MOLECULES

What is a molecule?

A molecule is an uncharged entity where atoms are bonded by the Valence forces. The molecule is a composition of two or more atoms that are linked together by chemical bonds. It is the smallest particle of an element or a compound that can exist freely. The absence of any electric charge distinguishes them from an ion. It is a neutral entity.

The chemical bonds in a molecule can be classified into- 

• Covalent Bonding: Attains stability via mutual sharing of electrons
• Ionic bonding: Attains stability through the transfer of electrons.
• Coordinate bonding: Stability is attained by partial transfer and partial sharing of electrons.

CLASSIFICATION OF A MOLECULE: 

A molecule can be classified into two categories ie.

• Molecules of an element: 

They constitute only one type of atom hence called homonuclear or homoatomic molecules.

On the basis of the number of atoms, a molecule can be classified into monoatomic, diatomic, or polyatomic.

• Molecules of a compound:  

They constitute atoms of different elements, thereby called heteronuclear or heteroatomic molecules. On the basis of the number of atoms, a molecule can be classified into Monoatomic, Diatomic, or polyatomic.

STRUCTURE OF MOLECULE AS PER -KOSSEL-LEWIS’S ELECTRONIC THEORY OF CHEMICAL BONDING: 

Rules for writing Lewis dot structure:

Step 1: calculate the total number of valence electrons of the atom:

• For ex- in CO molecule, the total number of valence electrons is 4+6=10

Step 2:

Case I: Anion – the no. of electrons equal to the charge on the ion must be added from the total no. of electrons.

Case II: Cation – subtract no. of electrons equal to the charge on the ion from the total no. of electrons.

Step 3: Central position in a molecular structure is occupied by the least electronegative atom. Write a skeletal structure for the given molecule.

• Here, it is CO.

Step 4: by utilising a pair of electrons draw single bonds between every two atoms.

• Here, draw a single bond (one shared electron pair) between C and O. 

Step 5: use the remaining pairs of electrons to complete the octet of each atom of the molecule. 

• Now complete the octet on O. The remaining two electrons will constitute a lone pair on C in this case.

• The lone pair is the pair of electrons that belong to only one atom.

The shortcoming of Octet Theory

• It doesn’t tell us about the shape and structure of the molecule.
• It doesn’t explain the stability and energy of molecules

MOLECULAR STRUCTURE OF NACl

The chemical name of NaCl is Sodium Chloride. It is an Ionic compound. The molecular structure of NaCl is shown as below: 

[Na]+[Cl]-

Sodium is assigned a positive electrovalence of one (Na+) and chlorine is assigned a negative electrovalence of one (Cl-)

STRUCTURE OF MOLECULE AS PER VALENCE SHELL ELECTRON PAIR REPULSION (VSEPR) THEORY

The structure of the molecule is best explained through the Valence Shell Electron Pair Repulsion (VSEPR) theory.

According to this theory, the valence shell electron pairs surround themselves around the central atom in such a way that the resulting structure possesses maximum stability and minimum energy. 

The following are assumptions of VSEPR theory:

• A molecule that has only 2 atoms is always linear in shape.
• In a molecule, with more than two atoms, one atom is referred to as a central atom, and other atoms are linked to it.

• If the central atom is linked to:

• Same atoms and only bond pairs of electrons surrounding it, the shape of the molecule is referred to as Symmetrical and possesses a regular geometry.
• Different atoms and surrounded by both bond pairs and lone pairs of electrons, the shape of the molecule is said to be Asymmetrical and possesses an irregular geometry.

• The order of repulsions should be as follows:

Lone pair- Lone pair > Lone pair- Bond pair > Bond pair- Bond pair

The different shapes as per VSEPR theory are as follows:

STRUCTURE OF ORGANIC COMPOUNDS:

The structure of organic compounds can be depicted in various ways:

• Lewis dot structures: it shows all the valence electrons.
• Dash structure formula: the covalent bond is shown by a dash. 
• A single dash represents a single bond.
• A double dash for double bond
• A triple dash for a triple bond.
• Condensed structural formula: in this type of representation, all the atoms bonded by carbon are written after C. Repeated groups are omitted by writing their number as a subscript. Forex: CH3(CH2)4CH3:  n-Hexane
• Bond line formula: in this, a series of carbon atoms are shown by a zig-zag line. Multiple bonds are also depicted in a bond-line formula.
• 3-D representation: this representation of the structure of organic compounds mainly helps to know the arrangement of atoms in space.
•  A solid wedge represents- bonds projecting out of the plane.
•  A Dashed wedge represents bonds behind the plane. 
• A dashed line represents bonds that lie in the plane of the paper. 

This is also known as Wedge and Dash Representation.

CONCLUSION

The structure of the molecule is a linkage of two or more atoms by chemical forces. Lewis’s theory of chemical bonding was initially proposed to give the structure of molecules which later got replaced by the VSEPR theory due to its limitations. Organic compounds are the structures that consist generally of carbon and hydrogen, but may also have a number of other elements e.g., halogens, nitrogen, oxygen, phosphorus, sulphur, silicon etc.