Types of Chemical Compounds

Periodic Table

The periodic table is divided into two major divisions: metals and non-metals. Metals (shown below in yellow) are located on the lower left and include typical metals such as iron (Fe) and nickel (Ni). Non-metals (shown in blue) are located in the upper right and include oxygen and nitrogen (common gases) as well as iodine (a solid). The position of elements on the periodic table can to used to predict with a high degree of accuracy the structure of a wide range of compounds.

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Classes of Binary Compounds

A binary compound is one that is formed from two types of elements. Three possibilities exist.

metal  +  metal --→ metallic compound
metal  +  non-metal --→ ionic compound
non-metal  +  non-metal --→ covalent compound

The physical properties of each of these types of compounds is summarized below.

  Metals Ionic Covalent
Melting points variable high low
Electrical
conductivity
high low
(solutions do conduct)
low
Hardness hard hard soft
Brittle malleable brittle brittle

Ionic Charges

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Rules for predicting ionic formulas

For simple binary compounds, it is possible to predict the formula of many ionic compounds by following the rules listed here.

  1. Is the compound ionic?
    If not, none of these rules will apply.
  2. Determine the charge of each element when present in an ionic compound.
    Use the table above to determine these charges. For example, O = -2, Rb = +1.
  3. Use the appropriate number of each ions such that:
    • The sum of all charges adds up to zero.
    • The simplest ratio of ions is used.

For example, if magnesium (Mg) and bromine (Br) are mixed:

  1. Metal (Mg) + Non-metal (Br) IS an ionic compound.
  2. Mg ⇒ Mg+2 and Br ⇒ Br-1
  3. For the final steps:
    • One +2 ion is exactly balanced by two -1 ions.
    • 2:1 is the simplest possible ratio.

Thus, the formula of the ionic compound formed is MgBr2.

Ionic Crystals

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Prentice-Hall site

In this image, the ions are represented as spheres, and a pair of ions is circled that corresponds to the ionic formula (NaCl). However, this choice is NOT unique: almost any pair of ions could have been selected. In solution, these ions will separate completely to create an electrically conductive solution. Water-soluble ionic compounds that dissolve in water to give conducting solutions are called electrolytes.

Molecular Crystals

molecularxtal.jpg

Prof. K. Leo, Dresden

Notice in this image that the individual molecules (represented as sticks) do not touch or bond with other molecules. Attraction forces between molecules (intermolecular forces) do exist, but are much weaker than the attractions (bonds) between atoms within a molecule.