Is it possible for the dispersion forces in a particular substance to be stronger than the hydrogen bonding forces in another substance?

I get that in general, LD is weaker than H-bonding, but LD does exist is both polar and nonpolar substances and what if it occurred between Fr and Fr (the biggest atom), would it be stronger than hydrogen bonding?

I think your understand is correct. It is unlikely that a Fr-Fr bond would be formed but if it were it would be a covalent bond as H-H is. And Fr does have dispersion forces as all atoms have.

In general, hydrogen bonding forces are stronger than dispersion forces. However, the strength of these intermolecular forces depends on various factors, including the size of the atoms or molecules involved.

Dispersion forces, also known as London dispersion forces or van der Waals forces, are present in all substances, regardless of their polarity. These forces arise from temporary fluctuations in electron density that induce a temporary dipole in one molecule, which in turn induces a dipole in a neighboring molecule. The strength of dispersion forces increases with the size and shape of the molecules involved.

Hydrogen bonding occurs between molecules that have a hydrogen atom bonded to a highly electronegative atom (e.g. nitrogen, oxygen, or fluorine) and another molecule with a lone pair of electrons on a highly electronegative atom. Hydrogen bonding is a form of dipole-dipole interaction and is generally stronger than dispersion forces.

In the case of two francium (Fr) atoms, which are the largest atoms on the periodic table, the strength of dispersion forces would indeed be significant due to their large size. However, due to the absence of hydrogen and highly electronegative atoms, hydrogen bonding would not occur. Thus, in this specific case, the dispersion forces between two francium atoms could be stronger than hydrogen bonding forces in a different substance. It's important to note that francium is a highly reactive and unstable element that is rarely found in nature, so studying its intermolecular forces is not very practical.

To determine whether the dispersion forces in a particular substance can be stronger than the hydrogen bonding forces in another substance, we need to consider the nature of these intermolecular forces.

Dispersion forces, also known as London dispersion forces (LD), occur between all molecules, including both polar and nonpolar substances. They arise due to temporary fluctuations in electron distribution, causing temporary partial charges. In nonpolar substances, these forces are the primary intermolecular forces. In polar substances, dispersion forces exist in addition to other intermolecular forces.

On the other hand, hydrogen bonding occurs when a hydrogen atom is bonded to a highly electronegative atom (usually nitrogen, oxygen, or fluorine) and experiences strong dipole-dipole interactions with another electronegative atom in a different molecule. Hydrogen bonding is a special type of dipole-dipole interaction that is particularly strong due to the large electronegativity difference between hydrogen and the electronegative atom.

In general, hydrogen bonding is stronger than dispersion forces because hydrogen bonding involves electrostatic attraction between charged particles (the slightly positive hydrogen atom and the electronegative atom). However, there can be instances where dispersion forces in one substance might be stronger than the hydrogen bonding forces in another substance.

In the case you mentioned, hypothetical Fr-Fr dispersion forces, it is unlikely that they would be stronger than hydrogen bonding forces. Although Francium (Fr) is a large atom with highly polarizable electrons, it is also highly reactive and can easily lose its outermost electron. Therefore, it is extremely rare to find Fr-Fr interactions since Francium atoms readily lose electrons to form ions instead of participating in intermolecular forces.

In summary, while dispersion forces can be significant in certain nonpolar substances, it is still generally true that hydrogen bonding is a stronger intermolecular force compared to dispersion forces. However, there are exceptions and unique scenarios that should be evaluated on a case-by-case basis.