Anions and Cations

Presentation notes (slides available to members on request)

Electrical Terminology

  • When invented (c1800), a battery’s +ve terminal, was named the anode (ana = up) and the –ve one, the cathode (kata = down)
  • “Something” was thought to flow downhill from anode (+) to cathode (-)
  • We now know that electrons flow from cathode (-) to anode (+)

Simplified Atomic Structure

  • Atoms have equal numbers of protons (+) and electrons (-), making them neutral
  • uncharged neutrons stop protons from flying apart
  • Electrons live in fixed orbits, each of which has a limited capacity (eg, 2 or 8)
  • Atomic number is no. of protons
  • Innermost electron orbits are always filled first
  • Atoms may gain or lose outer electron(s), making them charged, and are then known as ions
  • They ionise in order to end up with a full outer orbit of electrons, a lower energy state
  • If an atom were the size of the Albert Hall, the nucleus would only be about the size of a dried pea.

Ion formation

  • Sodium losing an electron, becoming a positively charged ion (Na+)
  • Chlorine gaining an electron, becoming a negatively charged ion (Cl-)
  • Sodium donating an electron to chlorine in a single step

Mendeleev’s Periodic Table (1860s)

  • Elements arranged by increasing atomic no., grouped by properties
  • Could predict properties of missing elements (Gallium, Scandium, Germanium)
  • Atoms like to have their outermost electron orbits full, like those on extreme RHS (inert gases), as this is a lower energy state
  • To left of table, atoms can lose electron(s) to achieve this
  • To right of table, atoms can gain electron(s)
  • Chemistry is almost all about these outermost (valence) electrons

Opposites Attract

  • Negatively charged ions are attracted the anode, so are called Anions
  • Positively charged ions are attracted to the cathode, and are called Cations

Anion + Cation = Mineral

  • Minerals are made of cations and anions, held together by electrical attraction, eg Na+ and Cl- (common salt)
  • Simple anions often use suffix ‘ide’, eg bromide, chloride, sulphide
  • Anions are often compound, eg Silicate, which is SiO4----, Carbonate (CO3--), or Phosphate (PO4---)
  • Suffix ‘ate’ indicates oxygen

Acids and Bases

  • Acids contain one or more H+ cations. Eg HCl, H2SO4, HNO3.
  • Bases (alkalis) contain one or more OH- anions. Eg NaOH, Ca(OH)2
  • In solution, the ions dissociate
  • pH (power of Hydrogen) is the (-ve log) concentration of H+ ions.
  • Water is H2O (both H+ and OH-) and therefore neutral (pH = 7)

Executive Summary

  • Atoms are neutral but can gain or lose outer electrons to become electrically charged ions
  • Negative ions are attracted to the +ve anode, and are known as anions
  • Positive ions are attracted to the –ve cathode, and are called cations
  • Salts (minerals) are an anion (eg Cl-, SiO4----) and one or more cations (eg Ca++, Na+, Mg++)
  • Acids liberate H+ ions in solution (low pH), bases OH- ions (high pH)

Further information