Chemical Bonding E-material

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CHAPTER - 2 STD. X

CHEMICAL BONDING

Chemical bond is the force which holds two or more atoms or ions together in a stable molecule.

AN ATOM

An atom is the smallest unit of matter taking part in a chemical reaction and is built up of subatomic

particles- protons, neutrons and electrons.

Two or more atoms [metallic or non metallic] combine to form a molecule.

The force which holds the atoms together as a stable molecule is a chemical bond.

Types of elements- involved in chemical combination.

METALS:have 1, 2 or 3 electrons in valence shell. They lose 1, 2 or 3 electrons and become positively

charged ions (cations).

NON METALS:have 4, 5, 6 or 7 electrons in valence shell. They gain 3, 2 or 1 electrons and become

negatively charged ions (anions).

Chemical combination:Atoms combine to form a molecule by two means:

1. Transfer of valence electrons form a metallic atom to a non metallic atom.

2. Sharing of valence electrons between two atoms (generally both non metallic).

SN Kansagra School

CHEMISTRY


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LEWIS THEORY OF CHEMICAL BONDING

In 1916, Lewis put forth a comprehensive theory of bonding based on the electronic concept of the

atom. According to him, bonding of an atom depends chiefly on the electrons in the outermost shell.

He observed that the number of electrons in the last orbits of inert gases formed a most stable

grouping. Except the inert gases, all other elements have atoms with unstable and incomplete outer

shell. They tend to lose or gain electrons so as to acquire an electronic configuration identical with

that of the nearest inert gas in the periodic table having 8 electrons in the outermost orbit, except

helium which contains 2 electrons. It is this tendency of atoms to complete and thus, stabilise their

outermost ring of electrons which causes them to combine chemically. This theory has been applied

to explain the three common types of bonds.

(1) Electrovalent bond (2) Covalent bond (3) Coordinate bond

METHODS FOR ACHIEVING CHEMICAL BONDING

A stable electronic configuration in two combining atoms resulting in chemical bonding between

them is achieved by:

Electron transfer

of valence electrons from one atom to another leading to the formation of

electrovalent bond and formation of electrovalent compound.

Electron sharingof pairs of electrons between two reacting atoms leading to covalent bonding and

formation of covalent compound.
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PERIODIC PROPERTIES WHICH AFFECT THE FORMATION OF CHEMICAL BOND

For the formation of ionic bond

Ionisation potential Lower the value of I.P. of a metallic atom greaterthe ease of formation of the cation.

Electron affinity Higher the value of E.A. of a non metallic atom,

greater the ease of formation of the anion.

Electronegativity Larger the difference between two combining

atoms, electron transfer takes place easily.

For the formation of Covalent bond

Ionisation potential, Electron affinity andElectronegativity

High between both the atoms.

Electronegativity difference Should be negligible between two combining

atoms.

TYPES OF CHEMICAL BONDS

1. ELECTROVALENT BOND

Electrovalent or ionic bond: The

chemical bond that is formed between

two atoms by transfer of one or more

electrons from the atom of an

electropositive or metallic element to

the atom of an electronegative or non

metallic element is called an

electrovalent or ionic bond.

1. An electrovalent bond is formed by the complete transfer of electrons from one atom to

another.

2. Transfer of electrons takes place from the atom of a metallic element to the atom of a non-

metallic element.

3. Due to transfer of electrons, one atom loses electrons while other atom gains electrons,

leading to the formation of charged particles called ions.

4. There exists a strong force of attraction between oppositely charged particles, resulting in

the formation of an electrovalent bond.


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FORMATION OF IONIC COMPOUNDS

1. SODIUM CHLORIDE

Sodium loses an electron to resemble the configuration of neon [2, 8] While chlorine takes up that

electron to resemble the configuration of argon [2, 8, 8]. Sodium forms a cation and chlorine an

anion. The oppositely charged ions are held together by strong electrostatic forces resulting in the

formation of the electrovalent compound, Sodium chloride.

Electron dot diagram:


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2. MAGNESIUM CHLORIDE

Magnesium has two valence electrons which it must lose to resemble the configuration of the inert

gas, neon. One chlorine atom requires one electron to form a stable octet. Hence, two chlorine

atoms, each with a capacity to take up one electron are required to form the compound, magnesium

chloride.

Electron dot diagram:


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3. CALCIUM OXIDE

Calcium needs to lose electrons from its outermost shell in order to resemble argon while oxygen

needs two electrons to attain the configuration of neon. Therefore, calcium atom loses twoelectrons which are taken up by oxygen and a stable, ionic compound, calcium oxide is formed.

COVALENT BOND

1. A covalent bond is formed by the mutual sharing of electron pairs between two atoms.

2. The sharing of electron pairs take between atoms of non metallic elements. This is because

both of them are deficient in electrons and both have a tendency to gain electrons.

Thus, transfer of electrons is not possible and the octet is satisfied by sharing of electrons.

3. Atoms which in their outermost shells have seven, six and five electrons share one, two and

three pairs of electrons respectively forming covalent molecules.


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4. The sharing of electrons between atoms results in the formation of non ionised molecules.

This is because electrons are not transferred from one atom to another, leading to the

absence of ions.

5. The chemical compound, formed as a result of mutual sharing of electrons or electron pairs

thereby establishing a covalent bond, is called a covalent or molecular compound.

Covalency of an atom is the number of its electrons forming shared pairs with other atoms.

Hydrogen molecule Oxygen molecule

Covalent bond:The chemical bond that is formed between two combining atoms by mutual sharing

of one or more electrons of atoms of non-metallic elements is called a covalent or a molecular bond.

FORMATION OF COVALENT MOLECULES:

1. HYDROGEN:

Each of two hydrogen atoms contributes one electron so as to have one shared pair of electrons

between them. Both atoms attain stable duplet structure, resulting in the formation of a single

covalent bond between them.

Atomic or orbit structural diagram of hydrogen molecule formation


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Electron dot diagram of hydrogen molecule formation

2. CHLORINE:

Each of the two chlorine atoms contributes one electron so as to have one shared pair of electrons

between them. Both atoms attain stable octet structure, resulting in the formation of a single


Atomic or orbit structural diagram of Chlorine molecule formation

Electron dot diagram of hydrogen molecule formation


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3. OXYGEN:

Each of the two oxygen atoms contributes two electrons so as to have two shared pair of electrons

between them. Both atoms attain stable octet structure, resulting in the formation of a double


Atomic or orbit structural diagram of oxygen molecule formation

Electron dot diagram of oxygen molecule formation


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4. NITROGEN:

Each of the two nitrogen atoms contributes three electrons so as to have three shared pair of

electrons between them. Both atoms attain stable octet structure, resulting in the formation of a

triple covalent bond between them.

Atomic or orbit structural diagram of Nitrogen molecule formation

Electron dot diagram of Nitrogen molecule formation

5. CARBON TETRACHLORIDE:

Four atoms of chlorine and one atom of carbon combine to form one molecule of carbon

tetrachloride. Carbon has four valence electrons and hence requires four electrons to complete its

octet. Chlorine has seven valence electrons and requires only one electron to complete its shell.

Therefore, four chlorine atoms share one electron each with carbon. Thus, both, carbon and

chlorine, complete their valence shells with eight electrons.
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6. METHANE:

Four atoms of hydrogen and one atom of carbon combine to form one molecule of methane. Carbon

has four valence electrons and hence requires four electrons to complete its octet. Hydrogen has

one valence electron and requires only one electron to complete its shell. Therefore, four hydrogen

atoms share one electron each with carbon. Thus, carbon completes its valence shells with eight

electrons and hydrogen completes its valence shell with two electrons.

POLAR AND NON-POLAR COVALENT COMPOUNDS

Non polar covalent compounds Polar covalent compoundsCovalent compounds are said to be non polar

when shared pair of electrons are equally

distributed between the two atoms.

Covalent compounds are said to be polar when

shared pair of electrons are unequally distributed

between the two atoms.

No charge separation takes place. The covalent

molecule is symmetrical and electrically

neutral.

Charge separation takes place. The atom which

attracts electrons more strongly develops a slight

negative charge.

e.g. H2, Cl2, O2, N2, CH4, CCl4. e.g. H2O, NH3, HCl.


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Coordinate bonds (Dative bonds)

Coordinate bond or Dative bond is formed by sharing of two electrons between two atoms where

both the electrons of the shared pair are contributed by one atom and another atom merely

participates in sharing.

This is a special type of covalent bond.

The atom which contributes the shared pair of electrons is called the donor, while the other which

simply shares the electron pair contributed by donor, is known as acceptor. The donor atom has a

complete octet in the valence shell but it still has one or more unshared pairs of valence electrons

called lone pairs of electrons. The donor contributes such a pair of electrons for being shared by the

acceptor, which is short of two electrons in its valence shell. The bond formed in this way is generally

represented by an arrow ( ) pointing from donor atom to the acceptor atom.

FORMATION OF HYDRONIUM ION:

A water molecule contains an oxygen atom having two lone pairs of electrons linked to two

hydrogen atoms through covalent bonds.

The hydrogen ion (H+) in hydronium ion shares one lone pair of electrons of the oxygen atom of

water molecule. This results in the formation ofH3O+ion.

Lewis electron-dot representation:

FORMATION OF AMMONIUM ION:

An Ammonia molecule contains a nitrogen atom having one lone pair of electron linked to three

hydrogen atoms through covalent bonds.

The hydrogen ion (H+

) in ammonium ion shares one lone pair of electrons of the nitrogen atom ofammonia molecule. This results in the formation ofNH4

+ion.

Lewis electron-dot representation:

Chemical Bonding E-material

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