Covalent bond between atoms in a molecule: electrostatic force of attraction between the localized shared pair of electrons and the 2 positively charged nucleus. See diagram below
Intermolecular bonds: as the name suggests are bonds between the molecules. See diagram below
How do intermolecular bonds come about?
Even though we draw the dot-cross diagrams as shown above, please remember that electrons are not stationary but are always in constant random motion!
(Diagram above shows an electron cloud orbiting a nucleus.. Note that the continuously changing pattern shows random motion of the electrons! – For some reason, the gif is not moving.. will find a better pic soon..)
Therefore, at any point in time, there will always be an uneven distribution of electrons in a molecule. This results in one part of the molecule having a positive charge while the other part must have a negative charge. (The fancy A level term for this is instantaneous dipole)
The instantaneous dipole in one molecule can cause the formation of dipoles in nearby unpolarized molecules.
As a result, a weak electrostatic attraction forms between the positive portion of one molecule and the negative portion of another molecule. This type of attraction is called the intermolecular bond/ Van Der Waals forces of attraction (fancy A level term = instantaneous dipole-induced dipole)
*Please note that these intermolecular bonds are fleeting in nature because the electrons are in constant motion. However, the process of bond forming and bond breaking keeps repeating such that on average, there will be attractive forces between the molecules.
How does Mr affect the strength of the intermolecular bonds?
1. The higher the Mr, the stronger the intermolecular bond (more important reason at O levels)
When Mr increases, number of protons and hence number of electrons increases. Hence, the dipoles formed by the random motion of electrons will, on average have larger +ve and larger –ve charges.
This increases the electrostatic forces of attraction between the positive portion of one molecule and the negative portion of the other molecule resulting in a stronger intermolecular bond.
2. Molecules with larger Mr have higher surface area and hence more dipole interactions (less important at O levels)
When Mr increases, the molecular size tends to increase due to increasing number of electron shells. Therefore, molecules have larger area in contact with other molecules. This increases the amount of the dipole interactions, hence forming more intermolecular bonds.
In turn, how does Mr affect m.p. and b.p. of a simple covalent substance?
Therefore, from the explanation above, higher Mr results in stronger intermolecular bonds which will require more energy to break. Therefore, m.p. and b.p. increases when Mr increases!
However, this is not the entire story. In the next post, I will discuss why some molecules with lower relative molecular masses have higher m.p./b.p. than molecules with higher relative molecular masses.
1. The above explanation only applies to simple covalent substances
2. Covalent bonds between the atoms are never broken during melting or boiling!!! (i.e. H2O after boiling still remains as H2O molecules just that the molecules are more widely spaced. It does not become hydrogen and oxygen atoms!) Therefore, simple covalent molecules have low m.p./b.p. because only the weak intermolecular bonds are broken during melting and boiling. The covalent bonds between the atoms which are very strong are not broken.