Chapter
2…..
Q1.
Define Molecules.
Molecules:
“Molecules is the combination of two or more atoms
forms a chemical bond with each other.” It doesn’t matter if the atoms are
the same or are different from each other.
Q2. Explain concept of ionic bonding
and summarize the properties of ionic bonds.
OR
Q2. Explain ionic bonding in NaCl and
summarize the properties of ionic bonds.
OR
Q2. Give the physical properties of
ionic compounds [4
M]
Ionic
Bonding:
1) Definition
– “An ionic bond is a type of chemical bond formed due to electrostatic
attraction between positive ions and negative ions.”
2) One
or more electrons from one atom may transfer to the other and the resulting
positive and negative ions attract each other and forms an ionic bond.
3) Ionic
bonds are formed between an electropositive element (i.e. metal) and electronegative
element (i.e. non-metal).
4) In
ionic bond formation, an electropositive atom (metal), releases some of its
electron(s) to achieve a stable electronic configuration and after releasing
some of its electron(s) the atom becomes a cation. The electronegative atom
(non-metal) accepts the electrons to achieve a stable electronic configuration
again and after accepting electron(s), the atom becomes an anion.
Ionic
bonding in NaCl-
1) Common
table salt is sodium chloride (NaCl) which is very good example of ionic
bonding.
2) The
Na atom has electronic configuration 1s2 2s2 2p6
3s1 and has low ionization energy and hence easily loses an
electron. The Cl atom has electronic configuration 1s2 2s2
2p6 3s2 3p5.
3) When
sodium (Na) and chlorine (Cl) are combined, the sodium atom lose an electron,
forming cation (Na+) and the chlorine atom gain accept an electron
to form anions (Cl-).
4) The
formation of NaCl molecule is shown below:
Na + Cl →
Na++ Cl- → NaCl
b)
Sodium and Chlorine ions both have stable outer shells that are filled with
electrons
Fig.
Formation of NaCl molecule
Properties /
characteristics of ionic compound:
1) Ionic bonds are very strong.
2) They have high melting point and boiling point.
3) They are hard and brittle.
4) Ionic bonds are polar in nature.
5) They conduct electricity but only when they are dissolved in water.
6) They are soluble in water or other polar solvent but insoluble in non-polar solvent.
Q3. What is meant by covalent bond.
Give properties of covalent crystal? [4
M]
1.
Covalent bond- A covalent bond is formed between the two atoms by
shearing of pair of electrons between atoms.
2.
Two atoms that
are covalently bonded will contribute at least one electron to the bond and
sheared electron may be consider belong to both the atoms.
3.
The simplest
covalent bond is form in hydrogen molecule, the hydrogen atoms shear the two
electrons.
Fig.a) Two
separate atoms at large distance.
4.
An atom with the
several electrons in its outermost shell can form several covalent bonds with
other atoms.
e.g.
Carbon atom having four electrons in its outermost cell falls four covalent
bonds with other four atoms.
Fig. Covalent bonding in methane (CH4)
molecule
Properties of covalent
bond/ compounds:
1.
The covalent
bonds have strongly directional properties. The bond orientation depends upon
the orientation of overlapping charge density.
2.
Covalent
compounds are mostly liquids and gases.
3.
Melting point
and Boiling point of covalent compounds are low as compare to that of ionic
compounds.
4.
Solubility of
covalent compound in water varies from compound to compound but it is low as
compare to ionic crystal.
5. When
dissolve in water they don’t conduct electricity. Some crystals are insulator
at ordinary temp. In semiconductors like germanium and silicon the electrical
conductivity increases with increase in temp.
Q.4 Draw the diagram of covalent bonds
in Silicon Semiconductor.
Q.5 Draw the diagram of covalent bonds
in Germanium Semiconductor.
Q 6. Explain the formation of water
molecule by shearing of electrons.
1. Water is made up
of two hydrogen atoms covalently bonded to an oxygen atom.
2. While this bonding involves the shearing of electrons
3. The oxygen atom attract the electrons more strongly
i.e. the oxygen has higher electronegativity.
Fig. Formation of water molecule by shearing of
electrons
Q 7. Define polar molecules ?
The formation of molecule between two atoms with
different electronegativity is known as polar molecule and such a
bond is called as polar covalent bond.
e.g. H-Cl
Q 8. Define non-polar molecules ?
The formation of molecule between two atoms with equal
(same) electronegativity is known as non- polar molecule and such
a bond is called as polar covalent bond.
e.g.
H-H, Cl-Cl.
Q 9. What are types of covalent bond ?
There are three types of covalent bond depending
upon the number of shared electron pairs
1. Single
covalent bond
2. Double
covalent bond
3. Triple
covalent bond
1. Single
Covalent bond:- “A covalent bond formed by the mutual
sharing of one electron pair between tow atoms is called a single covalent
bond.” It is denoted as single short line (-).
Example: H – Cl, Br – Br
2. Double
Covalent bond:- “A covalent bond formed by the mutual
sharing of two electron pairs between two atoms is called a double covalent
bond.” It is denoted as double short line (=).
Example: O = O
3. Triple
Covalent bond:- “ A covalent bond formed by the mutual
sharing of three electron pairs between two atoms is called a triple covalent
bond.” It is denoted as triple short line (≡).
Example: N ≡ N, -C ≡ C-
Q 10. Explain Van der Waal’s bonding in
molecules with suitable example. [4
M]
1. Definition-
All atoms, molecules even inert gas atoms exhibit weak, short range attraction
for one another due to van der Waal’s forces. These interaction are called van
der Waal’s bonds.
2. These
bonds are due to interaction between electric dipoles of atoms or molecules.
3. Van
der Waal’s forces include attraction and repulsion between atoms, molecules and
surfaces as well as other intermolecular forces.
4. Let
us consider a polar molecule which has permanent electric dipole moment. This
molecule interacts with other molecules having electric dipole moment. This
interaction is attractive and called dipole-dipole interaction as shown in fig.
Fig.
Different possible interactions between polar molecules.
Fig.
Polar molecule induces dipole moment in non-polar molecule and attracts it.
5. The
potential energy that arises due to dipole-dipole interaction is negative, so
the force between them is attractive. It is observed that the dipole-dipole
interaction energy between two molecules is proportional to 1/r6 and the force
is proportional to 1/r7, where r is the separation between two molecules.
6. Van
der Waal’s bond in polyvinyl chloride (PVC) are shown in fig.
Fig.
Van der waals bonds in PVC.
Properties of van der Waal’s bonding:
- 1. They are weaker than normal covalent and ionic bond.
- 2. Van der Waal’s forces are additive and cannot be saturated.
- 3. They have no directional characteristics.
- 4. They are short range forces and hence only interaction between the nearest particles.
- 5. Van der Waal’s attraction is greater if the molecules are closer.
- 6. Low melting point and boiling point as well as less mechanical strength.
- 7. Bond energies is 0.1eV or less.
Q 11. Explain hydrogen bonding and
discuss formation of hydrogen bonds in water.
[4 M]
1.
Definition- The
hydrogen bond is electrostatic attraction between two polar molecules that
occurs when hydrogen atom covalently bound to highly electronegative atom.
2.
A hydrogen bond
is form due to attractive dipole-dipole interaction between an electronegative
atom and hydrogen atom bonded to another electronegative atom. This bond always
involves a hydrogen bond.
3.
These hydrogen
bond attractions can occur between molecules (intermolecular) or within
different parts of a single molecule (intra-molecular).
4.
Molecules that
have non polar covalent bonds do not form hydrogen bonds.
5.
This type of
bond can occur in both organic molecules, such as DNA and inorganic molecules,
such as water.
Formation
of hydrogen bond in water
1.
Every water
molecule in bonded to four adjacent water molecules through hydrogen bonding.
2.
The two hydrogen
atom attached to every oxygen form hydrogen bonds to the oxygen molecules in
adjacent water molecule.
3.
This
intermolecular hydrogen bonding is responsible for relatively high boiling
point of water.
4.
A hydrogen bond
consists of a hydrogen bond acceptor, the target atom and a hydrogen bond
donar, the hydrogen atom itself.
Properties of hydrogen bonding:
1. A
hydrogen bond tends to be stronger than van der waals forces, but weaker than
covalent bonds or ionic bonds.
2. Bond
energy is of the order of 0.1 eV.
3. The
length of the hydrogen atom varies depending on bond strength, temp and
pressure.
Q 12. What is meant by metallic
bonding? Explain the properties of metallic bonding. [4 M]
1. Definition-
The metal atom becomes positively charged ions and are attracted to the free
electrons. This attraction is called metallic bonding.
2. Metallic
bonding is the force of attraction between valence electrons and the metal
atoms.
3. Metallic
bonds are formed by the sharing of valence electrons of all the atoms of a
metal.
4. The
valence electrons move randomly in different direction and are called free
electrons. Except free electrons, all other electrons are tightly bound to the
nuclei.
Fig. Metallic bonding
5. The
metallic bonding depends upon following types of forces-
a. The
force of attraction between positive ions and free electrons.
b. The
force of mutual repulsion of positive ions.
c. The
force of repulsion of free electrons.
Properties of metallic bonding:
1) Metallic bonds are weaker than the ionic
bonds and covalent bonds.
2)
Due to large number of available free electrons, they are good conductors of
electricity.
3)
Metals are good conductors of heat.
4)
They have high melting point and high boiling point.
5)
Metal have low ionization energy.
