Periodic trends

Seven properties of the elements, each drawn as a colour-coded periodic table from PubChem’s data and ranked element by element. Every page says how its property changes across a period and down a group, and where the data breaks the pattern.

• Electronegativity 95 elements, led by fluorine at 3.98.
• Atomic radius 99 elements, led by francium at 348 pm.
• First ionisation energy 102 elements, led by helium at 24.587 eV.
• Electron affinity 57 elements, led by chlorine at 3.617 eV.
• Melting point 103 elements, led by carbon at 3823 K (sublimes at 1 atm).
• Boiling point 93 elements, led by rhenium at 5869 K.
• Density 96 elements, led by osmium at 22.57 g/cm³.

Why the trends run the way they do

Almost every trend on these pages comes from two competing effects. Across a period, each element adds a proton to the nucleus and an electron to the same outer shell, and electrons in one shell shield each other poorly, so the outer electrons feel a steadily stronger pull: atoms get smaller, and they hold their electrons harder.

Down a group, each period starts a new shell further out, behind more electrons, and the extra nuclear charge does not make up for the distance. Atoms get larger and let go of their outer electrons more easily.

Ionisation energy and electronegativity follow those two rules closely in the main groups. Atomic radius follows them too, though the van der Waals figures on its page show it less cleanly than covalent radii would, and electron affinity follows them only roughly. Melting point, boiling point and density depend on how atoms bond to each other in bulk as well, so they follow the rules only in places, and each page says where.

The interactive periodic table gives every element’s details in one place, and each element has a page of its own with these properties listed.