Periodic Table History and Chemical Bonding Basics

Posted on Mar 30, 2025 in Chemistry

History of the Periodic Table

The classification of elements began in the nineteenth century following their gradual discovery. Initially, the distinction was simply between metals and nonmetals.

  • Johan Döbereiner (1817): Introduced the concept of Triads.
  • John Newlands (1863): Proposed the Law of Octaves, sorting elements according to mass.
  • Lothar Meyer (1869): Sorted elements based on atomic volume.
  • Dmitri Mendeleev (1869): Sorted elements based on mass and properties, leaving gaps for undiscovered elements.

The current periodic table sorts elements based on their atomic number.

Element Classification by Metallic Character

Metals

  • Good conductors of heat and electricity.
  • Present a characteristic luster.
  • They are malleable and ductile.
  • Have a tendency to lose electrons and form positive ions (cations).

Nonmetals

  • Bad conductors of heat and electricity.
  • Can be found in any state (solid, liquid, gas) at room temperature.
  • Have a tendency to gain electrons and form negative ions (anions).

Noble Gases

  • Exist as isolated atoms.
  • They are very stable chemically and generally do not form compounds.

Representative Element Groups

  • Hydrogen: Despite being the first element, it is unique and not typically considered part of any single group.
  • Alkali Metals (Group 1): Have 1 electron in the last energy level.
  • Alkaline Earth Metals (Group 2): Have 2 electrons in the last energy level.
  • Boron Group (Group 13): Have 3 electrons in the last energy level.
  • Carbon Group (Group 14): Have 4 electrons in the last energy level.
  • Nitrogen Group (Pnictogens) (Group 15): Have 5 electrons in the last energy level.
  • Oxygen Group (Chalcogens) (Group 16): Have 6 electrons in the last energy level.
  • Halogens (Group 17): Have 7 electrons in the last energy level.
  • Noble Gases (Group 18): Have 8 electrons in the last energy level (with the exception of Helium, which has only 2). They do not easily lose or gain electrons.

Atom Clustering and Chemical Bonds

Elements and Compounds

  • Element: Formed by identical atoms that cannot be decomposed into simpler substances by chemical means. Can exist as isolated atoms (e.g., noble gases), molecules (e.g., O₂, H₂), or crystalline networks (e.g., Fe, C diamond).
  • Compound: Formed by atoms of different elements chemically bonded together. Can be decomposed into simpler substances (elements). Can exist as molecules (e.g., water H₂O), macromolecules (e.g., DNA, insulin), or ionic crystalline networks (e.g., NaCl).

Why Atoms Bond

Atoms unite because they are more stable together than when separated. Most elements have a tendency to form compounds or groups readily, except for the noble gases, which possess a very stable electronic configuration.

According to the Lewis theory (1916), atoms combine to achieve a more stable electronic configuration, typically resembling that of the nearest noble gas.

Chemical Bonds

A chemical bond is the force of attraction that holds particles (atoms, ions, molecules) together in a substance.

Ionic Bond

The electrostatic force of attraction between oppositely charged ions. Typically involves electron transfer from a metal to a nonmetal.

  • Metallic elements tend to lose one or more electrons from their valence shell to achieve a noble gas configuration, forming positive ions (cations).
  • Nonmetallic elements tend to gain one or more electrons to achieve a noble gas configuration, forming negative ions (anions).
  • Summary: Electron transfer (Metal + Nonmetal → Ions)

Covalent Bond

The union of two atoms that share one or more pairs of electrons. Occurs when the tendency of atoms to lose or gain electrons is similar (typically between nonmetals). A stable configuration is achieved through electron sharing.

  • Summary: Electron sharing (Nonmetal + Nonmetal)

Metallic Bond

The attraction between positive metal ions and a surrounding ‘sea’ of delocalized electrons, forming crystalline networks. This involves the collective sharing of valence electrons among many metal atoms.

  • Summary: Electron pooling (Metal + Metal → Cations in electron sea)