons from one mole of atoms of an element in the gaseous state to form one mole of gaseous ions
    • Eg. the first ionisation energy of Na is:

Na(g) → Na+(g) + e

IE1 values of the Period 3 elements table

The Periodic Table - Table_Variation in First Ionisation Energy, downloadable AS & A Level Chemistry revision notes

 

  • There is a general increase in IE1 across a period
    • The nuclear charge increases
    • The atomic radius decreases
    • There are stronger attractive forces between the nucleus and outer electrons
    • It therefore gets harder to remove any electrons
  • Small ‘dips’ are observed between Mg – Al and P – S

Ceramics

Structure of ceramics

  • A ceramic is a rigid material that is made up of an infinite 3D network of sintered metals bonded to carbon, nitrogen or oxygen
  • Sintering is when a powdered material is heated below its melting point
    • This results in the formation of new bonds between the powder grains to form one large mass
  • Examples of common ceramics are magnesium oxide, aluminium oxide and silicon dioxide
    • Though silicon is a non-polar silicon dioxide it is still considered a ceramic
  • These compounds all have giant ionic (magnesium oxide and aluminium oxide) or giant covalent (silicon dioxide) structures
  • There are strong bonds and electrostatic forces between the atoms or ions that hold the giant lattice structures together
  • This affects the physical properties of ceramics

Physical properties

  • Ceramics are very hard and strong but brittle
    • This is due to the strong bonds and forces that hold the atoms and ions together in the 3D structure
    • However, if the 3D structure is distorted the ceramic will easily break
  • The presence of strong covalent or ionic bonds in the structure also means that ceramics have high melting points
    • A lot of energy is required to break these bonds
  • Ceramics are good electrical insulators as they do not conduct electricity
    • Covalently bonded ceramics have no delocalised electrons that can move around
    • Ionic bonded ceramics have no ions that can freely move around

Applications of ceramics

  • Magnesium oxide ceramics are used as:
    • Furnace linings due to their high melting points
    • Heating elements – for example, electrical cookers as ceramics are good electrical insulators
  • Aluminium oxide is used as:
    • High temperature and voltage electrical insulators due to its high melting point and electrical insulating properties
    • Replacement hip joints as aluminium oxide is highly durable
  • The uses of silicon dioxide are:
    • Furnace linings due to the high melting point of the giant covalent structure
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