# Covalent Bonds

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Covalent bonds are usually formed between atoms of non-metals.

• There is a sharing of electrons between the two atoms in order for them to achieve the stable noble gas configuration.
• Each atom contribute the same number of electrons for the bond
• If the two atoms share one pair of electrons, it is a single bond
• Share two pairs of electrons $\rightarrow$ double bond
• Share three pairs of electrons $\rightarrow$ triple bond

Before we go into more details about covalent bonds, we will need to distinguish between molecules and compounds.

• A molecule is a group of two or more atoms chemically bonded together. Examples: Hydrogen molecules ($\text{H}_{2}$), Oxygen molecules ($\text{O}_{2}$)
• A compound is a group of atoms created by chemically combining two or more elements together. Examples: Water ($\text{H}_{2}\text{O}$), carbon dioxide ($\text{CO}_{2}$)

### Some examples to illustrate covalent bonding in molecules and compounds

Covalent bond in hydrogen molecules:

• Electron configuration of hydrogen is 1
• Number of valence electron: 1
• Each hydrogen atom will contribute 1 electron for sharing.
• This will result in both atoms sharing 1 pair of electrons, hence achieving the stable noble gas configuration. (Because they will now have 2 valence electrons $\rightarrow$ similar to helium)
• A single bond has formed between the two hydrogen atoms

Covalent bond in oxygen molecules:

• Electron configuration of oxygen is 2.6
• Number of valence electron is 6
• Each oxygen atoms will contribute 2 electrons for sharing
• This will result in both atoms sharing 2 pairs of electrons. Number of valence electrons will then be 8.
• Stable noble gas configuration is achieved
• A double bond has formed between the two oxygen atoms

Covalent bond in nitrogen molecules:

• Electron configuration of nitrogen is 2.5
• Number of valence electron is 5
• Each nitrogen atoms will contribute 3 electrons for sharing
• This will result in both atoms sharing 3 pairs of electrons. Number of valence electrons will then be 8.
• Stable noble gas configuration is achieved
• A triple bond has formed between the two nitrogen atoms

Covalent bond in water:

• Electron configuration of oxygen is 2.6 while electron configuration of hydrogen is 1
• Each hydrogen atom can share 1 electron
• Oxygen need 2 electrons to achieve the stable noble gas configuration
• Hence, two hydrogen atoms will pair with one oxygen atom
• The oxygen atom will form 2 covalent bonds – 1 for each hydrogen atom.
• Two single bonds has formed between the two hydrogen atoms and one oxygen atom

### Properties of covalent compounds

• Low melting and boiling points
• Covalent compounds do not conduct electricity
• Covalent compounds dissolve in organic solvent but do not dissolve in water

Reason for low melting and boiling points: The molecules are held together by very weak Van der Waals’ forces of attraction. Hence, very little energy is required to overcome the weak intermolecular forces of attraction.

Reason for non-conductance of electricity: Covalent compounds do not carry charges. Hence, there are no ions to carry electricity