Periodic Trends in Chemistry

Periodic Trends

• Dmitri Mendeleev organized the periodic table by increasing atomic mass.
• Henry Mosely organized the periodic table by increasing atomic number.
• Periodic Law: Elements arranged in order of increasing atomic number show a periodic recurrence of their properties.
• Groups (columns) on the periodic table represent families of elements with similar properties.
• Periods (rows) on the periodic table show elements with varying properties as you move across the row.
• Alkali Metals (Group 1) are the most reactive.
• Noble Gases (Group 18) are the least reactive.
• Metals are located on the left side of the periodic table.
• Metalloids are located along the staircase line.
• Nonmetals are located on the right side of the periodic table.
• Representative Elements are located in the s and p blocks.
• Sub-levels have specific shapes:
  • s: Sphere
  • p: Dumbbell
  • d: Clover
  • f: Double Clover
• Malleable means a substance can be hammered into shapes.
• Ductile means a substance can be drawn into wires.
• Brittle means a substance is hard but can break or shatter easily.
• Coulomb's Law:
  • Charge directly affects the force of attraction: Greater charge, greater force.
  • Distance inversely affects the force of attraction: Greater distance, weaker force.
• Electron Configuration describes the arrangement of electrons in an atom's sublevels.
  • Nitrogen (N): 1s² 2s² 2p³
  • Sodium (Na): 1s² 2s² 2p⁶ 3s¹
  • Cobalt (Co): 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d⁷
  • Sulfur (S): 1s² 2s² 2p⁶ 3s² 3p⁴
• Atomic Radius is half the distance between the nuclei of two identical atoms.
• Electronegativity measures an element’s tendency to attract electrons when chemically bonded.
• Ionization Energy is the energy needed to remove an electron from a gaseous atom.
• Removing a Second Electron: Requires more energy than removing the first.
• Cations are positively charged ions.
• Anions are negatively charged ions.

Periodic Trends in the Periodic Table

• Atomic Radius: Increases down a group and decreases across a period (towards the right).
• Ionization Energy: Decreases down a group and increases across a period (towards the right).
• Electronegativity: Decreases down a group and increases across a period (towards the right).

Identifying Trends

• Larger Atomic Radius:
  • Ca (Calcium) is larger than Mg (Magnesium)
  • Si (Silicon) is larger than N (Nitrogen)
  • Br (Bromine) is larger than Ga (Gallium)
  • P (Phosphorus) is larger than Rb (Rubidium)
• Larger 1st Ionization Energy:
  • S (Sulfur) is larger than O (Oxygen)
  • F (Fluorine) is larger than K (Potassium)
  • Cl (Chlorine) is larger than Al (Aluminum)
  • Fe (Iron) is larger than Al (Aluminum)
• Larger Electronegativity:
  • In (Indium) is larger than B (Boron)
  • Ga (Gallium) is larger than P (Phosphorus)
  • Li (Lithium) is larger than K (Potassium)
  • Al (Aluminum) is larger than N (Nitrogen)
• Larger Atom/Ion:
  • F^-1 (Fluoride ion) is larger than F (Fluorine atom)
  • Mg (Magnesium atom) is larger than Mg⁺² (Magnesium ion)
  • Cr⁺³ (Chromium(III) ion) is larger than Cr⁺² (Chromium(II) ion)
  • P^-3 (Phosphide ion) is larger than P (Phosphorus atom)