KEY2CHEM

Relationship between Structure and Properties

The chemical formula of a substance provides insight into its chemical and physical properties. For example, ionic compounds are generally formed between a metal and a nonmetal and have properties of high melting point, brittleness, and electrical conductivity when melted or in aqueous solutions. Covalent compounds are generally formed between nonmetals and have more moderate melting points and more often do not conduct electricity when melted or in solution.

Example 1.

Which compound is most likely to conduct an electrical current when dissolved in water?

A. $$\require{mhchem}\ce{NaCl}$$

B. $$\require{mhchem}\ce{CO2}$$

C. $$\require{mhchem}\ce{BrCl3}$$

Solution

A. $$\require{mhchem}\ce{NaCl}$$

Ionic compounds dissociate into ions in water and will conduct an electrical current.

Example 2.

Which compound’s solid state structure is best described by the diagram below?

A. $$\require{mhchem}\ce{OF2}$$

B. $$\require{mhchem}\ce{NaCl}$$

C. $$\require{mhchem}\ce{MgCl2}$$

Solution

C. $$\require{mhchem}\ce{MgCl2}$$

The charged particles that make up the compound indicate it is an ionic compound. The charges on the ions best fit with $$\require{mhchem}\ce{MgCl2}$$.

Example 3.

Which compound is likely to have the lowest melting point?

A. $$\require{mhchem}\ce{NaCl}$$

B. $$\require{mhchem}\ce{AlCl3}$$

C. $$\require{mhchem}\ce{PCl3}$$

Solution

C. $$\require{mhchem}\ce{PCl3}$$

Ionic compounds tend to have the highest melting points while covalent compounds have lower melting points. Aluminum is a metalloid, so its compounds will have properties intermediate between the two.