Question

Consider HF and HCl: a) Which is the bigger atom, \(\mathrm{F}\) or \(\mathrm{Cl}\) ? b) Which has the longer bond length, \(d\), HF or HCl? c) Which has the greater partial charge \(-\mathrm{F}\) in \(\mathrm{HF}\) or \(\mathrm{Cl}\) in \(\mathrm{HCl}\) ? (Hint: which is more electronegative \(-\mathrm{F}\) or \(\mathrm{Cl} ?\) ) d) Why is the dipole moment of HF larger than the dipole moment of HCl? That is, which appears to be the more important factor in determining the dipole moment: the bond length or the partial charge?

Short answer

a) Cl is the bigger atom. b) HCl has a longer bond length. c) F in HF has the greater partial charge. d) The dipole moment of HF is larger than HCl due to the greater partial charge (difference in electronegativity) in HF, demonstrating that partial charge is the more important factor in determining dipole moment.

Step-by-step solution

Compare the Size of F and Cl

Looking at the periodic table, both fluorine (F) and chlorine (Cl) are in the same group (Group 17, the halogens), but fluorine is in period 2 while chlorine is in period 3. Since atomic size or radius tends to increase down a group, chlorine (Cl) is the bigger atom.

Compare the Bond Length of HF and HCl

Considering step 1, where Cl was established as the larger atom compared to F, the bond between H and Cl would be longer. Bond length increases with atomic size. Hence, HCl has a longer bond length than HF.

Compare the Partial Charge (Electronegativity) of F in HF and Cl in HCl

Electronegativity is a measure of the ability of an atom to attract electrons in a bond. It typically increases across a period. Therefore, as fluorine (F) is positioned to the right of chlorine (Cl) on the periodic table, fluorine is more electronegative. Consequently, fluorine in HF will have a greater partial charge than chlorine in HCl.

Determine What Factors Influence the Dipole Moment

The dipole moment of a molecule is determined by both the difference in electronegativity and the separation distance between the atoms, or the bond length. Despite having a greater bond length, HCl's smaller difference in electronegativity (lower partial charge) results in a smaller dipole moment than that of HF. The more important factor in determining the dipole moment between HF and HCl is the electronegativity difference (the partial charge), not the bond length.

Key concepts

Atomic Size and Periodic Trends

Understanding the concept of atomic size and how it changes within the periodic table is fundamental in chemistry. The periodic table is arranged in such a way that provides a visual representation of these trends. Atomic size, or atomic radius, generally increases as we move down a group due to the addition of more electron shells. Conversely, atomic size tends to decrease across a period from left to right as the effective nuclear charge pulling the electrons closer increases.

For instance, when comparing the size of fluorine (F) and chlorine (Cl), which are in the same group but different periods, chlorine is the larger atom. This increase in size impacts properties such as bond lengths in molecules. Larger atoms like chlorine will form longer bonds with other atoms, such as hydrogen, compared to smaller atoms like fluorine. The trend in atomic size is crucial in predicting and explaining the behavior and properties of different elements and compounds in chemical reactions.

Bond Length Comparison

Bond length is a key parameter in molecular structure and can greatly influence a molecule's physical and chemical properties. It refers to the distance between the nuclei of two bonded atoms. Several factors can affect bond length, including atomic size and the type of bond formed (single, double, or triple).

As a rule of thumb, larger atomic sizes result in longer bond lengths. This is due to the larger orbitals that have to overlap to form the bond, which are naturally more diffuse, and therefore, the bond length is increased. Comparing molecules such as HF and HCl, we can see this principle at work. Since chlorine has a larger atomic size than fluorine, the bond length between hydrogen and chlorine in HCl is longer than that between hydrogen and fluorine in HF. Therefore, knowing the relative sizes of atoms provides a good first approximation for comparing bond lengths in molecules.

Electronegativity and Partial Charges

Electronegativity is a measure of an atom's tendency to attract the shared electrons in a chemical bond. The concept of electronegativity is paramount when predicting the type of bond that will form between atoms and how the electrons will be distributed in a molecule. High electronegativity corresponds to a greater ability to attract electrons, leading to the formation of partial negative charges on the more electronegative atom in a polar bond.

For example, when examining HF versus HCl, the partial charges that develop within these molecules are directly tied to the electronegativities of fluorine and chlorine. Fluorine is more electronegative than chlorine, meaning that in the molecule HF, fluorine will attract the bonding electrons more strongly than hydrogen. This results in a significant partial negative charge on the fluorine atom and a corresponding partial positive charge on the hydrogen atom. This uneven distribution of charge creates a dipole moment, where the magnitude is influenced by both the electronegativity and the distance between the involved atoms (bond length).