Covalent compounds or molecular compounds are chemical compounds made of elements connected by covalent bonds. Covalent bonds only form between nonmetallic elements because these elements have the same or similar electronegativity values. Here are examples of covalent compounds and a look at their common properties.
Examples of Covalent Compounds
You can recognize covalent compounds because they consist only of nonmetals. Alternatively, consult a table of electronegativity values. If the electronegativity values of the elements in a compound are identical or similar, then they form covalent compounds. As a general rule, if the electronegativity difference is less than 2 on the Pauling scale, the atoms form covalent bonds. If the electronegativity difference is 2 or more, the elements form ionic bonds.
Examples of covalent compounds include:
- O2 – oxygen
- Cl2 – chlorine
- PCl3 – phosphorus trichloride
- CH3CH2OH – ethanol
- O3 – ozone
- H2 – hydrogen
- H2O – water
- HCl – hydrogen chloride
- CH4 – methane
- NH3 – ammonia
- CO2 – carbon dioxide
- Carbohydrates (sugars and starch)
- Lipids (fats)
- Nucleic acids (DNA and RNA)
Note that organic compounds are all covalent compounds.
The Exception to the Rule
In general, if a molecule consists entirely of nonmetals, you can assume it is a covalent compound. However, there is one notable exception. The ammonium cation (NH4+) is so electropositive that it forms ionic bonds with nonmetals rather than covalent bonds. At the same time, the bonds between the nitrogen and hydrogen atoms are covalent. So, compounds such as ammonium chloride (NH4Cl) and ammonium nitrate (NH4NO3) contain both ionic and covalent bonds.
Covalent Compound Properties
Covalent compounds share some common properties:
- Low melting points
- Low boiling points
- Poor electrical conductors
- Poor thermal conductors
- Form brittle or soft solids
- Low enthalpies of fusion
- Low enthalpies of vaporization
Because of their low melting points, many molecular compounds are liquids and gases at room temperature. Solid covalent compounds tend to be soft or brittle because the covalent bonds (and sometimes hydrogen bonds) are relatively easy to break. The shared electrons within the compounds limit electron mobility, so covalent compounds don’t conduct heat or electricity as well as ionic compounds.
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