Periodic Table Notes

Atoms, Elements, and Molecules

  • Atom: Smallest indivisible particle in an element that can take part in a chemical reaction.
  • Element: Pure substance consisting of only one kind of atom with the same number of protons.
  • Diatomic molecule: Molecule consisting of two atoms of the same element (Hydrogen, Nitrogen, Oxygen, Fluorine, Chlorine, Bromine, Iodine).

Periodic Table

  • Period: Horizontal row of elements with different chemical properties.
  • Group: Vertical column of elements with similar chemical properties.

Groups Names

  • Group 1: Alkali metals
  • Group 2: Alkali earth metals
  • Group 17: Halogens
  • Group 18: Noble gases

Valence Electrons and Energy Levels

  • Elements in the same group have the same number of valence electrons.
  • Elements in the same period have valence electrons in the same energy level.
  • Number of valence electrons and the energy level matches the group number and period number, respectively.
  • Valence electrons determine valency of an element.

Atomic Radius

  • Definition: Average distance from the nucleus to the edge of the highest occupied orbital.
  • Trend across a period: Atomic size decreases from left to right due to increasing nuclear charge.
  • Screening: Core electrons reduce the attractive force between the nucleus and valence electrons.
  • Trend down a group: Atomic size increases due to increased screening and higher energy levels of valence electrons.

Ionization Energy

  • Definition: Energy needed to remove an electron from an atom in its gaseous state (kJ/mol).
  • First ionization energy: Removes the first electron.
  • Second ionization energy: Removes the second electron, etc.
  • Trend across a period: Ionization energy increases due to increasing nuclear charge.
  • Trend down a group: Ionization energy decreases due to increased screening and higher energy levels.
  • Ionization energy increases after each electron is removed because shielding decreases and attraction of the nucleus increases.
  • Sudden jump in ionization energy indicates removal of a core electron.

Electron Affinity

  • Definition: Energy released when an electron is added to an atom or molecule to form a negative ion.
  • Increases from left to right across a period.
  • Decreases down a group.

Electronegativity

  • Definition: Measure of the tendency of an atom in a molecule to attract a bonding pair of electrons.
  • Measured on the Pauling scale; Fluorine has the maximum value of 4.
  • Trend across a period: Electronegativity increases due to increasing nuclear charge.
  • Trend down a group: Electronegativity decreases due to increased screening and distance from the nucleus.

Trends Summary

  • Ionization energy is generally higher for non-metals.
  • Group 17 elements have the highest first ionization energy in each period.
  • Group 1 elements have the lowest ionization energy in each period.