2.5 The Periodic Table

The periodic table organizes elements by atomic number, revealing patterns in their properties. It's divided into periods and groups, with elements arranged based on electron configuration. This layout helps predict atomic size, ionization energy, and electronegativity trends.

Metals, nonmetals, and metalloids are distributed across the table, each with distinct characteristics. The periodic table's structure allows scientists to understand element behavior, predict chemical reactions, and discover new elements. It's a powerful tool for chemistry.

The Periodic Table

Organization of the periodic table

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• Elements arranged in order of increasing atomic number
  • Atomic number represents the number of protons in an atom's nucleus (H has 1, He has 2, Li has 3)
• Each element represented by a unique chemical symbol
  • Usually one or two letters, with the first letter capitalized (H, He, C, Fe)
• Elements organized into periods (rows) and groups (columns)
  • Periods represent the number of electron shells in an atom (period 1 has 1 shell, period 2 has 2 shells)
  • Groups represent the number of valence electrons in an atom
    • Valence electrons are the electrons in the outermost shell of an atom (group 1 has 1, group 2 has 2)
    • Valence electrons determine an element's chemical properties and reactivity (group 1 elements are highly reactive)
• Periodic table divided into four blocks based on electron configuration
  • s-block: Groups 1 and 2 (except hydrogen) (elements with s-orbital electrons)
  • p-block: Groups 13 to 18 (elements with p-orbital electrons)
  • d-block: Groups 3 to 12 (elements with d-orbital electrons)
  • f-block: Lanthanides and actinides located below the main table (elements with f-orbital electrons)

Predicting properties from position

• Atomic size (atomic radius) generally decreases from left to right across a period
  • Increasing number of protons in the nucleus attracts electrons more strongly (Na larger than Cl)
• Atomic size generally increases from top to bottom within a group
  • Additional electron shells increase distance between electrons and nucleus (Cs larger than Li)
• Ionization energy (energy required to remove an electron) increases from left to right across a period
  • Stronger attraction between electrons and nucleus makes removal more difficult (Na easier to ionize than Cl)
• Ionization energy decreases from top to bottom within a group
  • Larger atomic size and increased shielding effect make electron removal easier (Cs easier to ionize than Li)
• Electronegativity (ability to attract electrons in a chemical bond) increases from left to right across a period
  • Higher effective nuclear charge attracts electrons more strongly (Cl more electronegative than Na)
• Electronegativity decreases from top to bottom within a group
  • Larger atomic size reduces the ability to attract electrons (F more electronegative than I)

Metals vs nonmetals vs metalloids

• Metals located on the left side of the periodic table (except hydrogen)
  • Characteristics: Shiny, good conductors of heat and electricity, malleable, ductile
  • Examples: Sodium (Na), iron (Fe), gold (Au)
• Nonmetals located on the right side of the periodic table
  • Characteristics: Poor conductors of heat and electricity, brittle, dull appearance
  • Examples: Carbon (C), nitrogen (N), oxygen (O)
• Metalloids (semi-metals) located along the "staircase" line between metals and nonmetals
  • Characteristics: Properties intermediate between metals and nonmetals
  • Examples: Boron (B), silicon (Si), germanium (Ge)
• Noble gases (Group 18) are nonmetals with unique properties
  • Characterized by their full outer electron shells, following the octet rule

Additional Concepts

• Atomic mass: The average mass of an element's atoms, considering all its naturally occurring isotopes
• Isotopes: Atoms of the same element with different numbers of neutrons
• Electron configuration: The arrangement of electrons in an atom's orbitals, which determines its position in the periodic table
• The periodic table was developed by Dmitri Mendeleev, who organized elements based on their properties and atomic masses