Chapter No 4 Structure of molecules
Superior Lalazar Public School and College Thana
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Chemical Bond: The force of attractions which holds atoms or ions together is called
chemical bonds.
Q. Why do atoms form chemical bond? Or what are the causes of chemical
bonds formation? OR why chemical bonds are essential?
Ans:
Reason: Atoms form chemical bond to get stability.
Explanation: Noble gases are more stable due to complete outer shell. Other atoms
try to complete their outer shell like noble gases to get stability for which they form
chemical bonds. Chemical bond can be formed by loss, gain or sharing of electrons.
Similarly lower energy state is more stable for which atoms form chemical bonds as
energy released when chemical bonds are formed.
Duplet Rule: When atoms complete their outer shell by two electrons like noble gas
helium is called Duplet Rule. Example hydrogen completes their outer shell by duplet
rule.
Octet Rule: When atoms complete their outer shell by eight electrons, the rule followed
by atoms is called octet Rule. Example. Chlorine(Cl) follows octet rule.
Q2. What is ionic bond? How it is formed? Give example of a formation of
compounds which form ionic bond.
Ans:
Definition: The bond which is formed by the complete transfer of electron form one
atom to another atom is called ionic bond.
OR
The electrostatic force of attraction between positive and negative ions is called ionic
bond.
Explanation: Ionic bond is always formed by the complete transfer of electron from
one atom to another. Those atoms which have low ionization energy easily give
electrons and those atoms which have high electronegativity and electron affinity easily
accept electrons. Therefore one atom loses electrons and becomes positive ions called
cation and other atom gains electron and becomes negative ions called anion. The
opposite ions formed are held together by electrostatic force of attraction called ionic
bond.
Example. Formation of ionic bond in sodium chloride (NaCl)
Sodium atom has one electron in outer shell and its electronic configuration is 2, 8, 1
while chlorine has seven electrons in outer shell and electronic configuration is 2,8,7.
When these atoms come close together, the sodium transfers its one valence electron
to chlorine due to which sodium form positive ion and chlorine forms negative ion.
Na 2,8,1 -------------- Na+2.8 + 1 e
Cl 2,8,7 + 1e ------- Cl- 2,8,8
The above two appositively charge ions are held together by electrostatic force of
attraction called ionic bond.
Q. Define covalent bond and discuss types of covalent bond?
Ans:
Covalent Bond:
Definition: A chemical bond formed between atoms by the mutual sharing of electrons
is called covalent bond.
Explanation: Covalent bond is formed when transfer of electrons from one atom to
another atom is not possible due to smaller difference in their electronegativity. To
achieve noble gases configuration some atoms share their valence electrons which is
called covalent bond. In covalent bond formation each atom provides equal number of
electrons for sharing but no transfer of electrons takes place. Each electron pair is
attracted by both the nucleus.
REPRESENTATION:
Covalent bond between two atoms is represented by a short line
Example: Two hydrogen atoms when combines by mutual sharing it forms hydrogen
molecules H-H (H2)
Types of covalent bond: There are three types of covalent bond depending upon the
number of shared pair electrons between bonded atoms.
1. Single covalent bond:
Definition: A covalent bond formed by mutual sharing of one pair of electrons is called
single covalent bond OR The covalent bond in which each bonding atom share only
one electron is called single covalent bond
Representation: A single bond is represented by a single dash (-) between the two
atoms.
Example, The two hydrogen atoms in H2 molecules are bonded by a single bond,
because two H atoms share only one pair of electrons.
2. Double covalent bond:
Definition: A covalent bond formed by mutual sharing of two pair of electrons is called
double covalent bond OR The covalent bond in which each bonding atom share only
two electrons is called double covalent bond
Representation: Double covalent bond is represented by two dashes (=) between the
two atoms.
Example, Two oxygen atoms combine together to form a molecule of oxygen by mutual
sharing of two pair of electrons.
3. Triple covalent bond:
Definition: A covalent bond formed by mutual sharing of three pair of electrons is called
triple covalent bond OR The covalent bond in which each bonding atom share only
three electrons is called triple covalent bond
Representation: Triple covalent bond is represented by three dashes (≡) between the
two atoms.
Example, Two nitrogen atoms share three pair of electrons to form a molecule of
Nitrogen (N2).
Q. What are polar and non polar covalent bonds?
Ans:
Polar covalent bond:
Definition: The type of covalent bond in which the shared pair electrons are attracted
unequally by both bonded atom is called polar covalent bond
Explanation: when covalent bond is formed between two different atoms having
different electronegativity then the shared pair electrons are more attracted by high
electronegative atom. As a result partial positive charge appears on low electronegative
atoms and partial negative charge on high electronegative bonded atom. Such
molecules are called polar molecules and the bond is called polar covalent bond. The
more polar the molecules, the higher is its boiling point and melting point.
Example: when a Covalent bond is formed between H and Cl , it is polar in nature
because Cl is more electronegative than H atom . Due to this reason a partial -ve
charge appeared on Cl atom and an equal +ve charge on H atom
Non Polar covalent bond:
Definition: The type of covalent bond in which the shared pair electrons are attracted
equally by both bonded atom is called non polar covalent bond
Explanation: when covalent bond is formed between two same atoms having same
electronegativity then the shared pair electrons are attracted equally by both bonded
atoms. As a result no partial charges appear on any atom and the molecule remain
neutral. Such molecules are called non polar molecules and the bond is called polar
covalent bond.
Example: H2, Cl2, O2
Q. what is coordinate covalent bond? Explain it with examples.
Ans;
Coordinate Covalent Bond:
Definition: A covalent bond in which both of the shared pair electrons are supplied by
one of the bonded atom is called coordinate covalent bond.
Explanation: coordinate covalent bond is formed when one atom has lone pair
electrons and other atoms has short pair electrons. The atom which donates pair of
electrons is called Donor and atom which accepts pair of electrons is called acceptor.
Coordinate covalent bond is represented by short arrow (
) which is always from
Donor towards acceptor. Coordinate covalent bond is always polar in nature because
partial positive charge appears on donor and partial negative charge on acceptor.
Example 1: Formation of Ammonium ions
When nitrogen of ammonia donates its lone pair to hydrogen ions it form coordinate
bond where nitrogen is Donor and hydrogen ion is acceptor
Example 2: Formation of hydronium ions (H3O+)
When oxygen of water donates its lone pair to hydrogen ions it form coordinate bond
where water is Donor and hydrogen ion is acceptor
Example 3: Formation of ammonia boron triflouride
When nitrogen of ammonia donates its lone pair to boron in boron triflouride it forms
coordinate bond where nitrogen is donor and boron is acceptor
Q. What is metallic bond? Explain with examples.
Metallic Bond: The special type of bond which holds metal atoms together is called
metallic bond.
OR
The force of attraction between the positive metal ions and the mobile sea of electrons
is called metallic bond
Explanation: metallic bonds exist only in metals. Due to low ionization energy metal
atoms lose its valence electron and form positive ions which are surrounded by sea of
free electrons. The free valence electrons do not belong to an individual ion or atom but
belong to all atoms of metal due to which keep all positive metal ions together in a
definite position. The valence electrons move freely from on part of metal to another but
do not escape from their structure. Due to metallic bond, metals are good conductor of
electricity.
Examples. The metallic bond exist between metal atoms such as iron, copper,
aluminum etc
Q. What are intermolecular forces?
Definition: The force of attraction between molecules which keeps molecules close
together is called intermolecular forces
Explanation: Intermolecular forces are attractions between molecules and present in
all molecules but different at different temperature. These forces keep molecules
together and affect physical properties of substances. For example, melting point ,
boiling point, viscosity etc of compounds depend upon intermolecular forces.
Temperature, pressure, nature of molecules etc affects intermolecular forces. These
forces are represented by dots or dashes between molecules
Example. The force of attraction between HCl molecules are intermolecular forces
Q. what is dipole to dipole interaction?
Definition: The positive pole of one polar molecule to attract the negative pole of
another polar molecule is called dipole-dipole interaction
Explanation: It is the type of intermolecular forces which present only in polar
molecules. Polar molecules have two poles i.e partial positive and partial negative
poles. These poles arrange themselves in such way that positive pole of one molecule
is directed towards negative pole of another molecule. The partial positive pole of one
molecule is then attracted by the negative pole of another molecule which is called
dipole dipole ineraction. In other words, the forces of attraction between polar molecules
are called dipole to dipole interactions.
Example. Dipole-Dipole interaction exists between HCl molecules
Q. Explain hydrogen bonding and give examples.
Hydrogen bond or bonding:
Definition: The force of attraction between hydrogen atom of one polar molecule and
high electronegative atom like oxygen, nitrogen or fluorine of another polar molecule is
called hydrogen bond or bonding.
Explanation: hydrogen bond is formed when hydrogen atom is covalently bonded to
only oxygen, nitrogen or fluorine. These high electronegative atoms (O, N or F) must
have lone pair electrons. The hydrogen bond formed because lone pair of O, N or F of
one polar molecule attracts the partial positive hydrogen of another polar molecule.
Hydrogen bond is not a chemical bond due to which weaker than covalent bond but
stronger than dipole-dipole interaction. It is represented by dotted lines (……)
Example1: hydrogen bond is present in water molecules due to which water is liquid at
room temperature.
Hydrogen bond is also present in NH3 and HF molecules.
Q. what are the properties of ionic compounds?
Ans. Ionic compounds are those compounds which contain ionic bond. The main
properties of ionic compounds are given below
1. Ionic compounds are solid at room temperature
2. Ionic compounds are hard and brittle
3. Ionic compounds have high melting and boiling point
4. Ionic compounds are soluble in water and other polar solvents
5. Ionic compounds are good conductor of electricity at molten or solution state
6. Ionic compounds reactions are very fast in solution
Q. what are the properties of covalent compounds?
Ans. covalent compounds are those compounds which contain covalent bond. The
properties of covalent compound depend upon shape, intermolecular forces, polarity
bond types etc of molecules. The main properties of covalent compounds are given
below
1. Covalent compounds are solid, liquid or gas at room temperature
2. Covalent compounds have low m.p and b.p
3. Covalent compounds are volatile in nature
4. Covalent compounds are solid, liquid or gas at room temperature
5. Polar covalent compounds are soluble in polar solvent while non polar covalent
compounds are soluble in non polar solvents
6. Covalent compounds reaction are generally slow
Q. Explain properties of metals.
Ans. Metals atoms are held together by metallic bond and the main properties of metal
are given below.
1. They have shine on their surface.
2. They are good conductors of electricity and heat.
3. They reflect light.
4. Metals are ductile.
5. Metals are malleable.
6. They are solid at room temperature except mercury
7. They have high m.p and boiling point
Shape of molecules
The molecule BF3 has a dot
symbol as:
Here the B atom has three bond pairs in its outer shell.
Minimizing the repulsion causes this molecule to have
a trigonal planar shape, where F-B-F bond angles are
all 120°, and all the atoms are in the same plane.
The CH4 molecule has a dot
structure as shown.
BF3
CH4
Carbon has 4 bond pairs. The four H atoms are
arranged about the C atom in a tetrahedral shape .
This shape minimizes the repulsion between the
bond pairs. The 109.5° angle is the same for all HC-H bond angles and is called the tetrahedral
angle.
The molecule NH3 has a dot symbol as
shown where a lone pair in the outer
shell of the central N atom.
NH3
In NH3 the N has 3 bond pairs and 1 lone pair, (4 total pairs).
The shape is called trigonal pyramidal (approximately
tetrahedral minus one atom).
The H2O molecule has this dot structure:
The O in H2O has 2 bond pairs and 2 lone pairs. The shape is
called bent or v shape. The H-O-H bond angle is 104.4°. This angle is
less than that in NH3, due in part to the greater repulsions felt
with two lone pairs
H2
O
Exercise
Q.1 Choose the correct answers
Ans. i.) c ii.) d iii.) b iv). b v). a vi). C vii) d. viii) c ix) c x) d
Q. what is polarity
Ans: when covalent bond is formed between two different atoms having different
electronegativity then the shared pair electrons are more attracted by high
electronegative atom. As a result partial positive charge appears on low electronegative
atoms and partial negative charge on high electronegative bonded atom. Such
molecules are called polar molecules and molecules are said to be having polarity
Q. What is meant by Lewis Structure?
A Lewis structure is a structural representation of a molecule where dots are used to
show electron position around the atoms and lines or dot pairs represent covalent
bonds between atoms.
Example . Lewis structure of water is given below
Q. write the electronic configuration for Na and Cl atoms
Q. 4 Give two reasons why valence electrons in sodium are less tightly bound
than valence electron in Cl atom. What will be consequences in chemical
reaction?
Ans. 1. Sodium atom has larger size than chlorine atom due to which valence
electron of sodium is faraway from nucleus and is less attracted by its nucleus as
compared to chlorine. Therefore the valence electron of sodium is less tightly bound
than valence electron in Cl atom
2. Sodium atom has 11 proton while chlorine has 17 proton due to which the nuclear
attraction for valence electron of sodium is much less than chlorine. . Therefore the
valence electron of sodium is less tightly bound than valence electron in Cl atom
The consequence of above observation is that when sodium react with chlorine the
sodium transfers its lose valence electron to chlorine and sodium form positive ion while
chlorine negative
ion.
Q. Draw lewis dot and bar structure for H2S, PCl3, NH3 and HCN
Give Reasons
Q i. Some atoms form ionic bond while other form covalent bond
Reason: Some atoms form ionic bond because complete transfer of electron from one
atom to another is possible due to low ionization energy of one atom and high electron
affinity of another atom. In contrast other atoms form covalent bond because complete
transfer of electrons is not possible due to high ionization energy of both atoms and
smaller electronegativity difference between them. Usually ionic bond is formed when
electonegativity difference between bonded atoms is greater than 1.7 and covalent
bond is formed when electronegativity difference is less than 1.7.
Q ii. Cl2 has non polar covalent bond while HCl has polar covalent bond.
Reason: Cl2 has non polar covalent bond because made from same atoms (chlorine )
having zero electronegativity difference and shared pair electrons are attracted equally
by both bonded atoms. In contrast HCl is polar because made from two different atoms
(Hydrogen and chlorine) having different electronegativity and shared pair electrons are
attracted unequally by both bonded atoms. As a result partial positive charge appears
on hydrogen and partial negative charge on chlorine due to which HCl molecules have
polar covalent bond.
Q. iii H2S is a gas while H2O is liquid
Reason: H2S is a gas while H2O is liquid because dipole dipole interaction is present in
H2S molecules while Hydrogen bonding is present in H2O molecules where dipoledipole interaction is much weaker than hydrogen bonding. Hydrogen bonding only
occurs when hydrogen is covalently bonded to N,O or F where sulphur cannot form
hydrogen bonding in its compound due to larger size and smaller electronegativity.
Q iv Dative or coordinate covalent bond is always polar
Reason: Dative or coordinate covalent bond is always polar because it is formed from
two different atoms having different electronegativity. Additionally the shared pair
electrons are supplied by one bonded atom due to which partial positive charge appears
on donor and partial negative charge on acceptor and molecule becomes polar.
Q v. Ionic compounds are good conductor as compared to covalent compound
Reason: Ionic compounds are good conductor of electricity in molten or solution state
because they are present in the form of positive and negative ions which are
responsible for conduction. When electric current is passed from ionic compounds in
molten or solution state the cations start motion toward cathode and anions towards
anode due to which conduction occurs. The pure covalent compounds do not dissociate
into ions so do not conduct electricity.