Why do compounds have fixed compositions?

Compounds have fixed compositions because they are made up of elements in a specific ratio that doesn't change.

In more detail, a compound is a substance that is formed when two or more elements chemically combine in a fixed ratio. This ratio is determined by the valency of the elements involved, which is the ability of an atom to combine with other atoms. The valency is determined by the number of electrons in the outermost shell of an atom. If an atom has a full outer shell, it is stable and does not need to combine with other atoms. However, if an atom does not have a full outer shell, it will try to achieve stability by either gaining, losing, or sharing electrons with other atoms. This results in the formation of chemical bonds and subsequently, compounds.

For example, water is a compound made up of hydrogen and oxygen. The ratio of hydrogen to oxygen in water is always 2:1. This means that for every one oxygen atom, there are two hydrogen atoms. This ratio does not change, regardless of the amount of water. Whether you have a drop of water or a whole ocean, the ratio of hydrogen to oxygen will always be 2:1. This is because the valency of oxygen is 2, meaning it needs two hydrogen atoms to achieve stability.

Therefore, the composition of a compound is fixed and does not change. This is a fundamental principle in chemistry known as the law of definite proportions or law of constant composition. It states that a given chemical compound always contains its component elements in a fixed ratio and does not depend on its source or method of preparation.