Question

For each pair of elements and compounds listed below, discuss their similarities and/or differences in terms of physical and chemical properties.

(a) Boron (B) and silicon (Si)

(b) BeCl2 and MgCl2;

(c) Al2O3 and In2O3;

Answer 1

a) B (5): [He]2s²2p¹

Si (14): [Na]2s²2p²

B belongs to Group 13, period 2 while Si occurs in Group 14, period 3. Both are non-metallic elements.

The differences are more since B exhibits properties of group 13 while Si shows properties of group 14. B can form 3 sigma bonds with other non-metallic elements or radicals. Since B can form maximum three bonds, compounds of B are electron-deficient (they have incomplete octet). Therefore, B can accept electron density into the vacant p orbital. In that sense, B acts as a Lewis acid.

Si can form tetravalent compounds with non-metallic elements or radicals. Si has a complete octet in all its compounds, hence Si can neither accept electron density or donate any. In that way, Si is electron toward Lewis acidity/basicity.

B is smaller in size than Si and hence more reactive.

b) The anions are the same and all similarities or dissimilarities stem from the cations.

Be (4); [He]2s²

Mg (12); [Ne]3s²

Be and Mg occur in group II of the periodic table. Both are metals and hence reducing. Both can easily lose the 2 electrons in the outer orbital and get oxidisied. Both the elements re alkaline earth metals and exhibit similar chemical properties.

The elements differ in physical properties. Be is smaller in size than Mg and hence the crystal packing in BeCl₂ is less effective (since Be is much smaller than Cl atoms). Therefore, BeCl₂ should be more reactive and undergoes hydrolysis easily.

MgCl₂ on the other hand forms good crystals since both Mg and Cl have similar sizes and hence, the close packing of atoms into the crystal structure is more effective. Therefore, MgCl₂ should be less reactive to hydrolysis and should have higher melting and boiling points.