Why does molecular weight only refer to ionic bonds?

Question

I thought that molecular weight applies to compounds of any type? Also what is the difference between molecular weight and formula weight? Any help is much appreciated.

Answer 1

The term "molecular weight" is commonly used to refer to the sum of the atomic weights in the formula for a compound. That USED to be the convention, and a lot of chemists still use it. The problem is, in many cases, what's called "molecular weight" isn't a molecular weight at all! Two cases come to mind: 1) that of ionic salts, like NaCl. Such compounds don't have molecules. They exist as ions distributed in a crystal lattice. 2) Many compounds do indeed exist in molecular form, but the actual molecules vary in size. For instance, the gas that's usually called "nitrogen dioxide" can have molecules of NO2, N2O4, and higher multiples, of the form NnO2n. Which species predominates depends on the temperature.
Today, one normally uses the term "formula weight" in order to be correct.

Answer 2

We use the term "molecular" mass for covalent compounds, and "formula" mass for ionic compounds.

Compounds that are covalently bonded form distinct molecules, like H2O, CO2, C6H12O6, and we can talk about the mass of one molecule ("molecular" mass).

Covalent bonds form between atoms when their electronegativity difference is less than 1.7, so electrons are shared betwen them.
If the difference in electronegativity is greater than 1.7, one atom has a stronger pull on the electron that the other, and removes it from the atom, making itself a negative ion, and the other a positive ion.

The result is a bunch of positive and negative ions all attracted to each other by electrostatic forces. An ionic crystal consists of an equal number of positive and negative charges, arranged in a regular repeating pattern, eg.

Na+ Cl- Na+ Cl-
Cl- Na+ Cl- Na+
Na+ Cl- Na+ Cl-

There are no distinct molecules of NaCl.
The formula "NaCl" just means that there are an equal number of sodium and chlorine ions, so we say "formula" mass.

Answer 3

usually the order is covalent bonds>ionic bonds> hydrogen bonds>Van der Waals interactions covalent bonds rapidly connect atoms and are stable (think of diamond), ionic bonds are elctrostatic interactions between + and - ions and are additionally stable, yet no longer as stable as covalent bonds. Hydrogen bonds are a sort of dipole-dipole intermolecular interaction between molecules with partial ionic character (so are weaker than ionic bonds) and van der Waal's interactions are the intermolecular interactions that arise from quantum fluctuations of the electrons interior the atoms/molecules. there is great version in van der Waals interactions, yet in addition they perform in independent molecules/atoms and are regularly the weakest.