4 · The periodic table as a map
In 1869 Dmitri Mendeleev arranged the 63 elements then known into a grid, and he left gaps. He predicted that the gaps were undiscovered elements, and even forecast their weights and densities. Within seventeen years, gallium and germanium turned up, matching his predictions closely. A data table that predicts missing entries is a model, not a list.
Chapter 3 explains why the grid works. Each row is a new electron shell being filled. Each column holds elements whose outer electrons are arranged the same way, and since only the outer electrons do chemistry, elements in a column behave alike.
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Show the selected property as a table
The numbers come from PubChem's compiled table, which draws on several sources. Electronegativity can't be measured directly (it's a derived scale, chapter 5). And "atomic radius" depends on what you measure: the van der Waals radius used here, the covalent radius or the metallic radius all differ, though they follow the same trends.
Why the trends run the way they do
Almost every trend on the table comes from one quantity: how strongly the outer electrons feel the nucleus. Inner electrons partly shield the outer ones from the nucleus's charge.
- Across a row, each step adds a proton to the nucleus but adds the electron to the same shell, where it shields the others poorly. The outer electrons feel a stronger pull, so atoms get smaller, harder to ionise and more electronegative, all the way to the noble gas at the end.
- Down a column, each step starts a new shell further out, behind a full set of inner electrons. The outer electrons are further away and well shielded, so atoms get bigger and give up electrons more easily.
The clearest signature is the energy needed to remove the outermost electron, plotted against atomic number:
That sawtooth is the periodic law in one picture. The pattern repeats with periods 2, 8, 8, 18, 18, 32: the shell capacities from chapter 3.
Families
- Alkali metals (lithium, sodium, potassium…) have one loose outer electron. They're soft, shiny and react with water: lithium fizzes, sodium skates around and can catch fire, potassium bursts into lilac flame. It gets more violent going down, exactly as the ionisation energies predict.
- Halogens (fluorine, chlorine, bromine, iodine) are one electron short of a full shell and grab electrons hard. Going down the column: fluorine is a pale yellow gas, chlorine a greenish gas, bromine a red-brown liquid, iodine a violet-black solid. Bigger atoms mean stronger dispersion forces (chapter 1), so they condense more easily.
- Noble gases have full shells and high ionisation energies, so they're reluctant to react, but not unable (see below).
- Transition metals fill d orbitals. Many form coloured compounds (the blue of copper sulfate, chapter 13) and make excellent catalysts (chapter 10), because d electrons can be lent and borrowed easily.
- Lanthanides fill f orbitals. They're very similar to each other and hard to separate, and they make the strongest permanent magnets (neodymium, in phone speakers and electric motors).
Metals sit at the bottom left, where electrons are easy to remove, and non-metals at the top right, where they're held tightly. A staircase of in-between metalloids (silicon, germanium) divides them, and those happen to make the best semiconductors. Only two elements are liquid at room temperature: mercury and bromine. Gallium and caesium melt on a warm day.
"Noble gases never react." For decades textbooks said so. In 1962 Neil Bartlett made a xenon compound, and since then dozens have followed (xenon fluorides are stable white crystals). Full shells make reactions hard, not impossible. The bigger, more shielded noble gases, xenon and krypton, are the easiest to persuade.
Each column is the same "interface": same number and type of outer electrons, so the same reactions, with sizes and energies scaling down the column. Chemists swap elements within a column the way you'd swap implementations behind an interface.
The table is also a low-dimensional embedding that data recovers on its own. A 2019 study trained word2vec-style embeddings on materials-science abstracts. Without being told any chemistry, the element vectors organised themselves along the periodic table's structure. Graph neural networks for molecules and crystals likewise learn element embeddings in which similar elements end up close together.
Chapter 13's electron chemical potential connects here too. Mulliken defined electronegativity as the average of the energy to remove an electron and the energy released by adding one, (IE + EA)/2, which is approximately the negative of the electrons' chemical potential: the atom's own Fermi level.
Electronegativity scales (Pauling's from bond energies, Mulliken's from IE and EA, others) disagree on the numbers while agreeing on the trends. The layout itself is still debated at the edges: where hydrogen and helium belong, and which elements go in group 3.
- Look
- A pile of things from around the house: a spoon, a paperclip, a coin, foil, a pencil, a plastic toy, a glass marble, a wooden block. Plus a fridge magnet.
- Wonder
- "Which of these are metal? Which ones will the magnet pick up?"
- Try
- Sort them into metal and not-metal. Then test each one with the magnet. Some metals stick and some don't!
- True sentence
- "There are about 100 kinds of tiny pieces, and the periodic table is a map of all of them. Metals are on one side of the map. Only a few metals, like iron, stick to magnets."
Iron, nickel and cobalt are the magnetic elements; steel paperclips stick, aluminium foil and copper don't (and many coins contain little iron). For a picture version of the table, print Keith Enevoldsen's illustrated periodic table (elements.wlonk.com), which shows an everyday use for every element. It's a great poster to point at.
Play more
- Ptable. The best interactive periodic table: properties, orbitals, isotopes, compounds.
- Keith Enevoldsen's illustrated table. Printable, and loved by children.
- Theodore Gray, The Elements. A photograph of every element.
- Sam Kean, The Disappearing Spoon. Stories behind the elements (for you).