Elements, Atoms and Molecules

Matter around us — a stick of chalk, the air we breathe, the water we drink — is built from a small set of

fundamental building blocks. Understanding those building blocks, and the precise language chemists use for

them, is the first step in the study of chemistry.

Element. An element is the simplest form of matter that cannot be broken down into any simpler substance

by ordinary chemical means. Every atom of a given element has the same number of protons in its nucleus (its

atomic number). Iron ($\text{Fe}$), oxygen ($\text{O}$), gold ($\text{Au}$) and sodium ($\text{Na}$) are all

elements — no chemical reaction can split iron into anything simpler than iron. As of today, 118 elements are

known, of which about 94 occur naturally on Earth; the rest have been made artificially in laboratories.

Atom. An atom is the smallest particle of an element that retains all the chemical properties of that

element. Atoms themselves are made of even smaller sub-atomic particles — protons and neutrons in a dense

central nucleus, and electrons occupying the space around it — but for the purposes of a chemical reaction, the

atom is treated as the fundamental, indivisible unit. Atoms are extraordinarily small: the radius of a typical

atom is of the order of $10^{-10}\ \text{m}$, and a full stop on this page contains more atoms of carbon than

there are stars in the observable universe.

Molecule. A molecule is formed when two or more atoms join together by chemical bonds. It is the smallest

particle of an element or a compound that can exist independently under ordinary conditions and still show all

the properties of that substance.

• A molecule made of atoms of the same element is called a homoatomic molecule — for example, oxygen gas exists as $\text{O}_2$ (two oxygen atoms bonded together), and elemental sulphur commonly exists as $\text{S}_8$ (eight sulphur atoms in a ring).
• A molecule made of atoms of different elements is called a heteroatomic molecule, or a molecule of a compound — for example, water, $\text{H}_2\text{O}$, is made of two hydrogen atoms bonded to one oxygen atom, and carbon dioxide, $\text{CO}_2$, is made of one carbon atom bonded to two oxygen atoms.

Compound vs. mixture. It is important not to confuse a compound (atoms of different elements chemically

bonded in a fixed ratio, like $\text{H}_2\text{O}$) with a mixture (two or more substances simply mixed

together, in no fixed ratio, with no new chemical bond formed, like salt stirred into water). A compound has

properties completely different from the elements that form it — sodium is a reactive silvery metal and

chlorine is a poisonous greenish gas, yet together as $\text{NaCl}$ they form ordinary, edible table salt.

Atomicity. The number of atoms present in one molecule of a substance is called its atomicity. Noble gases

like helium ($\text{He}$) exist as single, unbonded atoms and are said to be monatomic (atomicity 1). Oxygen

($\text{O}_2$), nitrogen ($\text{N}_2$) and hydrogen ($\text{H}_2$) are diatomic (atomicity 2). Ozone

($\text{O}_3$) is triatomic, and glucose ($\text{C}_6\text{H}_{12}\text{O}_6$) is polyatomic, with an

atomicity of 24 (6 carbon + 12 hydrogen + 6 oxygen atoms).

These three words — element, atom, molecule — are the vocabulary the rest of this chapter is built on: the

laws of chemical combination describe how atoms combine, the mole concept lets us count them by weighing them,

and stoichiometry uses balanced equations written in terms of atoms and molecules to predict how much product

a reaction will give.