Question

What are the main properties of covalent compounds?

Answer 1

Covalent compounds are those compounds which are formed by a covalent bond is a chemical bond that involves the sharing of electron pairs between atoms. These electrons are known as shared pairs or bonding pairs.

Complete answer: Covalent bonds are only formed between nonmetallic elements because these elements have the same or similar electronegativity values. Here are some examples of covalent compounds,
${O_2}$ -Oxygen, $C{l_2}$ -Chlorine, ${H_2}$ -Hydrogen, proteins, carbohydrates etc.
For many molecules, the sharing of electrons allows each atom to attain the equivalent of a full valence shell, corresponding to a stable electronic configuration. The stable balance of attractive and repulsive forces between atoms, when they share electrons, is known as covalent bonding.
Properties of covalent compounds:
$\bullet$ Most covalent compounds have relatively low melting point and boiling points. When the ions in an ionic compound are strongly attracted to each other, covalent bonds create molecules that can separate from each other when a lower amount of energy is added to them. Therefore, molecular compounds usually have low melting and boiling point.
$\bullet$ Covalent compounds usually have lower enthalpies of fusion and vaporization than ionic compounds. The enthalpy of fusion is the amount of energy needed, at constant pressure, to melt one mole of a solid substance. The enthalpy of vaporization is the amount of energy, at constant pressure, required to vaporize one mole of a liquid. On average, it takes only $1\%$ to $10\%$ as much heat to change the phase of a molecular compound as it does for an ionic compound.
$\bullet$ Covalent compounds tend to be soft and relatively flexible. This is largely because covalent bonds are relatively flexible and easy to break. The covalent bonds in molecular compounds cause these compounds to take form as gases, liquid and soft solids. As with many properties, there are exceptions, primarily when molecular compounds assume crystalline forms.
$\bullet$ Covalent compounds tend to be more flammable than ionic compounds. Many flammable substances contain hydrogen and carbon atoms which can undergo combustion, a reaction that releases energy when the compounds react with oxygen to produce carbon dioxide and water. Carbon and hydrogen have comparable electronegativities so they are found together in many molecular compounds.
$\bullet$ When dissolved in water, covalent compounds don’t conduct electricity. Ions are needed to conduct electricity in an aqueous solution. Molecular compounds dissolve into molecules rather than dissociate into ions, so they typically do not conduct electricity very well when dissolved in water.
$\bullet$ Many covalent compounds don’t dissolve well in water. There are many exceptions to this rule, just as there are many salts that don’t dissolve well in a polar solvent, such as water. Examples of molecular compounds that dissolve well in water are sugar and ethanol. Covalent compounds that don’t dissolve well in water are oil and polymerized plastic.

Note:
Organic compounds are all covalent compounds. If a molecule consists of nonmetals we can assume it is a covalent compound however, there is one notable exception. The ammonium cation $(N{H_4}^ + )$ is so electropositive that it forms ionic bonds. At the same time, the bonds between the nitrogen and hydrogen atoms are covalent. So, compounds such as ammonium chloride $(N{H_4}Cl)$ and ammonium nitrate $(N{H_4}N{O_3})$ contain both ionic and covalent bonds.