Question

Covalent molecules that contain an \(\mathrm{O}-\mathrm{H}, \mathrm{N}-\mathrm{H}\), or \(\mathrm{F}-\mathrm{H}\) bond have very strong intermolecular forces. Explain why.

Short answer

Covalent molecules with O-H, N-H, or F-H bonds have strong intermolecular forces due to the highly electronegative nature of oxygen, nitrogen, and fluorine atoms. These atoms create highly polar bonds, with hydrogen carrying a partial positive charge (δ+) and the other atom carrying a partial negative charge (δ-). The strong intermolecular forces result from hydrogen bonding, a special type of dipole-dipole interaction where the already covalently bonded hydrogen forms an additional weak bond with another electronegative atom on a neighboring molecule.

Step-by-step solution

Introducing Covalent Bonds

In covalent molecules, atoms share electrons to form covalent bonds. The strength of these bonds depends on the electronegativity of the atoms involved. Electronegativity is the ability of an atom to attract shared electrons in a covalent bond towards itself.

Electronegativity of Involved Atoms

In the given covalent molecules (O-H, N-H, and F-H), oxygen, nitrogen, and fluorine atoms are very electronegative elements. This means that they have a strong ability to attract electrons towards themselves when they form covalent bonds with hydrogen.

Polarity of O-H, N-H, and F-H Bonds

Due to the high electronegativity of oxygen, nitrogen, and fluorine, the O-H, N-H and F-H covalent bonds are highly polar. The electron density in these bonds is unevenly distributed, creating partially positive (δ+) and partially negative (δ-) charges. In these cases, the hydrogen atom carries a partial positive charge (δ+) while the oxygen, nitrogen, or fluorine atom carries a partial negative charge (δ-).

Hydrogen Bonding

The strong intermolecular forces observed in molecules containing O-H, N-H, or F-H bonds are due to a special type of dipole-dipole interaction called hydrogen bonding. Hydrogen bonding occurs when a hydrogen atom, already covalently bonded to a highly electronegative atom (such as O, N, or F), forms an additional weak bond with another electronegative atom on a neighboring molecule. The highly polar nature of the O-H, N-H, and F-H bonds allows for this strong intermolecular attraction.

Explaining the Strong Intermolecular Forces

In conclusion, covalent molecules containing O-H, N-H, or F-H bonds have very strong intermolecular forces due to the highly electronegative nature of oxygen, nitrogen, and fluorine atoms. The electronegativity results in highly polar bonds and the formation of hydrogen bonds, which contribute to stronger intermolecular forces compared to other covalent molecules.

Key concepts

Covalent Bonds

Covalent bonds are the glue that holds molecules together in the world of chemistry. When two atoms come together and share electrons, we have what is known as a covalent bond. This sharing allows each atom to achieve a more stable electronic configuration, often akin to the coveted noble gas structure. Imagine two friends sharing a delicious pie; both savour the sweetness without completely owning it—this is the essence of covalent bonding.

It's not always an equal share, though. Depending on the atoms' needs and powers of persuasion, or electronegativity (more on that shortly), one might get a larger slice of the electron pie than the other. These intricacies give rise to a variety of covalent bonds, from nonpolar, where sharing is caring and equal, to polar, where one atom strong-arms electrons more toward itself.

Electronegativity

Electronegativity is essentially a measure of how much an atom behaves like the kid who wants all the toys. In technical terms, it measures an atom's ability to attract and hold onto the electrons in a bond. High electronegativity means the atom is like a magnet for electrons; they just can't resist its pull.

On the periodic table, electronegativity increases as you move from left to right and from bottom to top. Thus, elements like fluorine, oxygen, and nitrogen sit at the high end of the scale, greedily attracting electrons in covalent bonds. Understanding electronegativity is crucial for predicting how electrons will be distributed in a molecule and therefore, how molecules will interact with each other.

Polarity of Molecules

When we talk about polarity in molecules, we're basically discussing the tug-of-war over electrons between different atoms. If one atom has a higher electronegativity, it will pull the shared electrons closer, creating a polar covalent bond. This inequitable sharing creates a dipole, with partially positive and negative ends—just like a magnet.

Different regions of the molecule carry these 'partial charges,' symbolized by δ+ (slightly positive) and δ- (slightly negative). Molecules with polar covalent bonds, such as water (H2O), have this molecular magnetism. As a result, they can cling to each other or to other charged particles. This property is central to the behavior of many substances, from the water in your glass to the proteins in your body.

Intermolecular Forces

Intermolecular forces are akin to the social ties between people; they dictate how molecules recognize and stick to one another. These forces point to the attraction or repulsion between molecules or between parts of the same molecule. Among the types of intermolecular forces, there's one standout in terms of strength—the hydrogen bond. It's more like a special handshake than an actual bond.

The hydrogen bond occurs when the hydrogen atom from one molecule (which carries a δ+ charge from being in a polar bond with a highly electronegative atom like oxygen) plays footsie with an electronegative atom (carrying a δ- charge) from a neighboring molecule. This interaction is pretty strong, albeit weaker than a covalent bond but significantly stronger than other intermolecular forces, such as London dispersion forces or dipole-dipole interactions. These mighty hydrogen bonds are the reason behind water's high boiling point and the unique properties of DNA.