Why do elements combine in fixed ratios in compounds?

Elements combine in fixed ratios in compounds due to the Law of Constant Composition, which governs chemical reactions.

The Law of Constant Composition, also known as the Law of Definite Proportions, states that a chemical compound always contains the same elements in the same proportions by mass, regardless of the amount or source of the compound. This law is a fundamental principle in chemistry and it explains why elements combine in fixed ratios in compounds.

To understand this, let's consider water (H2O) as an example. Water is a compound made up of hydrogen and oxygen. No matter where you find water, whether it's from a river, a tap, or even from a different planet, the ratio of hydrogen to oxygen in water is always 2:1. This means that for every one oxygen atom, there are always two hydrogen atoms. This ratio is fixed and does not change, which is why we say that elements combine in fixed ratios in compounds.

This fixed ratio is due to the way atoms bond to form compounds. Atoms combine in such a way as to achieve a stable electron configuration, usually that of a noble gas. This is often achieved by sharing or transferring electrons between atoms, which forms a chemical bond. The type and number of bonds an atom can form is determined by its electron configuration, which is unique for each element. This results in a fixed ratio of elements in a compound.

For example, in carbon dioxide (CO2), one carbon atom bonds with two oxygen atoms. This is because carbon needs four electrons to achieve a stable electron configuration, and each oxygen atom can provide two electrons. Therefore, the ratio of carbon to oxygen in carbon dioxide is always 1:2.

In summary, elements combine in fixed ratios in compounds due to the Law of Constant Composition, which is a result of the unique electron configurations of each element and the way atoms bond to achieve stability.