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