Why do isomers have the same molecular formula but different properties?

Isomers have the same molecular formula but different properties due to their different structural arrangements.

Isomers are compounds that have the same molecular formula, meaning they have the same number and type of atoms, but they differ in the way these atoms are arranged. This difference in structural arrangement leads to different physical and chemical properties.

The concept of isomerism is a fundamental aspect of organic chemistry. There are two main types of isomerism: structural (or constitutional) isomerism and stereoisomerism. Structural isomers have their atoms connected in a different order. For example, butane and isobutane are structural isomers. They both have the formula C4H10, but the arrangement of carbon atoms differs. This difference in structure results in different boiling points, melting points, and other physical properties.

Stereoisomers, on the other hand, have the same connectivity but a different arrangement of atoms in space. This type of isomerism includes geometric isomers (also known as cis-trans isomers) and optical isomers. Geometric isomers occur in compounds with restricted rotation (like double bonds or ring structures), where atoms can occupy different positions relative to each other. Optical isomers are mirror images of each other, like left and right hands. They can rotate plane-polarised light in different directions, hence the name 'optical'.

The different structural arrangements of isomers can also lead to differences in chemical reactivity. For instance, the reactivity of an organic compound often depends on the functional group (like OH, COOH, NH2 etc.) present in the molecule. In structural isomers, the position of the functional group can vary, which can significantly affect the compound's reactivity.

In summary, isomers have the same molecular formula but different properties because their atoms are arranged differently. This difference in arrangement can affect both the physical properties (like boiling point, melting point) and chemical properties (like reactivity) of the isomers.