When nonmetals react with each other, they attain noble gas configuration by sharing their valence electrons between themselves and completing their valency. This type of chemical bonding is called covalent bonding. Two or more than two atoms join together to form a molecule, and they have a covalent bond between them. 

Dot and cross diagram is used to represent the covalent bonding amongst the atoms in a molecule. These atoms can be identical or non- identical as well. 

H2   

 

Hydrogen has one electron in its shell and needs one more to attain duplet configuration. It shares its valence electron with another hydrogen atom to achieve its duplet configuration H-H single covalent bond is formed in the hydrogen molecule. 

Cl2    

 

Chlorine is in Group 7 of the Periodic table, which means it has seven valence electrons in its valence shell and needs one more to attain octet (8) configuration. It shares its valence electron with another chlorine atom to achieve its octet configuration Cl-Cl single covalent bond is formed in chlorine molecule. 

O2  O=O

 

Oxygen is in Group 6 of the Periodic table, which means it has six valence electrons in its valence shell and needs two more to attain octet (8) configuration. It shares its valence electron with another oxygen atom to achieve its octet configuration O=O double covalent bonds are formed in oxygen molecule. 

HCl H-Cl

 

Hydrogen has one electron in its shell and needs one more to attain duplet configuration. It shares its valence electron with a chlorine atom to achieve it's noble gas configuration H-Cl single covalent bond is formed in hydrogen chloride molecule. 

N2

 

Nitrogen has five valencies and needs three more to attain duplet configuration. It shares its three electrons in its valence with another nitrogen atom to achieve its octet configuration. A triple covalent bond is formed in a nitrogen molecule. 

H2O

 

Oxygen has six valency needs two more electrons to attain octet configuration. It shares its two valence electrons (two pairs of valence electrons form two lone pairs) with two hydrogen atoms to achieve its octet configuration, single covalent bonds are formed between hydrogen and oxygen atom, and a water molecule is formed.

CH4

 

Carbon has four electrons in its valence shell and needs four more to attain octet configuration. It shares its valence electron with four hydrogen atom to achieve its octet configuration C-H single covalent bond is formed in the methane molecule. 

C2H4

 

Carbon has four electrons in its valence shell and needs four more to attain octet configuration.  When two carbon atoms form a covalent bond, they share two pairs of electrons that are C=C. The remaining two electrons of carbon in its valence shell are shared by two hydrogen atoms to form a single carbon-hydrogen bond. 

Note:  When two electrons are shared, a single bond is formed, represented using a single line in the structural formula of the compound/ molecule.

When four electrons or two pairs of electrons are shared, a double bond is formed, represented using a double line in the structural formula of the compound/ molecule.

When six electrons or three pairs of electrons are shared, the triple bond is formed, represented using three lines in the structural formula of the compound/ molecule.

Allotropes are the same elements with different structures called allotropes. For instance, O2 and O3 are allotropes. CO and CO2:- both are not allotropes.

Covalent compounds can be simple or giant covalent structures. Both have varying properties because of their structure and bonding.

 

 

Simple Molecular Substances: Iodine molecules are formed by two iodine atoms sharing a covalent bond; however, intermolecular forces exist between iodine molecules. That is why the little amount of heat can break the weak intermolecular forces (van der Waals forces). This implies that simple molecular structures are highly volatile and are usually liquids or gases at room temperature and pressure. Those with larger molecules are solids at R.T.P. They have low melting and boiling points. They are unsolvable in water and solvable in organic solvents. These substances cannot conduct electricity due to unavailable free moving electrons to carry charges; exceptions include graphite.

Giant molecules Graphite, diamond, and silicon dioxide are all giant molecules because they have lots of atoms held together in strong covalent bonds to form giant molecular structures. Their properties are also different. 

Silicon dioxide: It is a 3D tetrahedral structure. Si has four valencies and shares its single covalent bonds with four oxygen atoms to form a giant SiO2 compound. It can only melt at high temperatures.