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Periodic Trends

Periodic trends are general patterns in the properties of elements that can be observed across the periodic table. These trends arise because of the periodic repetition of electron configurations as atomic numbers increase. By understanding these trends, chemists can predict the behavior of elements and their compounds.

Atomic Radius​

  • Trend down a group: Atomic radius increases because new electron shells are added, increasing the distance between the nucleus and the outermost electrons.
  • Trend across a period: Atomic radius decreases from left to right. Although more protons are added to the nucleus, pulling electrons inward, no new shells are added, so the effective nuclear charge increases and the atom becomes smaller.

Ionization Energy​

Ionization energy is the energy required to remove an electron from an atom.

  • Trend down a group: Ionization energy decreases because the outer electrons are farther from the nucleus and experience less attraction.
  • Trend across a period: Ionization energy increases from left to right due to stronger nuclear attraction as effective nuclear charge increases.

Electronegativity​

Electronegativity is the ability of an atom to attract shared electrons in a bond.

  • Trend down a group: Electronegativity decreases as atomic size increases, reducing the pull on bonding electrons.
  • Trend across a period: Electronegativity increases from left to right as atoms become smaller and nuclear charge increases.

Electron Affinity​

Electron affinity measures the energy change when an atom gains an electron.

  • Trend down a group: Electron affinity generally becomes less negative (less favorable) due to increased distance between the nucleus and the added electron.
  • Trend across a period: Electron affinity generally becomes more negative (more favorable), especially for halogens, as atoms more strongly attract extra electrons to complete their valence shell.

Metallic Character​

Metallic character refers to the tendency of an element to lose electrons and form cations.

  • Trend down a group: Metallic character increases because ionization energy decreases.
  • Trend across a period: Metallic character decreases from left to right as elements become less likely to lose electrons and more likely to gain them.

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