Ionic and Covalent Compounds & Chemical Formulae

Ionic Compounds

  • Ionic compounds, like sodium chloride, are solids at room temperature.
  • Ions arrange themselves into a regular lattice where each ion is surrounded by ions of the opposite charge (Figure C3.18).
  • The giant ionic structure is held together by electrostatic forces of attraction between oppositely charged particles.

Ionic Groups

  • Metal ions can combine with negative ions containing a group of atoms (e.g., SO4^{2-}, NO3^{-}, CO_3^{2-}. These ionic groups are covalently bonded.
  • These groups have a negative charge due to gained electrons.
  • There is one important positively charged ionic group: the ammonium ion, NH_4^{+}.
  • The overall charge of a formula is zero; the total positive charge must equal the total negative charge.

Types of Chemical Bonding

  • Three major types of chemical bonding:
    • Metallic bonding
    • Ionic bonding
    • Covalent bonding

Physical Properties of Ionic and Covalent Compounds

  • Knowledge of atomic combination helps understand substances' physical properties.
  • Ionic and simple covalent compounds have broad differences in properties (Table C3.04).
  • Ionic compounds interact through electrostatic attraction of full chemical bonding in all directions, strongly holding ions in place.
  • Simple covalent compounds are made of molecules and full chemical bonding works within molecules. This bonding doesn't act between molecules; forces between molecules are weak intermolecular forces.
  • Simple covalent compounds have lower melting and boiling points because less energy is needed to separate molecules than ions in ionic compounds.

Valency

  • For elements in Groups I-IV:
    valency = group \ number
  • For elements in Groups V-VII:
    valency = 8 - group \ number
  • Elements in Group VIII/0 have a valency of 0.

Examples of Valency

  • Carbon (Group IV) has a valency of 4.
  • Oxygen (Group VI) has a valency of 8 - 6 = 2.

Naming Chemical Compounds

  • Naming classifies compounds; modern systems aim for consistency.
  • If there is a metal in the compound, it is named first.
  • If the metal can form more than one ion, the name indicates which ion is present [e.g., iron(II) chloride contains Fe^{2+}, iron(III) chloride contains Fe^{3+}].
  • Compounds containing only two elements have names ending in -ide (e.g., sodium chloride NaCl, calcium bromide CaBr2, magnesium nitride Mg3N_2).
  • Hydroxides contain the hydroxide (OH^-) ion.
  • Compounds containing an ionic group (usually containing oxygen) have names ending with -ate (e.g., calcium carbonate CaCO3, potassium nitrate KNO3, magnesium sulfate MgSO4, sodium ethanoate CH3COONa).
  • Prefixes indicate the number of atoms in the molecule (e.g., carbon monoxide (CO), carbon dioxide (CO2), nitrogen dioxide (NO2), dinitrogen tetraoxide (N2O4), sulfur dioxide (SO2), sulfur trioxide (SO3)).
  • Names for mineral acids are systematic but should be memorized (e.g., sulfuric acid H2SO4).
  • Oxidizing agents containing polyatomic negative ions involving metal and oxygen atoms [e.g., potassium manganate(VII) (KMnO4) and potassium dichromate(VI) (K2Cr2O7)] include the oxidation state of the metal.

Chemical Formulae

  • The 'cross-over' method can be applied to covalent compounds in two situations:
    • Simple molecules with a central atom, for example water, methane, carbon dioxide and ammonia.
    • Giant covalent molecules, where the formula is simply the whole-number ratio of the atoms present in the giant lattice, for example silica.

Formulae of Ionic Compounds

  • Ionic compounds' formulae represent the whole-number ratio of positive to negative ions.
  • In magnesium chloride, there are two chloride ions (Cl^-) for each magnesium ion (Mg^{2+}). Thus, the formula is MgCl_2.
  • The overall structure must be neutral; positive and negative charges must balance.
  • The size of the charge on an ion measures its valency or combining power.

Formulae of Compounds with Ionic Groups

  • The same rules apply to ionic groups.
  • It is useful to put the formula of the ionic group in brackets. This emphasises that it cannot be changed. For example, the formula of the carbonate ion is always CO_3^{2-}.

Properties of Ionic Compounds

  • Crystalline solids at room temperature.
  • High melting and boiling points.
  • Ions are arranged regularly in a lattice; ions with opposite charges are next to each other.
  • Large amounts of energy are needed to separate them.
  • Ions are attracted to each other by strong electrostatic forces.
  • Often soluble in water (not usually soluble in organic solvents).
  • Water is attracted to charged ions; therefore, many ionic solids dissolve.
  • Conduct electricity when molten or dissolved in water (not when solid).
  • In liquid or solution, ions are free to move; they can move toward electrodes when a voltage is applied.

Properties of Simple Covalent Compounds

  • Often liquids or gases at room temperature.
  • Low melting and boiling points.
  • Substances are made of simple molecules; atoms are joined by covalent bonds.
  • Forces between molecules (intermolecular forces) are very weak; not much energy is needed to move molecules further apart.
  • Soluble in organic solvents (very few are soluble in water).
  • Covalent molecular substances dissolve in covalent solvents.
  • Do not conduct electricity.
  • There are no ions present to carry current.