Question

Sodium oxide, \(\mathrm{Na}_{2} \mathrm{O}\), is a white-gray powder that sublimes (goes directly from solid to gas) at \(1275{ }^{\circ} \mathrm{C}\). Is sodium oxide a molecular or nonmolecular solid?

Short answer

Sodium oxide (Na₂O) is a nonmolecular (ionic) solid, as it is formed from a metal (sodium) and a non-metal (oxygen) with a significant electronegativity difference of 2.51, indicating ionic bonding.

Step-by-step solution

Analyze the elements involved in the compound

Sodium (Na) is a metal from group 1 (alkali metals) in the periodic table, while oxygen (O) is a nonmetal from group 16 (chalcogens). Metals and nonmetals usually form ionic compounds.

Examine the electronegativity difference between the elements

Electronegativity is the ability of an atom to attract electrons around it. The electronegativity difference between sodium and oxygen can give us a clue about the type of bonding in the compound. The electronegativity values for Na and O are 0.93 and 3.44, respectively. We can calculate the difference in electronegativity between Na and O as follows: Difference = \(|3.44 - 0.93|\) = 2.51 A high electronegativity difference (typically above 1.7) usually indicates an ionic bond.

Determine the type of solid

Based on the electronegativity difference (2.51) and that sodium is a metal and oxygen a non-metal, we can conclude that the bonding in sodium oxide is ionic. Therefore, sodium oxide (Na₂O) is a nonmolecular (ionic) solid.

Key concepts

Molecular vs Nonmolecular Solids

When we classify solids, we distinguish between two types: molecular and nonmolecular. Molecular solids are composed of discrete molecules held together by relatively weak forces, such as van der Waals forces, hydrogen bonds, or dipole-dipole interactions. Examples include ice (frozen water) and dry ice (solid carbon dioxide). On the other hand, nonmolecular solids include ionic solids, metallic solids, and covalent network solids. These are characterized by strong bonds such as ionic bonds in salt, covalent bonds in diamond, or metallic bonds in copper. In the case of sodium oxide, because it's composed of ions held by the electrostatic forces known as ionic bonds, it is classified as a nonmolecular solid.

This distinction is important because it affects the properties of the solids such as their melting points, hardness, and conductivity. Nonmolecular solids usually have high melting points due to the strong bonding between atoms, whereas molecular solids tend to have lower melting points.

Ionic Bonding

Ionic bonding occurs between metals and nonmetals on the periodic table. It involves the transfer of electrons from the metal atom to the nonmetal atom, resulting in the formation of positively charged ions (cations) and negatively charged ions (anions). These opposite charges attract each other, and this electrostatic attraction constitutes an ionic bond.

Sodium oxide is an exemplary case of ionic bonding. Sodium (Na) loses an electron to become Na⁺, and oxygen (O) gains two electrons to become O^2-. The resultant Na⁺ and O^2- ions are packed in a crystal lattice where each sodium ion is surrounded by oxide ions and vice versa, creating a stable structure with strong bonds between the ions.

Electronegativity

Electronegativity is a fundamental concept in chemistry that describes the tendency of an atom to attract a shared pair of electrons. This property varies across the periodic table and helps in predicting the type of bond that will form between different elements. Higher electronegativity values indicate a stronger attraction for electrons.

The Pauling scale is commonly used to measure electronegativity, with fluorine being the most electronegative element (3.98). When predicting bond types, if the difference in electronegativity between two atoms is significant—usually greater than 1.7—the bond tends to be ionic. As in our example, sodium has a low electronegativity (0.93) while oxygen has a high one (3.44), indicating a large difference (2.51) which confirms the presence of ionic bonding in sodium oxide.

Periodic Table

The periodic table is a comprehensive chart that organizes all known elements by increasing atomic number and similar chemical properties into groups and periods. Groups are the columns that run from top to bottom, and periods are the rows that run from left to right.

Each element's location in the periodic table gives insight into its chemical behavior, including electronegativity. For instance, elements grouped on the left side (like sodium in group 1) are metals and typically have low electronegativity, being more prone to lose electrons and form cations. Conversely, elements on the right side (like oxygen in group 16) are nonmetals with high electronegativity, tending to gain electrons and form anions. Understanding the period and group trends in electronegativity is essential for predicting the types of bonds that elements will form.