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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
covalent bond between them.
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
covalent bond between them.
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:
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