Atoms and Molecules: First Look

50 lessons, pages 271–320.

What Is an Atom? — The smallest particle of an element that keeps its identity
The Idea of Indivisible Particles — Democritus, 'atomos' and early philosophical atoms
How Small Is an Atom? — Atomic radii in picometres and scale comparisons
Counting Atoms in a Grain of Matter — Why even tiny samples contain enormous numbers of atoms
Seeing Atoms Indirectly — Evidence from diffusion, Brownian motion and microscopes
Atoms and Elements — One element, one kind of atom
Symbols for Atoms — Using element symbols to represent single atoms
Dalton's Atomic Theory — The main postulates set out in the early 1800s
Dalton's Postulates Explained — Indivisible, identical, combining in whole numbers
Dalton and the Conservation of Mass — Atoms rearrange but are neither created nor destroyed
Dalton and Constant Composition — Why a compound always has the same mass ratio
The Law of Multiple Proportions — Carbon oxides and simple whole-number ratios
Dalton's Symbols and Models — Circle symbols and early ball models of compounds
Limitations of Dalton's Theory — Subatomic particles, isotopes and nuclear change
Dalton's Theory Today — Which ideas survive in modern chemistry
What Is a Molecule? — Two or more atoms joined chemically as a unit
Molecules of Elements — H₂, O₂, N₂ and other diatomic elements
Molecules of Compounds — H₂O, CO₂, NH₃ and different atoms combined
Atomicity — Monatomic, diatomic, triatomic and polyatomic molecules
Noble Gases as Single Atoms — Why helium and argon exist as separate atoms
Allotropes: Same Atoms, Different Molecules — O₂ and O₃, diamond and graphite
Atoms, Molecules and Ions Compared — Neutral particles versus charged particles
Substances Without Discrete Molecules — Giant lattices such as sodium chloride and diamond
Reading a Chemical Formula — Symbols, subscripts and what they count
Counting Atoms in Formulae — Brackets, coefficients and totals of each atom
Molecular Models: Ball-and-Stick — Showing atoms and bonds in three dimensions
Molecular Models: Space-Filling — Showing the true shape and size of molecules
Exploring Molecules in 3D — Rotating and comparing models in the atom simulator
Molecules in Everyday Life — Water, air, sugar and familiar molecular substances
Physical and Chemical Change at the Particle Level — Molecules staying intact versus atoms rearranging
Why Atoms Need a Mass Scale — Atoms are far too light to weigh in grams
Relative Atomic Mass: The Idea — Comparing atom masses with a standard
The Carbon-12 Standard — One-twelfth of a carbon-12 atom as the unit
The Atomic Mass Unit — The unified atomic mass unit u and its size
Reading Atomic Masses from the Periodic Table — Common values such as H 1, C 12, O 16
Why Atomic Masses Are Not Whole Numbers — A first look at isotopes and averages such as chlorine 35.5
History of Atomic Mass Scales — Dalton's hydrogen scale to the modern carbon scale
Relative Molecular Mass — Adding the atomic masses in a molecule
Calculating Relative Molecular Mass — Worked examples for H₂O, CO₂ and NH₃
Formula Mass of Ionic Compounds — Relative formula mass for NaCl and CaCO₃
Masses of Formulae with Brackets — Calculating for Ca(OH)₂ and Mg(NO₃)₂
Percentage by Mass of an Element — How much of a compound is each element
Using Relative Masses to Compare Substances — Heavier and lighter molecules and their properties
From Atoms to Amounts: A Preview — Why chemists will count particles in large groups
Common Errors in Atomic Mass Calculations — Forgotten subscripts, brackets and diatomic elements
Atoms and Molecules in Reactions — Particle diagrams of reactants and products
Atoms and Molecules in Scientific Thinking — How evidence built the particle model
Problem Solving with Atoms and Molecules — Mixed questions on formulae, atomicity and masses
Atoms and Molecules: Key Vocabulary — Precise use of atom, molecule, element, compound and mass terms
Atoms and Molecules: Unit Review — Dalton's ideas, molecules and atomic mass brought together