Atoms and Molecules

📚 Explained

Laws of Chemical Combination

Two beautiful laws were discovered by experiments by Lavoisier and Proust before anyone could even see an atom. The Law of Conservation of Mass states that mass can neither be created nor destroyed in a chemical reaction. This means the total mass of the reactants (the substances you start with) is always exactly equal to the total mass of the products (the substances you make). If 12 g of carbon burns with 32 g of oxygen, you always get exactly 44 g of carbon dioxide — not a milligram more or less.

The Law of Constant Proportions (also called the Law of Definite Proportions) states that in a pure chemical compound, the elements are always present in a fixed ratio by mass. For example, pure water always contains hydrogen and oxygen in the mass ratio 1:8, whether it comes from a river, a tap, or a lab. Carbon dioxide always has carbon and oxygen in the ratio 3:8. These fixed ratios are a fingerprint of every compound.

Dalton's Atomic Theory

In 1808, John Dalton explained these laws with a brilliant idea: all matter is made of tiny indivisible particles called atoms. His postulates were: (1) all matter is made of atoms; (2) atoms cannot be created, divided or destroyed; (3) atoms of a given element are identical in mass and chemical properties; (4) atoms of different elements have different masses and properties; (5) atoms combine in small whole-number ratios to form compounds. The idea that atoms are indivisible is the one part we now know is not fully true (atoms have electrons, protons and neutrons), but the rest of Dalton's theory is still the backbone of chemistry.

What is an Atom?

Atoms are unimaginably small. The radius of a hydrogen atom is about 10^-10 m. If you placed atoms side by side, you would need crores of them to make a line one centimetre long! Because they are so tiny, we cannot see them with normal microscopes. Modern instruments like the scanning tunnelling microscope (STM) can now show us the positions of atoms. The IUPAC (International Union of Pure and Applied Chemistry) approves the names of elements and their symbols. A symbol is a short way to write an element — usually the first one or two letters of its name, like H for hydrogen, O for oxygen, and Ca for calcium. Some symbols come from Latin names: sodium is Na (Natrium), potassium is K (Kalium), iron is Fe (Ferrum).

Atomic Mass

Because atoms are so light, we do not measure their mass in grams. Instead we compare them to a standard. The modern standard is the carbon-12 atom: one atomic mass unit (1 u) is defined as exactly one-twelfth of the mass of one carbon-12 atom. So the atomic mass of an element tells us how heavy its atom is compared to 1/12 of a carbon-12 atom. For example, hydrogen has an atomic mass of 1 u, oxygen 16 u, and carbon 12 u. These numbers let us do calculations without ever weighing a single atom.

What is a Molecule?

A molecule is the smallest particle of an element or compound that can exist independently and shows all the properties of that substance. Molecules can be made of the same kind of atom (molecules of an element) or different kinds (molecules of a compound). The number of atoms in one molecule is called its atomicity. For example, helium is monatomic (He), oxygen is diatomic (O₂), ozone is triatomic (O₃), and phosphorus is tetra-atomic (P₄), while sulphur exists as S₈. Molecules of compounds include water (H₂O), carbon dioxide (CO₂) and ammonia (NH₃).

Ions and Polyatomic Ions

Some compounds, especially metals combined with non-metals, are made of charged particles called ions. A positively charged ion is a cation (e.g. Na⁺) and a negatively charged ion is an anion (e.g. Cl^-). A group of atoms carrying a charge is called a polyatomic ion, such as sulphate (SO₄^2-), nitrate (NO₃^-), carbonate (CO₃^2-) and ammonium (NH₄⁺). The combining capacity of an element or ion is called its valency, and valency is what decides the formula of a compound.

Writing Chemical Formulae

To write a formula, write the symbols of the elements and then balance their valencies by the criss-cross method: the valency of one element becomes the subscript of the other. For aluminium (valency 3) and oxygen (valency 2), criss-crossing gives Al₂O₃. For calcium (2) and chlorine (1) we get CaCl₂. When using a polyatomic ion more than once, enclose it in brackets, e.g. calcium hydroxide is Ca(OH)₂.

Molecular Mass and Formula Unit Mass

The molecular mass of a substance is the sum of the atomic masses of all the atoms in one molecule, expressed in u. For water, it is (2 × 1) + 16 = 18 u. Formula unit mass is the same idea but used for ionic compounds, which exist as a lattice rather than separate molecules. For sodium chloride (NaCl) it is 23 + 35.5 = 58.5 u.

The Mole Concept

Chemists deal with huge numbers of atoms, so they use a special counting unit called the mole. One mole of any substance contains 6.022 × 10²³ particles — this number is called Avogadro's number (N_A). Just as "a dozen" means 12, "a mole" means 6.022 × 10²³. Brilliantly, the mass of one mole of a substance in grams is numerically equal to its atomic or molecular mass in u. This is called the molar mass. So one mole of oxygen atoms weighs 16 g, and one mole of water molecules weighs 18 g. The mole is the bridge that connects the world of tiny atoms to the grams we can actually weigh in the lab.