Periodic Trends

Overview

  • Periodic trends are patterns that can be observed among elements as you move across a period or down a group on the periodic table. These are clues to understanding the chemical behaviour of elements.

Atomic Size

  • Atomic size, also commonly known as atomic radius, is the distance between an atom’s nucleus and its outermost electron.
  • This size generally decreases from left to right across a period. This is because the increasing number of protons attracts the electrons more strongly, pulling them closer to the nucleus.
  • Atomic size increases from top to bottom down a group. The addition of energy levels (electron shells) increases the distance between the nucleus and outermost electrons, leading to a larger atomic radius.

Ionization Energy

  • The ionization energy is the energy required to remove an electron from an atom or ion in its gaseous state.
  • Ionization energy generally increases from left to right across a period. As the atomic radius decreases, electrons are closer to the nucleus and more tightly bound, thus requiring more energy to remove.
  • Ionization energy decreases down a group as the atomic radius increases. More distant (and less tightly bound) electrons are easier to remove, requiring less energy.

Electron Affinity

  • The electron affinity is the energy released or absorbed when an electron is added to a neutral atom to form a negative ion.
  • Electron affinity usually increases from left to right across a period. Atoms with smaller radii (and higher nuclear charge) more readily attract additional electrons.
  • Electron affinity generally decreases down a group due to increased atomic size; outer electrons are less attracted to (and therefore loosely bound by) the nucleus.

Electronegativity

  • Electronegativity measures the tendency of an atom to attract electrons in a chemical bond.
  • Electronegativity generally increases from left to right across a period as the atomic radius decreases. A smaller radius brings electrons closer to the positively-charged nucleus, increasing its pull.
  • Electronegativity tends to decrease down a group because the increasing atomic radius means electrons in bonded pairs are further from the nucleus, reducing its pull.