Question

(a) State whether or not the bonding in each substance is likely to be covalent: (i) iron, (ii) sodium chloride, (iii) water, (iv) oxygen, (v) argon. (b) A substance XY, formed from two different elements, boils at \(-33^{\circ} \mathrm{C}\) . Is XY likely to be a covalent or an ionic substance?

Short answer

(a) (i) Iron: not covalent, (ii) Sodium chloride: not covalent (ionic), (iii) Water: covalent, (iv) Oxygen: covalent, (v) Argon: no covalent bonding (noble gas). (b) Substance XY with a boiling point of \(-33^{\circ} \mathrm{C}\) is likely to be a covalent substance due to its low boiling point.

Step-by-step solution

(a) Identifying Covalent Bonds

(i) Iron (Fe) is a metal, and there are no non-metal atoms involved, so the bonding is not covalent. (ii) Sodium chloride (NaCl) consists of a metal (sodium) and a non-metal (chlorine) so the bonding is ionic, not covalent. (iii) Water (H2O) consists of non-metals (hydrogen and oxygen), so the bonding is covalent. (iv) Oxygen (O2) consists of non-metal atoms (oxygen), so the bonding is covalent. (v) Argon (Ar) is a noble gas and does not form bonds, so there is no covalent bonding.

(b) Determining the Bond Type in Substance XY

The substance XY has a boiling point of \(-33^{\circ} \mathrm{C}\). Covalent substances typically have lower boiling points than ionic substances because the intermolecular forces are weaker in covalent substances. Since the boiling point of XY is low, it is more likely to be a covalent substance.

Key concepts

Understanding Covalent Bonds

Covalent bonds are formed when two non-metal atoms share electrons to fill their outermost electron shells. This sharing allows each atom to attain a full outer shell, mimicking the stable electron configuration of noble gases. Covalent bonding typically occurs between atoms with similar electronegativities, allowing for an equal or nearly equal sharing of electrons.
In molecules like water (\(\text{H}_2\text{O}\)) and oxygen (\(\text{O}_2\)), covalent bonds are prevalent:

• Water is a prime example of polar covalent bonds, where oxygen and hydrogen share electrons, but oxygen has a greater pull, creating a dipole moment.
• In diatomic molecules like oxygen, electrons are shared equally, resulting in nonpolar covalent bonds.

Understanding covalent bonds helps explain many molecular properties, such as low boiling and melting points, as these molecules are held together by weaker intermolecular forces than ionic compounds.

Exploring Ionic Bonds

Ionic bonds differ from covalent bonds as they result from the complete transfer of electrons from a metal to a non-metal. This electron transfer creates ions: positively charged cations and negatively charged anions. These oppositely charged ions attract each other and form a strong ionic bond.
Sodium chloride (\(\text{NaCl}\)) is a textbook example of ionic bonding:

• Sodium (Na), a metal, loses an electron to become a positively charged ion (\(\text{Na}^+\)).
• Chlorine (Cl), a non-metal, gains this electron to become a negatively charged ion (\(\text{Cl}^-\)).

The strong attraction between the sodium and chloride ions results in a structured ionic lattice, which accounts for ionic compounds' high melting and boiling points, contrasting with the typically lower boiling points seen in covalent compounds.

Noble Gases and Bond Formation

Noble gases, like argon, helium, neon, and others, are unique due to their complete electron shells. This full valence shell makes them exceptionally stable and largely inert, meaning they have little tendency to form chemical bonds under normal conditions.
Their inertness is a critical aspect of understanding chemical bonding because:

• Noble gases are often used as a reference point for understanding the stability other elements seek through bonding.
• They help explain why certain actions, such as covalent or ionic bonding, occur in other elements, as these elements strive to achieve a noble gas-like electronic configuration.

In most conditions, noble gases remain unreactive, although under specific circumstances and with significant energy, they can form compounds, offering compelling insights into the nature of chemical bonds.

Boiling Point and Bond Types

The boiling point of a substance provides insight into the type of bonding present. For instance, a lower boiling point generally suggests covalent bonding, characterized by weaker intermolecular forces compared to ionic or metallic bonds. These weaker forces mean that less energy is needed to separate the molecules and transition the substance to a gaseous state.
Consider the example of substance XY, boiling at \(-33^{\circ} \text{C}\):

• This low boiling point implies the presence of covalent bonds, which typically result in substances with lower boiling points.
• Ionic compounds, on the other hand, usually have significantly higher boiling and melting points due to strong electrostatic forces in their crystalline lattice structure.

By understanding boiling points, one can deduce the type of chemical bonding and the nature of the intermolecular forces at play.

Intermolecular Forces and Their Impact

Intermolecular forces are the forces that exist between molecules, influencing physical properties such as boiling point and solubility. These forces are crucial in understanding how molecules interact in different states.

• In covalent compounds, these forces include dipole-dipole interactions, hydrogen bonds, and London dispersion forces.
• They are generally weaker than the ionic or covalent bonds within a molecule, yet they play a significant role in determining a substance's phase at a given temperature.

Intermolecular forces help explain why covalent compounds usually have lower boiling and melting points compared to ionic substances, which are held together by strong ionic bonds. Recognizing these forces allows chemists to predict the behavior of substances in various conditions, enhancing our understanding of their practical applications.