About this site
Sources
Where every number on the site comes from. Each lesson lists its own sources at the foot of the page, and they are gathered here too.
Element data
The periodic table and the 118 element pages are built by a script from three sources, and a second script checks them against each other and against values checked by hand. The build fails if they disagree without an explanation.
- IUPAC CIAAW, abridged standard atomic weights (2024). Atomic weights, with their uncertainties and notes. Elements with no stable isotopes have no standard atomic weight, and we show none.
- NIST Atomic Spectra Database, ionization energies. Ground-state electron configurations and first ionisation energies. Configurations for elements 109 onwards are predictions, and are marked as such.
- NIST Atomic Spectra Database, lines. The wavelengths, in air, of the emission lines drawn on the home page and in the flame colours lesson.
- PubChem periodic table. Families, states at room temperature and Pauling electronegativities, and a second check on configurations.
Where sources differ, we say so on the corrections page.
Constants
- BIPM, the SI Brochure (9th edition, 2019), for the definition of the mole and the exact Avogadro constant.
- NIST, CODATA recommended values of the fundamental physical constants, for the gas constant, the Planck constant, the speed of light and the Bohr radius.
Each lesson
Where are the electrons in an atom?
- NIST Atomic Spectra Database: ground states and ionisation energies
- AQA GCSE Chemistry (8462) specification, 4.1.1.7: electronic structure
- IUPAC: the periodic table of the elements
- IUPAC Gold Book: orbital (atomic)
- AQA A-level Chemistry (7405) specification, 3.1.1.3 Electron configuration
- Chemguide (Jim Clark): electronic structures of atoms
Why do metals burn with different colours?
- NIST Atomic Spectra Database: lines of H I, Li I, Na I, K I, Ca I and Cu I
- AQA GCSE Chemistry (8462) specification, 4.8.3.1 Flame tests and 4.8.3.7 Flame emission spectroscopy
- Royal Society of Chemistry: the discovery of helium in the solar spectrum
- NIST: CODATA values of the Planck constant, speed of light and Rydberg constant
- AQA A-level Chemistry (7405) specification, 3.1.1.3 Electron configuration and ionisation energies (emission spectra are not a named AQA A-level topic)
Why does salt dissolve in water?
- PubChem, Sodium chloride (CID 5234): melting point and solubility
- AQA GCSE Chemistry (8462) specification, 4.2.1.2, 4.2.1.3 and 4.2.2.3: ionic bonding, ionic compounds and their properties
- IUPAC Gold Book: solvation
- NIST Chemistry WebBook: sodium chloride, thermochemistry data
- PubChem, Sodium chloride (CID 5234)
- Chemguide (Jim Clark): enthalpies of solution and hydration
- AQA A-level Chemistry (7405) specification, 3.1.8 Thermodynamics
Why is a water molecule bent?
- NIST Computational Chemistry Comparison and Benchmark Database: experimental geometry of water (H–O–H 104.5°)
- NIST CCCBDB: experimental geometry of ammonia (H–N–H 106.7°)
- AQA GCSE Chemistry (8462) specification, 4.2.1.4: covalent bonding
- Chemguide (Jim Clark): shapes of molecules and ions
- NIST CCCBDB: experimental geometries (water, ammonia, hydrogen sulfide)
- R. J. Gillespie, The VSEPR model revisited, Chemical Society Reviews 21, 59 (1992)
- AQA A-level Chemistry (7405) specification, 3.1.3.5 Shapes of simple molecules and ions
How do you count atoms you can’t see?
- BIPM: the SI Brochure, 9th edition (2019), definition of the mole
- NIST: CODATA value of the Avogadro constant (exact)
- IUPAC CIAAW: abridged standard atomic weights 2024
- AQA GCSE Chemistry (8462) specification, 4.3: quantitative chemistry
- IUPAC Gold Book: relative atomic mass (atomic weight)
- AQA A-level Chemistry (7405) specification, 3.1.2 Amount of substance
Why do reactions go faster when it’s hot?
- AQA GCSE Chemistry (8462) specification, 4.6.1: rate of reaction and collision theory
- IUPAC Gold Book: activation energy (Arrhenius activation energy)
- Chemguide (Jim Clark): the effect of temperature on rates of reaction
- IUPAC Gold Book: Arrhenius equation
- IUPAC Gold Book: activation energy
- NIST: CODATA 2018 value of the molar gas constant
- Chemguide (Jim Clark): the Maxwell–Boltzmann distribution
- OCR A-level Chemistry A (H432) specification, 3.2.2 Reaction rates
Why does a reaction push back when you change it?
- AQA GCSE Chemistry (8462) specification, 4.6.2: reversible reactions and dynamic equilibrium
- NIST Chemistry WebBook: nitrogen dioxide, thermochemistry data
- NIST Chemistry WebBook: dinitrogen tetroxide, thermochemistry data
- Chemguide (Jim Clark): Le Chatelier's principle
- IUPAC Gold Book: equilibrium constant
- AQA A-level Chemistry (7405) specification, 3.1.6 Chemical equilibria, Le Chatelier’s principle and Kc
- Chemguide (Jim Clark): Le Chatelier’s principle and equilibrium constants
What does pH actually measure?
- AQA GCSE Chemistry (8462) specification, 4.4.2.4 to 4.4.2.6: the pH scale, neutralisation, strong and weak acids
- IUPAC Gold Book: pH
- US Geological Survey: pH and water
- NOAA: ocean acidification (surface ocean pH about 8.1)
- NIST Chemistry WebBook: water, and the ionisation of water
- PubChem, Acetic acid (CID 176): dissociation constant, pKa 4.76
- AQA A-level Chemistry (7405) specification, 3.1.12 Acids and bases
How we check
Every value in a lesson is checked against the source listed for it, and worked examples are recalculated. The date each lesson was last checked is at its foot. The lessons have not yet been reviewed by a practising chemistry teacher; until they have, every lesson says so.