OpenOChem Learn Allied Health Chem Atoms, Molecules and Ions Early Ideas of Atomic Theory

Early Ideas of Atomic Theory

Atom, from the Greek atomos which derived from the word ‘indivisible’.

Aristotle, et al. believed all matter was made up of one or more combinations of the ‘elements’ earth, air, fire, and water. This lasted for a few thousand years until people started the modern science movement – that is, hypothesizing and running experiments.

The first modern explanation of the atom arose from the work of John Dalton (British), first published in 1807.

Dalton’s Atomic Theory

Matter is composed of atoms, the smallest unit of an element.
An element consists of only one type of atom.
Atoms of one element differ in properties from atoms in a different element.
A compound has atoms of two or more elements in a small, whole-number ratio that is constant for that compound.
Atoms are neither created nor destroyed during a chemical change but are rearranged to make different substances.

The work of Dalton and Joseph Proust (France) led to the Law of Definite Proportions aka Law of Constant Composition.

The Law of Constant Composition

(Also known as the Law of Definite Proportions)

The Law of Constant Composition states that a given chemical compound always contains its component elements in a fixed ratio by mass, regardless of the source of the compound or how it was prepared.

In a clinical context, this law ensures that the molecules we interact with—whether they are hormones produced by the body or medications administered from a vial—maintain a predictable and unchanging chemical identity.

Key Principles:

Fixed Atomic Ratios: A specific compound is always composed of specific atoms in the exact same numerical ratio. For example, water is always $H_{2}O$ . This means it will always contain two hydrogen atoms for every one oxygen atom.
Identity is Tied to Structure: If you change the ratio, you change the substance. Adding just one extra oxygen atom to water ( $H_{2}O$ ) creates Hydrogen Peroxide ( $H_{2}O_{2}$ ), a compound with drastically different chemical properties and physiological effects.
Consistency of Matter: Whether a water molecule is purified from a natural spring, synthesized in a lab, or metabolic water produced inside a human cell, its composition is identical.

Why it Matters in Health Science

This law is the foundation for pharmacology and dosage calculations. When a nurse administers a glucose ( $C_{6}H_{12}O_{6}$ ) solution, the Law of Constant Composition guarantees that every molecule of glucose in that IV bag has the same ratio of carbon, hydrogen, and oxygen, ensuring the body metabolizes it in a consistent, predictable way.