Atoms and Molecules From invisible specks to the stuff of stars.

Law of Conservation of Mass

During any chemical reaction, total mass of reactants equals total mass of products.

Mixing barium chloride and sodium sulphate solutions in a corked flask shows no change in total mass before and after the reaction, confirming mass conservation.

diagram showing sealed conical flask with suspended ignition tube being tilted, illustrating conservation of mass experiment

Diagram: diagram showing sealed conical flask with suspended ignition tube being tilted, illustrating conservation of mass experiment

Law of Constant Proportions

A compound is always composed of the same elements combined in a constant mass ratio.

Examples

  • Water: 1 g H with 8 g O
  • Ammonia: 14 g N with 3 g H

Dalton’s Atomic Theory

  • Matter consists of tiny, discrete atoms.
  • Atoms cannot be created, divided or destroyed in chemical changes.
  • All atoms of one element share identical mass and properties.
  • Atoms of different elements differ in mass and properties.
  • Atoms unite in small whole-number ratios to form compounds.
  • Each compound contains a fixed set and number of its constituent atoms.
portrait of John Dalton with simple atomic symbols and whole number ratio sketches

Diagram: portrait of John Dalton with simple atomic symbols and whole number ratio sketches

How Small is an Atom?

Stack millions of hydrogen atoms and you get a layer thinner than paper; their radii are measured in nanometres (1 nm = 1×10⁻⁹ m).

Scale Comparison

  • Hydrogen atom ≈ 0.1 nm
  • Water molecule (H₂O) ≈ 0.3 nm
  • Haemoglobin protein ≈ 6 nm
atomic force microscope image of silicon surface revealing individual atoms

Diagram: atomic force microscope image of silicon surface revealing individual atoms

Modern Symbols of Elements

Each element is given a unique symbol: write the first letter in uppercase and the second, if any, in lowercase—for example H, Al, Cl.

  • Fe – Ferrum (iron)
  • Na – Natrium (sodium)
  • K – Kalium (potassium)

Atomic Mass & the amu (u)

watermelon cut into 12 slices, single slice on balance scale comparing masses of different fruits

Diagram: watermelon cut into 12 slices, single slice on balance scale comparing masses of different fruits

One atomic mass unit (u) equals one-twelfth of a carbon-12 atom’s mass; therefore, hydrogen ≈ 1 u, oxygen ≈ 16 u.

Imagine using one watermelon slice as a standard weight—this mirrors how 1 u lets us weigh other atoms relatively.

Molecules: Elements & Compounds

  • Molecule: group of bonded atoms acting as one unit.
  • Element molecules contain identical atoms – monoatomic He, diatomic O₂, triatomic O₃, octa-atomic S₈.
  • Compound molecules contain different atoms in a fixed ratio – H₂O (2 H:1 O), CO₂ (1 C:2 O).
  • Atomicity = total atoms in a molecule; helps classify and write correct chemical formulae.

Ions: Cations and Anions

Ions are atoms or bonded atom groups carrying a net charge; positive ones are cations (Na⁺) and negative ones are anions (Cl⁻).

Polyatomic ions:

  • Ammonium, NH₄⁺
  • Carbonate, CO₃²⁻
  • Sulphate, SO₄²⁻

Valency & Writing Chemical Formulae

Criss-Cross Steps

  1. Write element or ion symbols with their valencies.
  2. Criss-cross valencies to become subscripts.
  3. Simplify subscripts to the lowest whole numbers.
  4. Enclose a polyatomic ion in brackets if more than one is present.
diagram showing criss-cross of Mg2+ and Cl− ions forming MgCl2

Diagram: diagram showing criss-cross of Mg2+ and Cl− ions forming MgCl2

Build the Formula!

Instructions

  1. Review the compound name and recall its ions.
  2. Drag the right count of Ca²⁺ and NO₃⁻ into the Calcium nitrate box until charges cancel.
  3. Drag Al³⁺ and O²⁻ into the Aluminium oxide box until neutral.
  4. Press “Check” for instant feedback, adjust if needed.

Molecular & Formula Unit Mass

Molecular mass is the sum of atomic masses in a molecule; in ionic solids we quote formula-unit mass (e.g., H₂O = 18 u, NaCl = 58.5 u).

Key Takeaways

  • Mass remains constant; elements combine in fixed ratios.
  • Atoms are indivisible basics; molecules/ions build matter.
  • Symbols and valency guide accurate chemical formula writing.
  • Atomic and molecular masses let us measure and predict reactions.
minimalist icon set showing atom model, balance scale, and chemical formula

Diagram: minimalist icon set showing atom model, balance scale, and chemical formula