Problem 39 - General Chemistry: Principles and Modern Applications

Question

What is the percent ionic character of each of the following bonds: (a) \(\mathrm{S}-\mathrm{H} ;\) (b) \(\mathrm{O}-\mathrm{Cl} ;\) (c) \(\mathrm{Al}-\mathrm{O}\) (d) As - O?

Step-by-Step Solution

Answer

After doing all of the calculations, the percentage ionic characters for the given bonds (approx) are: (a) S-H: 9.4%, (b) O-Cl: 13%, (c) Al-O: 57%, (d) As-O: 6%.

Step 1: Determine the Electronegativity Values

Look up the electronegativity values of the atoms in each bond from a trusted source such as a textbook or reliable online resource. For instance, typical values are: S (2.58), H (2.20), O (3.44), Cl (3.16), Al (1.61), and As (2.18).

Step 2: Calculate the Difference in Electronegativity

For each bond (S-H, O-Cl, Al-O, As-O), calculate the absolute difference in electronegativity values. For example, for the bond S-H, the difference would be |2.58 - 2.20| = 0.38. Repeat this calculation for all bonds.

Step 3: Calculate the Percent Ionic Character

Using the formula: Percent ionic character = 16(XA - XB) + 3.5(XA - XB)^2 , substitute the calculated electronegativity differences from step 2 into the formula to determine the percent ionic character for each bond. Here, (XA - XB) is the difference in electronegativity. For the S-H bond, the percent ionic character would be \(16 \times 0.38 + 3.5 \times 0.38^2\). Calculate this for all bonds.

Key Concepts

Electronegativity

Electronegativity is a key concept in understanding how atoms interact in a molecule. It refers to an atom's ability to attract and hold onto electrons when it is part of a compound.
Different elements have different electronegativity values, which are often published in tables. For example, elements like fluorine have very high electronegativity, while alkali metals like sodium have lower values.

Understanding electronegativity is crucial because it helps predict how atoms will bond with each other.

• If two atoms have similar electronegativities, they'll likely share electrons fairly equally, forming a covalent bond.
• However, if there is a significant difference in electronegativity, the bond may become polar or ionic, with electrons being unequally shared or transferred entirely.

This concept is foundational for determining the nature of the bond and the percent ionic character, which indicates how electron sharing or transfer defines the bond.

Ionic Bonds

Ionic bonds are formed when there is a complete transfer of electrons from one atom to another. This typically occurs when there's a large difference in electronegativity between two atoms. The atom with the higher electronegativity will attract and gain the electron, becoming a negatively charged ion, or anion.
The atom with lower electronegativity will lose an electron, thus forming a positively charged ion called a cation.

For instance, in table salt (NaCl), sodium (Na) has a much lower electronegativity compared to chlorine (Cl). Sodium donates an electron to chlorine, resulting in a typical ionic bond formation.

• Ionic bonds are usually strong and lead to the formation of crystalline solids.
• They dissolve in water to produce ions, which are conductive to electricity.

While not all compounds have purely ionic or covalent bonds, understanding ionic bonds helps explain many properties of compounds.

Chemical Bonding

Chemical bonding encompasses all the types of attractions that hold atoms together in compounds. The main types are ionic, covalent, and metallic bonds, with ionic and covalent being most common in chemical compounds.
Covalent bonds involve the sharing of electron pairs between atoms. These can further be categorized into nonpolar (equal sharing) and polar covalent bonds (unequal sharing).

Ionic bonds, as mentioned earlier, involve electron transfer. The type of bond influences a compound's properties significantly.

• Covalently bonded compounds may have low melting and boiling points and might not dissolve easily in water.
• Ionic compounds often have higher melting and boiling points due to strong electrostatic forces and typically dissolve in water to form electrolytes.

Understanding chemical bonding is essential to foresee the reactivity, state of matter, and other physical properties of a compound.

Electronegativity Difference

The electronegativity difference between two atoms determines the type of bond they will form. By calculating this difference, we can estimate how ionic or covalent a bond will be.
For example, if two atoms have identical or very similar electronegativity values, they will share electrons pretty evenly, resulting in a nonpolar covalent bond.
On the other hand, a high electronegativity difference, usually above 1.7, indicates that the bond will likely be ionic, with electrons fully transferring to the more electronegative atom.

• For instance, the NaCl bond has a high electronegativity difference, leading to salt's typical ionic nature.
• These differences also contribute to understanding the percent ionic character of a bond, reflecting how "ionic-like" a covalent bond can be.

The electronegativity difference is therefore a practical tool that helps predict and explain the nature and behavior of chemical bonds.