Trends
Periodic trends, drawn out in full
A periodic trend is any property that changes in a predictable way as you move across a period or down a group. They are far easier to see as a line than as a grid of numbers, so every property below is plotted across all 118 elements.
Atomic Mass
The average mass of one atom, weighted across naturally occurring isotopes. Climbs steadily with atomic number.
1.008 to 294.000 u
Atomic Radius
Empirical radius of a neutral atom. Shrinks left to right across a period as nuclear charge pulls electrons in, and jumps at the start of each new period.
25 to 260 pm
Electronegativity
Pauling scale. How strongly an atom pulls bonding electrons toward itself. Rises to the right and falls down a group, peaking at fluorine.
0.79 to 3.98
First Ionisation Energy
Energy needed to strip the outermost electron. Sawtooths upward across each period and peaks at the noble gases.
376 to 2,372 kJ/mol
Electron Affinity
Energy released when an atom gains an electron. Largest for the halogens, near zero for the noble gases.
-223.2 to 348.6 kJ/mol
Melting Point
Temperature at which the solid becomes liquid. Peaks in the middle of the transition metals, where metallic bonding is strongest.
1 to 3,695 K
Boiling Point
Temperature at which the liquid becomes gas. Tungsten and rhenium sit at the extreme; the noble gases at the other end.
4 to 6,203 K
Density
Mass per unit volume at standard conditions. Osmium and iridium are the densest; gases are reported in g/L.
0.09 to 40.70 g/cm³
Molar Heat Capacity
Energy needed to raise one mole by one kelvin. Remarkably flat across the metals, which is what the Dulong-Petit law describes.
8.52 to 62.70 J/(mol·K)
Abundance in Crust
How much of Earth’s crust each element makes up. Spans twenty orders of magnitude, so this one is plotted on a log scale.
3.0e-20 to 461,000 mg/kg
Year Discovered
When each element was first isolated or identified. Elements known since antiquity are excluded from the plot.
1250 to 2010
Why the patterns repeat
Nearly every trend on this page comes back to two things pulling against each other. Going across a period, protons keep being added to the nucleus while the new electrons go into the same shell, so the pull on the outer electrons gets stronger. The atom ends up smaller, harder to ionise and more electronegative.
Going down a group, each element starts a new shell. The outer electrons sit further out and are screened from the nucleus by everything underneath, so the atom gets bigger and gives up that outer electron more easily. That is the whole reason caesium reacts violently with water and helium reacts with nothing.