Question

A compound of gallium with chlorine has a melting point of \(350 \mathrm{~K}\left(77^{\circ} \mathrm{C}\right)\) and a boiling point of \(474 \mathrm{~K}\left(201^{\circ} \mathrm{C}\right)\). Is the compound ionic or covalent? What is a likely formula?

Short answer

The compound is covalent with a likely formula of GaCl₃.

Step-by-step solution

Understanding Melting and Boiling Points

To determine whether a compound is ionic or covalent, we look at its melting and boiling points. High melting and boiling points generally indicate an ionic compound. A melting point of 350 K (77°C) and a boiling point of 474 K (201°C) are relatively moderate for melting and boiling, indicating a covalent compound.

Identifying the Likely Formula

Gallium typically forms compounds with oxidation states of +3. Chlorine, being a halogen, usually has an oxidation state of -1. Therefore, three chlorine atoms are needed to balance the +3 charge of one gallium atom, suggesting a likely formula of GaCl₃.

Key concepts

Covalent Compounds

Covalent compounds are formed when atoms share electrons. This electron-sharing often occurs between nonmetal atoms. In these compounds, atoms are bound together by strong covalent bonds, which occur when there is a mutual sharing of electrons from their outer electron shells. This sharing helps the atoms attain a stable electron configuration, similar to the noble gases.
Covalent compounds typically have relatively low melting and boiling points compared to ionic compounds. This is because the forces holding the molecules together, known as intermolecular forces, are not as strong as the ionic bonds holding ionic compounds together.
Characteristics of covalent compounds:

• Low melting and boiling points due to weak intermolecular forces.
• They often exist as gases or liquids at room temperature, though many can be solids.
• Poor conductors of electricity in any form since they don’t have free-moving ions or charged particles.

In the context of the exercise, with a melting point of 350 K and boiling point of 474 K, gallium chloride shows characteristics of a covalent compound.

Ionic Compounds

Ionic compounds are formed through the electrostatic attraction between positively and negatively charged ions. This typically happens between metal and nonmetal elements. Metal atoms, which have few electrons in their outer energy levels, tend to lose electrons, forming positively charged ions (cations). Nonmetals, with nearly full valence shells, tend to gain those electrons, forming negatively charged ions (anions).
Ionic compounds usually exhibit high melting and boiling points due to the strong attraction between ions. This strong bond requires more energy to break. As a result, ionic compounds are usually solid at room temperature.
Characteristics of ionic compounds:

• High melting and boiling points due to strong ionic bonds.
• Conduct electricity when dissolved in water or melted, as ions are free to move.
• Often form crystalline structures.

Given the information in the exercise, the compound does not exhibit high-temperature characteristics typical of ionic compounds, suggesting it is more likely covalent.

Oxidation States

Oxidation states indicate the degree of oxidation of an atom within a compound. They represent the hypothetical charge that an atom would have if all bonds to atoms of different elements were 100% ionic.
Oxidation states are useful in determining the formula of compounds. Each element tends to exhibit specific oxidation states in compounds based on their position in the periodic table and their electronic configuration.
For example:

• Metals like gallium usually have positive oxidation states because they tend to lose electrons.
• Nonmetals like chlorine often have negative oxidation states since they tend to gain electrons.

For gallium, a common oxidation state is +3, while chlorine usually has an oxidation state of -1. Therefore, to balance the +3 charge of one gallium ion, three chloride ions are needed, leading to the formula GaCl₃, as seen in the exercise solution.

Chemical Formulas

Chemical formulas provide essential information about the elements that make up a compound and the ratio in which these elements combine. These formulas are composed of element symbols from the periodic table and numerical subscripts.
Formulas are key in understanding the composition and properties of a compound. By knowing the chemical formula, one can infer the potential chemical behavior and interactions of the compound.
Some facts about chemical formulas:

• The subscript number indicates the number of atoms of each element in one molecule of the compound.
• If no subscript is present, it’s assumed to be 1.
• Empirical formulas show the simplest whole-number ratio of atoms in a compound.

The suggested formula GaCl₃ implies that there are three chlorine atoms for each gallium atom. Chemical formulas, such as GaCl₃ for gallium chloride, give insight into the compound’s bonding and structure, as detailed in the exercise solution.