Atomic Structure and Chemical Bonding

Thomson's Atomic Model

Chemical Bonding

Elements are rarely capable of free existence. In a compound, atoms of different elements are held together by bonds. The types of bonds present in a compound are largely responsible for its physical and chemical properties. The different bonds can be classified as strong and weak.

Why do elements undergo bond formation?

Elements are made of atoms, which comprise of protons, electrons, and neutrons. The protons and the neutrons reside in the nucleus and the electrons revolve around in definite paths called orbits. The electrons present in the last shell are called valence electrons. These electrons are responsible for all the chemical reactions of that element.

Every element has a tendency to attain a stable outer octet. To do so, it either gains or loses or shares its electrons; and in this process, it forms the bonds.

Types of strong bonds:

Ionic or electrovalent bond Covalent bond Metallic bond

Types of weak bonds:

Bonds formed due to van der Waal’s interaction Hydrogen bond

This representation of elements with valence electrons as dots around elements is referred to as Electron Dot structures for elements. The electron dot structure of some of the elements are: 

Chemical Bonding

Elements are rarely capable of free existence. In a compound, atoms of different elements are held together by bonds. The types of bonds present in a compound are largely responsible for its physical and chemical properties. The different bonds can be classified as strong and weak.

Why do elements undergo bond formation?

Elements are made of atoms, which comprise of protons, electrons, and neutrons. The protons and the neutrons reside in the nucleus and the electrons revolve around in definite paths called orbits. The electrons present in the last shell are called valence electrons. These electrons are responsible for all the chemical reactions of that element.

Every element has a tendency to attain a stable outer octet. To do so, it either gains or loses or shares its electrons; and in this process, it forms the bonds.

Types of strong bonds:

Ionic or electrovalent bond Covalent bond Metallic bond

Types of weak bonds:

Bonds formed due to van der Waal’s interaction Hydrogen bond

This representation of elements with valence electrons as dots around elements is referred to as Electron Dot structures for elements. The electron dot structure of some of the elements are: 

A chemical bond is an attractive force which holds various constituents (such as atoms, ions) together in different chemical species.

Kossel-Lewis Approach to Chemical Bonding

Lewis postulated that atoms attain the stable octet when they are chemically bonded.

Lewis symbols

Notations to represent valence electrons in an atom

Example:

Significance of Lewis symbols − The number of dots represents the number of valence electrons.

Octet rule- Atoms tend to gain, lose or share electrons so as to have eight electrons in their valence shells.

            

 

Lewis dot structure

Representation of molecules and ions in terms of the shared pairs of electrons and the octet rule

Steps to writing Lewis dot structure:

Add the valence electrons of the combining atoms. This will give the total number of electrons required to write the structure.

One negative charge means the addition of an electron. Similarly, one positive charge implies the removal of an electron from the total number of electrons.

The chemical symbol of the atoms and the skeletal structure of the compound should be known. Then, distribute the total number of electrons as bonding shared pairs between the atoms in proportion to the total bonds.

The least electronegative atom occupies the central position of the molecule/ion. For example in NF3, nitrogen occupies the central position whereas the three fluorine atoms occupy the terminal positions.

When the shared pairs of electrons have been accounted for single bonds, utilise the remaining electron pairs for either multiple bonding or count them as lone pairs. Here, the basic requirement is that each bonded atom gets an octet of electrons.

Lewis representation of some molecules

 

(*- Each hydrogen atom attains the electronic configuration of helium i.e. a duplet of electrons)

Covalent bond

Single covalent bond − Sharing of one electron pair

Double bond − Sharing of two electron pairs

Triple bond − Sharing of three electron pairs

Formal Charge

Example:

Lewis structure of O3 is

F.C. on the O-1 atom

F.C. on the O-2 atom

F.C. on the O-3 atom

Smaller the formal charge on the atoms, lower is the energy of the structure. The concept of formal charge is based on covalent bonding in which electron pairs are equally shared by neighbouring atoms.

Limitations of the octet rule:

Incomplete octet of the central atom

Examples: LiCl, BeH2, BCl3

Odd electron molecules

Examples: NO, NO2

Expanded octet

Examples: PF5, SF6, H2SO4

Some other drawbacks of octet rule:

It is based upon chemical inertness of noble gases. However, some noble gases can combine to form compounds such as XeF2, KrF2, XeOF2, etc.

It does not account for shape of molecules

It does not explain the relative stability of molecules Conditions for Formation of Covalent Bond  Presence of four or more electrons in the outermost shell of an atom (exception H, Be, B and Al) High electronegativity of both the atoms High electron affinity for both the atoms High ionisation energy of both the atoms Electronegativity difference between combining atoms should be zero or very low Formation of Some Covalently Bonded Molecules    Compound Molecule Type and Number of Covalent Bonds Hydrogen (H2) H−H One single bond  Chlorine (Cl2) Cl−Cl One single bond Nitrogen (N2) N≡N One triple bond Water (H2O) H−O−H Two single bonds between O and H Ammonia (NH3) H-N-H|H Three single bonds between N and H Carbon tetrachloride (CCl4)  Cl|Cl-C-Cl|Cl Four single bonds between C and Cl Methane (CH4) H|H-C-H|H Four single bonds between C and H Difference between Properties of Ionic and Covalent Compounds    Ionic Compounds Covalent Compounds The constituent particles are ions. The constituent particles are molecules. They exist as hard solids. They exist as gases, liquids or soft solids. They have high melting and boiling points They have low melting and boiling points. They are good conductors of electricity in the aqueous or molten state. They do not conduct electricity. They ionise in solution and behave as electrolytes. Only polar compounds form ions in aqueous solutions. They undergo dissociation. They do not undergo dissociation. They are soluble in water. They are soluble only in organic solvents. They undergo fast chemical reactions. They undergo slow chemical reactions. A chemical bond is an attractive force which holds various constituents (such as atoms, ions) together in different chemical species.

Kossel-Lewis Approach to Chemical Bonding

Lewis postulated that atoms attain the stable octet when they are chemically bonded.

Lewis symbols

Notations to represent valence electrons in an atom

Example:

Significance of Lewis symbols − The number of dots represents the number of valence electrons.

Octet rule- Atoms tend to gain, lose or share electrons so as to have eight electrons in their valence shells.

            

 

Lewis dot structure

Representation of molecules and ions in terms of the shared pairs of electrons and the octet rule

Steps to writing Lewis dot structure:

Add the valence electrons of the combining atoms. This will give the total number of electrons required to write the structure.

One negative charge means the addition of an electron. Similarly, one positive charge implies the removal of an electron from the total number of electrons.

The chemical symbol of the atoms and the skeletal structure of the compound should be known. Then, distribute the total number of electrons as bonding shared pairs between the atoms in proportion to the total bonds.

The least electronegative atom occupies the central position of the molecule/ion. For example in NF3, nitrogen occupies the central position whereas the three fluorine atoms occupy the terminal positions.

When the shared pairs of electrons have been accounted for single bonds, utilise the remaining electron pairs for either multiple bonding or count them as lone pairs. Here, the basic requirement is that each bonded atom gets an octet of electrons.

Lewis representation of some molecules

 

(*- Each…