Question

The melting points of the period 3 metals sodium and magnesium are shown below.What is the difference in the melting points of sodium and magnesium, using the model of metallic bonding?

Metal	Melting point/$^{\circ }C$
Sodium	98
Magnesium	649

Answer 1

The melting point is actually the point at which the substance with solid state is getting converted into liquid state and this conversion involves the breaking of several bonds. And we should also have an idea about metallic bonding to solve the question.

Complete answer:
In the question, a table consisting of two elements and its melting point is given and we have to give the reason for the difference in the melting point for the elements of the same period.
Before going into the solution of the question, first let us talk about bonding in compounds and specifically about metallic bonding in metals.
We know that when a chemical reaction takes place the reactants combine to form a product and the chemical reaction taking place involves the bond breaking and bond formation.
There are various types of chemical bonds by which the new products are formed in the chemical reaction.
If a compound is formed by the sharing of electrons between the atoms, then that type of bonding of atoms is called covalent bonding.
If a compound is formed by the loss or gain of electrons and the force that keeps the atom intact in their position is due to the electrostatic force of attraction between oppositely charged ions i.e. the bond formation is due to electrostatic force of attraction between the ions ad the bond formed is called as ionic bond.
And the next one is exclusively applicable for the metal s, the metallic bonding. It is the chemical bonding formed by the sharing of electrons by the delocalized electrons or the sea of electrons or the conduction electrons with the positively charged metal ions. The bonding is due to electrostatic force of attraction.
And melting point is the temperature at which a substance in the solid phase gets converted to a liquid phase in the atmospheric pressure. So when a solid structure melts to liquid phase, it involves breaking of chemical bonds. So for a structure in which the bonding is strong will have higher melting point as it needs more energy to break the bonds.
In Mg metal, it exists as a divalent ion as $M{{g}^{2+}}$ and hence two electrons are available for sharing and the bonding will be stronger and hence will have higher value for melting point than sodium. Since in sodium only one electron is involved and the bonding will be lesser than the Mg.

Note:
Aluminium has the greater melting point than sodium and magnesium, since the aluminium is trivalent ion, i.e. there are three valence electrons which is involved in bonding and the three conducting electrons are having electrostatic force of attraction with the positively charged metal ion. The bonding is stronger than the other metal hence greater value for melting point.