What “periodic trends” means
Element properties repeat predictably as you move across a period (row) or down a group (column) — the whole point of the periodic table. The heat-map above colors every element by one measurable property at a time, so you can see the gradient.
Switch between electronegativity, first ionization energy, and electron affinity. Each cell is shaded against the real minimum and maximum for that property; elements with no value in the dataset stay grey, never guessed.
Electronegativity
Electronegativity is how strongly an atom in a bond pulls shared electrons toward itself (Pauling scale). It increases across a period (left → right) and decreases down a group (↓). Fluorine is the most electronegative element at 3.98; alkali metals like cesium sit near the bottom (~0.79). The cause is nuclear charge and atom size — more protons and a smaller atom mean a stronger pull. Electronegativity differences make bonds polar, so this property drives much of chemistry.
First ionization energy
First ionization energy is the energy needed to remove the outermost electron from a neutral gas-phase atom (kJ/mol). It increases across a period and decreases down a group — a larger atom holds its outer electron more loosely, so it leaves more easily. Helium is highest (~2372 kJ/mol) because its full shell is very stable; group 1 metals are lowest because losing one electron gives them a noble-gas configuration. Small dips (boron below beryllium, oxygen below nitrogen) reflect subshell stability — a great link to the valence electrons lesson.
Electron affinity
Electron affinity is the energy change when a gas-phase atom gains an electron (kJ/mol; larger positive = energy released = the atom “wants” the electron). It generally increases across a period, peaking at the halogens. Watch the famous anomaly: chlorine (≈349) actually has a higher electron affinity than fluorine (≈328) because fluorine’s tiny 2p shell crowds the incoming electron. Many metals and the noble gases have near-zero or negative values, meaning they do not readily accept an electron.
Atomic radius (not on the heat-map)
Atomic radius is not in our dataset, so it has no color layer — but it explains the others. Radius decreases across a period (growing nuclear charge pulls the same shell inward) and increases down a group (each period adds a shell). Two anchors: across period 2, lithium is much larger than fluorine; down group 1, cesium is far larger than lithium. Smaller atoms hold electrons more tightly, so radius is the mirror image of ionization energy and electronegativity. Browse the full elements list to compare.
Using this with a class
Project the heat-map, switch properties, and have students predict the next color before you reveal it. Tie it to the Bohr model and atomic mass for the full atomic picture. It’s free to embed on your own site or LMS using the snippet below.