Question

Compare and contrast the bonding found in the \(\mathrm{H}_{2}(g)\) and \(\mathrm{HF}(g)\) molecules with that found in \(\mathrm{NaF}(s)\)

Short answer

H2(g) has a nonpolar covalent bond with a bond dissociation energy of 432 kJ/mol. HF(g) has a polar covalent bond with a bond dissociation energy of 568 kJ/mol, mainly due to fluorine's high electronegativity. In contrast, NaF(s) has a highly ionic bond with a lattice energy of 923 kJ/mol, as electrons are transferred from sodium to fluorine. Thus, the bonding in H2 is quite different from that in HF and NaF, with bond strengths differing significantly from one compound to another: NaF > HF > H2.

Step-by-step solution

Identify the type of bonds in each molecule

In H2(g) and HF(g) molecules, these are both covalent compounds formed by sharing electrons between atoms. In contrast, NaF(s) is an ionic compound, meaning it is formed by the transfer of electrons from one atom to another. Sodium (Na) donates an electron to fluorine (F), creating Na+ and F- ions that are attracted to each other, forming an ionic crystal lattice.

Analyze the electronegativity differences

Electronegativity is the ability of an atom to attract electrons in a bond. The greater the difference in electronegativity between two atoms, the greater the bond polarity, making the bond more ionic in character. The electronegativity difference in H2 is zero, as both hydrogen atoms have the same electronegativity, indicating a nonpolar covalent bond. In the HF molecule, the electronegativity of hydrogen is 2.2, and that of fluorine is 3.98. This gives a difference of 1.78, which indicates a polar covalent bond. Fluorine attracts the shared electrons more strongly, resulting in a partially negative charge on the fluorine and a partially positive charge on the hydrogen. In NaF, the electronegativity difference between sodium (0.93) and fluorine (3.98) is 3.05, indicating a highly ionic bond as electrons are transferred from sodium to fluorine.

Compare bond strengths

Bond strength can be quantified by bond dissociation energy which is the energy required to break the bond. In H2, the bond dissociation energy is 432 kJ/mol, indicating a strong covalent bond between the two hydrogen atoms. HF has an even stronger bond, with a bond dissociation energy of 568 kJ/mol. This difference arises mainly from the high electronegativity of fluorine, creating a stronger electron attraction in the covalent bond. In NaF, the bond strength is represented by the lattice energy, which is 923 kJ/mol. This is much higher than in H2 and HF, indicating that ionic bonds in NaF are stronger than the covalent bonds in H2 and HF. In summary: 1. H2 has a nonpolar covalent bond with a bond dissociation energy of 432 kJ/mol. 2. HF has a polar covalent bond with a bond dissociation energy of 568 kJ/mol. 3. NaF has an ionic bond with a lattice energy of 923 kJ/mol. Thus, the bonding in H2 is quite different from that in HF and NaF. H2 has a nonpolar bond, HF has a polar bond, and NaF has an ionic bond. Bond strengths also differ significantly from one compound to another, with NaF having the strongest bond, followed by HF, and then H2.

Key concepts

Covalent Bonds

Covalent bonds are formed when two atoms share electrons. This sharing allows each atom to achieve a stable electron configuration, similar to that of noble gases. In covalent bonding, the electrons involved in the bond do not belong to a single atom but are shared between the atoms.

These bonds can be classified into two types:

• Nonpolar covalent bonds: This occurs when the electrons are shared equally between the atoms. A classic example is the hydrogen molecule (\( ext{H}_2 \)). Here, both hydrogen atoms have the same electronegativity, resulting in equal sharing of electrons.
• Polar covalent bonds: These bonds form when there is an unequal sharing of electrons between atoms. This is due to a difference in electronegativity between the two atoms. In a polar covalent bond, the more electronegative atom will attract the shared electrons more strongly. For instance, in hydrogen fluoride (\( ext{HF} \)), the fluorine atom is more electronegative than hydrogen, resulting in a partial negative charge on fluorine and a partial positive charge on hydrogen.

Understanding covalent bonds is crucial, as they form the backbone of many organic and inorganic compounds that are vital to life.

Ionic Bonds

Ionic bonds form when electrons are transferred from one atom to another, leading to the creation of ions. One atom, usually a metal, loses an electron to become a positively charged ion. Meanwhile, the other atom, generally a non-metal, gains an electron, becoming a negatively charged ion. This electron transfer results in the formation of an ionic compound.

In the context of our example, sodium fluoride (\( ext{NaF} \)):

• Sodium (\( ext{Na} \)), a metal, donates an electron, thus becoming Na⁺.
• Fluorine (\( ext{F} \)), a non-metal, accepts an electron, forming F⁻.

These oppositely charged ions attract each other strongly, forming a robust ionic lattice structure.

It is in these dense lattice structures that ionic compounds often exhibit high melting and boiling points, alongside excellent electrical conductivity when dissolved in water. This is due to the strong electrostatic forces holding the ions together.

Electronegativity

Electronegativity is a measure of an atom's ability to attract and hold onto electrons within a chemical bond. Each element has a specific electronegativity value, which influences the nature of the bonds it forms.

The concept of electronegativity is essential for predicting bond types:

• A small or no difference in electronegativity between atoms results in nonpolar covalent bonds, as seen in hydrogen gas (\( ext{H}_2 \)
• Moderate differences lead to polar covalent bonds, like in hydrogen fluoride (\( ext{HF} \)
• Large differences result in ionic bonds, exemplified by sodium fluoride (\( ext{NaF} \)

Electronegativity not only determines how atoms bond but also how molecules interact. Stronger electronegative atoms can attract electrons more effectively. This creates polarity in the molecules, impacting their physical properties and behavior in various chemical reactions.