8 trend guides
Periodic Trends
How atomic radius, ionization energy, electronegativity and other properties change across the periodic table, and why.
- Atomic Radius pmHow large an atom is, measured here as van der Waals radius — the effective size an atom occupies when it isn't chemically bonded to anything. Across a periodShrinks as you move left to right across a period. Down a groupGrows as you move down a group.
- First Ionization Energy eVThe energy needed to remove the single most loosely held electron from a neutral, gas-phase atom. Across a periodRises as you move left to right across a period. Down a groupFalls as you move down a group.
- Electronegativity (Pauling scale)How strongly an atom pulls shared electrons toward itself when it's bonded to another atom. Across a periodRises as you move left to right across a period. Down a groupFalls as you move down a group.
- Electron Affinity eVThe energy change when a neutral, gas-phase atom gains one extra electron — how much an atom "wants" an additional electron. Across a periodGenerally rises left to right across a period (with real exceptions). Down a groupGenerally falls down a group (also with exceptions).
- Melting Point KThe temperature at which an element turns from solid to liquid at standard pressure. Across a periodRises through the early transition metals, then falls sharply toward the nonmetals and noble gases. Down a groupVaries by block — generally falls down the main-group metals, but the pattern is less consistent among nonmetals.
- Boiling Point KThe temperature at which an element turns from liquid to gas at standard pressure. Across a periodFollows roughly the same shape as melting point — peaks among the mid-transition metals, drops toward the nonmetals. Down a groupVaries by block, similarly to melting point.
- Density g/cm³Mass per unit volume at standard conditions — how tightly an element's atoms are packed together. Across a periodTends to rise through the transition metals, then fall toward the lighter nonmetals. Down a groupGenerally rises down a group.
- Atomic Mass uThe average mass of one atom, in atomic mass units — driven almost entirely by proton and neutron count. Across a periodRises steadily left to right (each element has one more proton, and generally more neutrons). Down a groupRises steadily down a group.