Periodic Trends

Periodic Trends

Periodic trends refer to tendencies of certain elemental characteristics to increase or decrease as one progresses along the periodic table.

Atomic Size

  • One important trend to consider is atomic size.
  • Generally, atomic radius decreases across a period from left to right and increases down a group. This is due to increased nuclear charge pulling electrons in closer without additional shielding, and the addition of electron shells, respectively.

Ionisation Energy

  • Ionisation energy refers to the amount of energy required to remove an electron from an isolated atom or ion.
  • Ionisation energy increases across a period due to increased nuclear charge without additional shielding, making it harder to remove an electron.
  • It decreases down a group because the outer electrons are located further from the nucleus, so less energy is required to remove them.

Electron Affinity

  • Electron affinity is the ability of an atom in a molecule to attract electrons to itself.
  • Generally, electron affinity increases across a period due to increased nuclear charge, and decreases down a group due to the increased distance of the outer electrons from the nucleus.


  • Electronegativity is a measure of how strongly atoms attract bonding electrons to themselves.
  • Similar to electron affinity, electronegativity increases across a period and decreases down a group.
  • It’s important to note that the noble gases are usually not included in electronegativity trends since they generally do not form compounds.

Understanding these trends can provide valuable insight into the properties of elements, their ions, and the compounds they form. They are central to predicting and explaining behaviours of atoms in chemical reactions.