Melting points of the elements across period 3

• A pattern is a little harder to see from the data, but you can see that it rises and falls:

Melting points of the period 3 elements

• The trends in melting point can be explained by looking at the bonding and structure of the elements

Bonding & Structure of the Period 3 Elements

• 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.

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

• 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 neighboring Si atoms by four?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 across Period 3 Elements

• 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