The Ionic Model

Forming Ions

    What are Ions?

  • An ion is an electrically charged species that is formed after an atom gains or loses an electron
  • When metal atoms lose electrons, they form positive ions called cations (fewer electrons than protons)
  • When non-metal atoms gain electrons, they form negative ions called anions (more electrons than protons)

    The Octet Rule:

  • The tendency of an atom to have a valence shell with a total of 8 electrons
  • This rule is helpful in deducing how many electrons an atom will gain or lose in order to become electronically stable
  • Examples:
    • Since sodium has one valence electron, it is energetically more favourable to lose one electron than to gain seven to achieve a full valence shell. This gives Na1+ or simply Na+.
    • As sulfur has six valence electrons, it is energetically more favourable to gain two electrons than to lose six to achieve a full valence shell. This gives S2–.

    Transition Elements

  • Transition elements are those located in groups 3-12 on the periodic table
  • Transition elements in period 4 have valence electrons in the 3d and 4s electron sublevels
  • When these period 4 elements lose electrons to form a cation, electrons are first lost from the 4s sublevel followed by the 3d sublevel, allowing them to have multiple common valence charges (multivalent)
    • Example: titanium has the electron configuration [Ar] 4s2 3d2, so it can lose two electrons from 4s giving Ti2+, and it can also lose two more electrons from 3d giving Ti4+

Ionic Bonds and Compounds

    Ionic Bonds

  • An ionic bond is formed by the electrostatic attraction between oppositely charged ions
  • To have an ionic bond, electrons are first transferred between neutral atoms to form cations and anions
  • While the individual ions in an ionic bond are charged, the overall ionic compound is electrically neutral

    Deducing Formulas for Ionic Compounds

  • This was discussed in detail here
  • Summary: find the common valence charge for both ions, then "cross" the charge of each element so it becomes the subscript of the other element

    Naming Ionic Compounds

  • This was discussed in detail here
  • Summary: take the name of the anion/metal and keep it the same, then take the name of the cation/nonmetal and replace the ending with -ide

    Polyatomic Ions

  • Finding the name and formulas for compounds with polyatomic ions was discussed in detail here
  • Polyatomic ions are groups of atoms covalently bonded together that have an overall net charge
  • Some common polyatomic ions that you should know:
    • Ammonium – NH4+
    • Acetate – C2H3O2–
    • Hydroxide – OH–
    • Carbonate – CO32–
    • Hydrogen Carbonate – HCO3–
    • Nitrate – NO3–
    • Sulfate – SO42–
    • Phosphate – PO43–

Ionic Structure and Bond Strength

    Lattice Structures

  • Electrostatic attraction occurs between many ions in three-dimensions, creating a highly ordered solid called a lattice
  • All ionic compounds exist as three-dimensional lattice structures, which have a repeating arrangement of the compound's ions
  • Even though the structure of ionic compounds contain many ions, their chemical formulas are empirical formulas meaning they show the simplest ratio of ions

    Ionic Radius

  • The radius of an atom or ion is determined based on the number of occupied energy levels, where more occupied energy levels results in a larger radius
  • The larger the ionic radius, the weaker the ionic bond, due to the greater distance between ions
    • This is because the force providing electrostatic attraction is inversely proportional to distance squared

    Ionic Charge

  • The charge of an ion is based on the difference between electrons and protons in an atom, where having significantly more/fewer electrons than protons results in a more negative/positive charge respectively
  • Ions with a greater charge will experience a stronger electrostatic attraction, and hence have a stronger ionic bond

    Lattice Enthalpy:

  • A measure of the strength of the electrostatic attraction between ions in an ionic compound, expressed in kJ mol–1
  • Can be thought of as the energy needed to break an ionic bond to form separate ions
  • The stronger the ionic bond, the higher the lattice enthalpy,
  • The weaker the ionic bond, the lower the lattice enthalpy,

Physical Properties of Ionic Compounds

    Melting Point:

  • The temperature at which a substance's state will change from solid to liquid
  • For ionic compounds, the process of melting involves breaking the electrostatic attractive forces between ions
  • Ionic compounds generally have a high melting point, as their electrostatic attractive forces are very strong compared to other types of bonding
    • For this reason, ionic compounds can generally be found in the solid state

    Volatility:

  • The ease at which a substance vaporises or becomes a gas
  • Because of their high melting points, and also high boiling points for similar reasons, ionic compounds do not easily vaporise and thus have a low volatility

    Solubility:

  • The ability of a substance to form a solution with, or dissolve in, another substance
  • Many ionic compounds are soluble in water, because the strength of attraction between water molecules and ions is usually greater than the ionic bond strength of the compound
  • In general, ionic compounds tend to be soluble in polar covalent solvents and not soluble in nonpolar covalent solvents (covered in covalent bonding)

    Electrical Conductivity:

  • The ability of charged particles to move through a region of space in a substance
  • Solid ionic compounds are not electrically conductive, as they have charged particles in the form of cations and anions that are fixed in a lattice structure (prevents movement)
  • However, molten and dissolved ionic compounds are electrically conductive, as the breaking of ionic bonds disrupts the lattice structure which allows cations and anions to move around (motion of charged particles)

    Brittleness:

  • A substance's tendency to fracture or break suddenly under stress
  • While ionic bonds are strong, ionic substances also fracture easily, making ionic compounds brittle
    • This is because applying stress to an ionic substance will cause layers of ions in the lattice structure to shift over, placing ions with like charges next to each other, resulting in strong repulsion and separation