Covalent bonding

​​In a nutshell

Covalent bonding occurs when atoms share electrons to gain a full outer shell. Dot and cross diagrams can be used to represent covalent bonding between atoms. Dative bonding is a type of covalent bond where one atom donates two electrons to another atom. 

Covalent bonding  

Covalent bonding is the sharing of a pair of electrons between two non-metal atoms in order to gain a full outer shell.

Important points:

• The bond is maintained by the strong electrostatic forces between the positively charged nuclei and negatively charged electrons.
  
• Outer-shell electrons are the electrons that are shared.
• Each atom involved in the covalent bond will share its own electrons. 

Dot and cross diagrams 

These diagrams show the arrangement of electrons in a covalent bond. 

Drawing dot and cross diagrams:

• Only draw the outer electrons. 
• Overlapping orbitals are drawn (electron shells overlap).
• Commonly the central atom contains eight electrons in its outer shell.
• Multiple bonds can be formed, where multiple pairs of electrons are shared. 

Example

The dot and cross diagram for an oxygen molecule where two oxygen atoms are covalently bonded together.  

A double covalent bond is formed between the two oxygen atoms, where two electron pairs are shared between the atoms. A full outer shell of electrons is formed for each oxygen atom. 

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Dot and cross diagrams for multiple covalent bonds 

Below are some examples of dot and cross diagrams for molecules that can contain single and multiple covalent bonds. 

Molecule	dot-cross diagram	Type of bond
Hydrogen, $$H_2$$​​		Single covalent
Oxygen, $$O_2$$​​	​	Double covalent
Nitrogen, $$N_2$$​​		Triple covalent

Bond enthalpy 

As well as an attraction between the positive nuclei and electrons, there is also a repulsion between the two positive nuclei in the covalent bond. The electrons will also repel one another, as they are all negatively charged. 

The bond length is the distance between the two nuclei in a covalent bond. At this distance, the attractive and repulsive forces cancel out. 

The more electrons contained in a bond, the higher the bond enthalpy and the shorter the bond length. There is more attraction towards the positive nuclei when the number of electrons is increased. Therefore, a double covalent bond will have a shorter bond length compared to a single covalent bond. 

Dative covalent bonding 

A dative covalent bond occurs when both electrons in a covalent bond is donated from one atom, instead of two. 

Note: A dative bond can also be known as a coordinate bond. 

Example

The dot and cross diagram for the dative covalent bonding in an ammonium ion. 

The nitrogen atom donates a pair of electrons to a hydrogen ion with an empty outer shell of electrons to form a dative covalent bond.