Problem 28 - Chemistry and Chemical Reactivity

Question

For each of the bonds listed below, tell which atom is the more negatively charged. (a) \(\mathbf{C}-\mathbf{N}\) (b) \(\mathbf{C}-\mathbf{H}\) (c) \(\mathrm{C}-\mathrm{Br}\) (d) \(\mathrm{S}-\mathrm{O}\)

Step-by-Step Solution

Answer

(a) N, (b) C, (c) Br, (d) O.

Step 1: Understand Electronegativity

Electronegativity is the ability of an atom to attract shared electrons in a chemical bond. The higher the electronegativity, the more strongly an atom can attract electrons.

Step 2: Recall Electronegativity Trends

Electronegativity increases across a period from left to right and decreases down a group in the periodic table. This means that atoms like fluorine, oxygen, and nitrogen are more electronegative compared to carbon and hydrogen.

Step 3: Compare Electronegativity of C and N

Nitrogen (N, electronegativity = 3.0) is more electronegative than Carbon (C, electronegativity = 2.5). Therefore, in the bond C-N, nitrogen will be more negatively charged.

Step 4: Compare Electronegativity of C and H

Carbon (C, electronegativity = 2.5) is slightly more electronegative than Hydrogen (H, electronegativity = 2.1). Therefore, in the bond C-H, carbon will be more negatively charged.

Step 5: Compare Electronegativity of C and Br

Bromine (Br, electronegativity = 2.8) is more electronegative than Carbon (C, electronegativity = 2.5). Therefore, in the bond C-Br, bromine will be more negatively charged.

Step 6: Compare Electronegativity of S and O

Oxygen (O, electronegativity = 3.5) is more electronegative than Sulfur (S, electronegativity = 2.5). Therefore, in the bond S-O, oxygen will be more negatively charged.

Key Concepts

Chemical Bonding

Chemical bonding is the phenomenon where atoms come together to form molecules, sharing or transferring electrons in the process. The type of chemical bond formed between atoms depends on their electronegativity, or their ability to attract electrons. There are different types of chemical bonds:

• Ionic Bonds: These occur when electrons are transferred from one atom to another, usually between metals and nonmetals. This transfer results in the formation of positively and negatively charged ions.
• Covalent Bonds: These bonds result from the sharing of electrons between two nonmetals. The electrons are shared to fill the outer electron shell of each atom, stabilizing them.
• Polar Covalent Bonds: In these bonds, electrons are shared unevenly due to differences in electronegativity. This results in one atom having a slight negative charge and the other a slight positive charge.

Understanding the type of chemical bond is essential to comprehend how substances interact at the molecular level. This interaction dictates the physical and chemical properties of the resulting compounds.

Periodic Trends

Periodic trends refer to the predictable pattern of variation of certain properties of elements in the periodic table. One such property is electronegativity, which shows us how atoms tend to attract electron pairs.
Electronegativity generally increases as you move from left to right across a period on the periodic table. This is because the number of charges on the nucleus increases, pulling the electrons closer to the nucleus.
Conversely, electronegativity tends to decrease as you move down a group. This decrease is due to the additional electron shells, which increase the distance between the nucleus and the valence electrons, reducing the nucleus' pull on these electrons.

• Elements like fluorine, that are high up and to the right on the periodic table, exhibit high electronegativity.
• Elements toward the bottom left, like cesium, show low electronegativity.

Understanding these trends helps us predict the behavior of atoms in bonds.

Atomic Charge

When we discuss atomic charge, we refer to the charge an atom carries when participating in a bond. This charge is often influenced by the difference in electronegativity between the bonded atoms.

• Atoms with higher electronegativity tend to pull shared electrons closer, acquiring a partial negative charge.
• Conversely, atoms with lower electronegativity become slightly positive by losing some electron density.

This concept of partial charges is what gives rise to polar bonds, where there is a distinct positive and negative side of the molecule. It's key to note that these partial charges greatly affect the molecule's interactions and behavior in chemical reactions.

Comparison of Electronegativity

Comparing electronegativity involves looking at the ability of different atoms to attract shared electrons. By comparing, we can determine which atom in a bond is more negatively charged.
Take the carbon-nitrogen bond as an example. Nitrogen, with an electronegativity of 3.0, is more electronegative than carbon with 2.5. Thus, nitrogen attracts the electrons more strongly, making it slightly more negative.

• C-N Bond: Nitrogen is more negative than carbon.
• C-H Bond: Carbon is more negative than hydrogen.
• C-Br Bond: Bromine is more negative than carbon.
• S-O Bond: Oxygen is more negative than sulfur.

This comparison helps us understand the polarity of bonds and predict their behavior in molecular interactions. Understanding which atom is more electronegative can give insights into the distribution of electrons and the resulting partial charges in the molecule.