KEY2CHEM

Using Periodic Trends to Design Compounds

The organization of the periodic table and its periodic trends can be useful for scientists to predict properties of new molecules. For example, the noble gases (Group 8A) have full subshells and do not tend to form compounds (note: there are exceptions for larger noble gases, especially $$\require{mhchem}\ce{Kr}$$ and $$\require{mhchem}\ce{Xe}$$). Members of the copper family ($$\require{mhchem}\ce{Cu}$$, $$\require{mhchem}\ce{Ag}$$, $$\require{mhchem}\ce{Au}$$ in particular) are often referred to as “coinage metals” since many coins are made out of these metals. The similar properties of these metals, given their similar valence electron count, make them practical for using as money.

Example 1.

Boron tends to form oxides with the formula $$\require{mhchem}\ce{B2O3}$$. Which other element is predicted to form an oxide with a similar formula ($$\require{mhchem}\ce{M2O3}$$)?

A. Aluminum

B. Magnesium

C. Zinc

Solution

A. Aluminum

Aluminum and boron are in the same group, so they are expected to share similar properties, since they have the same valence electron count.

Example 2.

Gallium nitride ($$\require{mhchem}\ce{GaN}$$) has applications in transistors. Which other compound is predicted to find applications in transistors?

A. $$\require{mhchem}\ce{GaCl}$$

B. $$\require{mhchem}\ce{GaS}$$

C. $$\require{mhchem}\ce{GaAs}$$

Solution

C. $$\require{mhchem}\ce{GaAs}$$

Since $$\require{mhchem}\ce{N}$$ and $$\require{mhchem}\ce{As}$$ are in the same group, they are expected to have similar properties. $$\require{mhchem}\ce{GaAs}$$ finds wide application in LEDs, semiconductors, and high-efficiency solar cells.

Example 3.

Cadmium sulfide ($$\require{mhchem}\ce{CdS}$$) nanoparticles are used in solar cells. Which other compound might be useful in solar cells?

A. $$\require{mhchem}\ce{CdTe}$$

B. $$\require{mhchem}\ce{CdF2}$$

C. $$\require{mhchem}\ce{CdAr}$$

Solution

A. $$\require{mhchem}\ce{CdTe}$$

$$\require{mhchem}\ce{Te}$$ and $$\require{mhchem}\ce{S}$$ are in the same family. $$\require{mhchem}\ce{CdTe}$$ is used in solar cells in combination with $$\require{mhchem}\ce{CdS}$$.