Isomers have the same molecular formula but different structural or spatial arrangements of atoms. This difference in arrangement can lead to varying strengths and types of intermolecular forces. For instance, straight-chain isomers have more surface area in contact with neighbouring molecules than branched isomers, leading to stronger van der Waals forces and higher boiling points. Additionally, the position or type of functional group in structural isomers can drastically change the compound's reactivity. Hence, the unique structural arrangement of atoms in isomers directly influences their physical and chemical properties.

As the size of the molecule increases within a homologous series, its viscosity generally also increases. Viscosity is the measure of a fluid's resistance to shear or flow. Larger molecules have greater surface areas, leading to stronger intermolecular forces and more sites for temporary dipoles. These forces create resistance to flow, making the substance more viscous. This is why, for example, longer-chain alkanes (like motor oil) are more viscous than shorter-chain alkanes (like petrol).

The presence of double or triple bonds introduces areas of unsaturation into the molecule. Such compounds are typically more reactive than their saturated counterparts. The π bonds in double or triple bonds are more exposed and weaker than the σ bonds, making them easier targets for reactions. Physically, compounds with multiple bonds have different shapes and bond angles compared to single-bonded compounds, influencing their physical properties. Additionally, the π electrons can create temporary dipoles, impacting the molecule's intermolecular forces and, consequently, its physical properties such as boiling point and solubility.

Functional groups introduce sites of electronegativity difference or polar bonds within organic compounds. This can lead to the presence of additional intermolecular forces like dipole-dipole interactions or hydrogen bonding, alongside the usual van der Waals forces. These additional forces usually have a stronger impact than van der Waals forces alone, leading to significant changes in physical properties, like higher boiling or melting points. Furthermore, functional groups can make a compound more reactive or confer specific chemical properties upon it, distinguishing it from other members of the homologous series without that functional group.

Organic compounds within a homologous series vary in molecular size and mass. Those at the beginning of the series have smaller molecular sizes and thus weaker intermolecular forces, specifically the van der Waals or London dispersion forces. Consequently, these compounds require less energy to break these forces and are often gases at room temperature. As we progress in the series, molecular size and mass increase, strengthening the intermolecular forces. This results in the compound needing more energy to overcome these forces, and they exist as liquids or even solids at room temperature.