Question

Based on electronegativity differences, would you expect bonds between the following pairs of atoms to be largely ionic or largely covalent? (a) \(\mathrm{Be}\) and \(\mathrm{F}\) (b) \(\mathrm{Ca}\) and \(\mathrm{Cl}\) (c) \(\mathrm{O}\) and \(\mathrm{H}\) (d) \(\mathrm{Be}\) and \(\mathrm{Br}\)

Short answer

(a) Ionic, (b) Ionic, (c) Covalent, (d) Covalent.

Step-by-step solution

Understanding Electronegativity

Electronegativity is a measure of the tendency of an atom to attract a bonding pair of electrons. Generally, if the difference in electronegativity between two atoms is greater than 1.7, the bond is considered largely ionic. If the difference is less than 1.7, the bond is considered largely covalent.

Find Electronegativity Values

Look up the electronegativity values for each atom: - Be: 1.57 - F: 3.98 - Ca: 1.00 - Cl: 3.16 - O: 3.44 - H: 2.20 - Br: 2.96 These values allow us to calculate the difference between each pair of atoms.

Calculate Electronegativity Differences

Calculate the differences:(a) \( \mathrm{Be - F} \): \( 3.98 - 1.57 = 2.41 \)(b) \( \mathrm{Ca - Cl} \): \( 3.16 - 1.00 = 2.16 \)(c) \( \mathrm{O - H} \): \( 3.44 - 2.20 = 1.24 \)(d) \( \mathrm{Be - Br} \): \( 2.96 - 1.57 = 1.39 \)

Determine Bond Type

Based on the electronegativity differences calculated:(a) \( \mathrm{Be} \) and \( \mathrm{F} \): 2.41 > 1.7, largely ionic.(b) \( \mathrm{Ca} \) and \( \mathrm{Cl} \): 2.16 > 1.7, largely ionic.(c) \( \mathrm{O} \) and \( \mathrm{H} \): 1.24 < 1.7, largely covalent.(d) \( \mathrm{Be} \) and \( \mathrm{Br} \): 1.39 < 1.7, largely covalent.

Key concepts

Ionic bond

An ionic bond is a type of chemical bond that is formed due to the transfer of electrons from one atom to another. This usually occurs between metals and non-metals. In an ionic bond, one atom donates an electron (or electrons), becoming a positively charged ion, while another atom accepts the electron, becoming negatively charged. The attraction between these oppositely charged ions holds the compound together.

Here are some key characteristics of ionic bonds:

• Generally form between metal and non-metal elements.
• Result from large differences in electronegativity (typically greater than 1.7).
• Form compounds that are usually solid at room temperature.
• Have high melting and boiling points.
• Conduct electricity when dissolved in water, as ions are free to move.

In the exercise, we see examples like beryllium and fluorine (Be-F), and calcium and chlorine (Ca-Cl), where the electronegativity differences are significant. This indicates that these pairs of atoms would form largely ionic bonds.

Covalent bond

A covalent bond is another fundamental type of chemical bond where atoms share electrons. Unlike ionic bonds that involve the transfer of electrons, covalent bonds involve the sharing of electron pairs between atoms. This typically occurs between non-metal elements.

Covalent bonds have these notable features:

• Form between atoms with similar electronegativities (difference less than 1.7).
• Can form single, double, or triple bonds depending on the number of shared electrons.
• Usually result in the formation of molecules rather than ionic compounds.
• Have lower melting and boiling points compared to ionic compounds.
• Do not conduct electricity, as there are no free ions or electrons.

The original exercise includes the pairs oxygen and hydrogen (O-H), and beryllium and bromine (Be-Br), both having electronegativity differences below 1.7. This suggests that these atoms will largely form covalent bonds by sharing electrons rather than transferring them.

Electronegativity difference

Electronegativity difference is crucial in determining the type of bond that will form between two atoms. It is defined as the absolute difference in the electronegativity values of two bonded atoms. This value helps predict whether a bond will be ionic or covalent.

Let's understand how electronegativity difference defines bond type:

• If the difference in electronegativity is greater than 1.7, the bond is typically ionic. This means that one atom strongly attracts electrons, leading to electron transfer.
• If the difference is less than 1.7, a covalent bond is more likely, indicating sharing, rather than transfer, of electrons.

The exercise guides you through calculating these differences, using electronegativity values like those of oxygen (3.44), hydrogen (2.20), and others, which are found in tables and reference materials. Knowing these values empowers chemists to predict molecular behavior and interactions.