Chapter 2…..

Physics of Molecules

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

a) Sodium atom give its one electron to the Chlorine atom

b) Sodium and Chlorine ions both have stable outer shells that are filled with electrons

c) Na⁺ and Cl^- ions are attracted towards  each other because of opposite charges.

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.

Fig.b) H₂ molecule

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.