Structure & Bonding of the Period 3 Elements (Cambridge (CIE) AS Chemistry)

Explaining Physical Properties of the Period 3 Elements

Melting point

Melting points of the elements across Period 3 table

Graph of melting points across Period 3

There is a general increase in melting point from Na to Si, followed by a sharp drop to the lower melting points of P to Ar

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

Bonding and structure of the Period 3 elements table

• 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 structure of metals

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

• Due to 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

Trends in electrical conductivity across Period 3 table

• 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