Short questions with answers First year chapter 6

Chapter  # 06 .                           Chemical Bonding                                          F.Sc Chemistry 1^st Year

1.    What is chemical Bond?

Ans:  the force which holds two or more atoms or ions together is known as chemical bond.e.g  ionic bond, covalent bond, coordinate covalent bond.

1.    Give the causes of chemical combination.

Ans:  all the Nobel gases has the configuration ns² np⁶. (except He has ns²) All the atoms have tendency to achieve the configuration of Nearest Nobel gas to  be stabilized. All the atoms of elements loss or gain electrons or share electrons to get the configuration of Nobel gas configuration, which are causes of chemical combination.

2.    Why Bond length is also called compromise distance?

Ans:  when atoms approach each other for  bond formation, forces of attraction and repulsion operates simultaneously. When they reached at a certain distance where the forces of attractions are maximum and potential energy is minimum, this distance is called compromise distance

e.g

bond distance(which is 75.4pm in case of H atoms in H₂ Molecule).

3.    What is bond formation Energy?

Ans:  the amount of energy released during the formation of Molecule is called bond formation Energy. Which is 436.5kj/mol in case of formation of H₂ Molecule.

4.    Define Atomic radii.

Ans: the average distance between the nucleus of an atom and the valance shell considering it to be spherical, is called atomic radius.

Unit: the atomic radii usually measured in pm

1pm = 10^-12 m

5.     Why atomic radii decreases along the period in periodic table?

Ans: Atomic radii decrease along the period because the nuclear charge increases and shielding effect remains the same. Nucleus   attracts the electrons more strongly results the decrease in atomic radii. While in transition elements this decrease is small due to the presence of intervening electrons. e.g. from Sc(21) to Zn(30).

6.     Why atomic radii increases along the group in periodic table?

Ans: Atomic radii increase along the group because number of shells increases down the group and the shielding effect also increases.

7.    What  is shielding Effect or Screening Effect?

Ans:  the decrease in the force of attraction between outermost electrons and the nucleus due to inner shell is called Shielding effect or Screening Effect.

8.    What is ionic radius?

Ans: : The average distance between the nucleus of an Ions  and the valance shell considering it to be spherical, is called Ionic radius.

Unit: the Ionic  radii usually measured in pm

1pm = 10^-12 m

9.    What is Covalent  radius?

Ans:  The covalent radius of atoms of elements is half of the single bond length between two similar atoms which are bonded covalently in a molecule. e.g. the bond length of Cl2 molecule is 198.8pm. so covalent radius is 198.8/2 =  99.4pm.

10.                       Define ionization energy?

Ans: Minimum amount of energy  required for the removal of one electron from the valance shell of the individual gaseous atom.

11.                       Define Electron affinity?

Ans:  Amount of energy released when an electron is added to an empty or partially filled orbital of an atom in its valance shell to form an anion. (having negative charge.)

12.Why  the electron affinity value of fluorine is less  then chlorine?

Ans: The electron affinity value of fluorine is less then chlorine because fluorine has very small size. It contains two orbits which form thick electronic cloud, which repels the incoming electrons.

13.                       Define Electronegativity?

Ans:  tendency of an electron to attract the shared pair of electrons towards itself in a molecule is called electronegativity. It has no units. Measured by Pauling’s  scale.

14.How  Electro negativity determines the nature of a chemical bond?

Ans: If the electronegativity value is zero then the bond will be non-polar.e.g  H₂ Molecule

If electronegativity difference between the bonded atoms is 1.7 or greater,

bond is Ionic e.g, NaCl.

If electronegativity difference between the bonded atoms is less than  1.7,

bond is covalent  e.g, HCl

15.                       Why 2^nd E.A value is positive ? or Endothermic?

Ans: when 2^nd electron is added in the valence shell of uninegative ion at that time already present electrons strongly repelled the incoming electron now incoming electron absorbs energy to overcome this repulsion so 2^nd Electron affinity value is  positive or endothermic.

16.                       No bond is 100 % ionic in Nature. Justify.

Ans: when cations and anions come close to each other then two forces become operate  Nucleus of cation attract the electrons of Anion  II. Nucleus of cation repell the nucleus of anion. By the result of both forces distortion of electronic cloud takes place and polarizability involved in anion so polarizability is only covalent character.e.g NaCl has 72% ionic character and CsF has 92% ionic character.

17.            Differentiate between polar and non polar Covalent bond.

Ans: Polar Covalent Bond	Non-Polar Covalent  bond
The covalent bond in which bonding electrons are not equally shared between two atoms.	The covalent bond in which bonding electrons are equally shared between two atoms.
This bond is formed between two different atoms.	This bond is formed between two similar atoms.
E.N difference range 0.5-1.6	E.N difference range 0-0.4
Comparatively stronger	Comparatively weaker
e.g. H-Cl, H-F	e.g. H-H, Cl-Cl

18.                       Limitations of Lewis Model.

Ans.It does not tells us about the shapes of molecules.

It does not tells us about bond distance.

It does not tells us about bond angles.

It does not tells us about the behavior of molecule during a chemical reaction.

19.Distinction between coordinate covalent bond and covalent bond vanishes during the formation of NH₄⁺ or H₃O⁺

Ans: NH₃ or H₂O donates its electron pair to H⁺ to form NH₄⁺ or H₃O⁺.All the bonds between Oxygen and Hydrogen or Nitrogen and Hydrogen behave exactly alike, although one of the bond is coordinate covalent bond. Every bond in Hydronium ion (H₃O⁺)  is 33% coordinate and 66% covalent.

Similarly all bonds in Ammonium ion (NH₄⁺ )is  25% coordinate and 75% covalent.

20.                       Why atomic radius cannot measure precisely?

Ans: Atomic radius cannot measure precisely because there is no sharp boundry of an atom, the probability of finding an electron never becomes exactly zero even at large distance from nucleus.

2.the electronic probability distribution is affected by neighboring atoms. Hence size of an atom is changed from compound to compound.

21. why second ionization energy value is greater than first ionization value?

Ans: second ionization energy value is greater than first ionization value due to the following reasons.

1.Increase of the effective nuclear charge.

2.Size of ion reduces.

3.Force of attraction between nucleus and remaining electrons increases and 4.large amount of energy is required for the removal of second electron from uni-positive ion.

Q.22.          I.E is index of metallic character why?

Ans:Metals have loosely held electrons which are delocalized and are responsible for the properties of metals. So, metals have low ionization energies.

Q.23. Why NH₃ and PH₃ give coordinate covalent bond?

Ans:NH₃ and PH₃ have lone pairs of electrons, which can be donated to H+ to make a coordinate covalent bond. In this way, NH₄⁺ and PH₄⁺ are produced which have perfect tetrahedral structure and all the four bonds have perfectly equal status.

Q24.What is difference between sigma & pi bond?

Ans:SIGMA BOND:

The bond which is formed by the head to head overlapping called sigma bond. The electron density is present between two nuclei.

Pi- BOND:

The bond which is formed by the sideways overlapping of two half  p-filled orbitals. The electron density is present above and below the line joining the two nuclei.

Q.25. Why the size of an atom cannot be measured directly?

Ans: The size of an atom cannot be measured directly due following reasons:

(i)There is no sharp boundary of an atom. The probability of finding an electron never becomes zero even at larger distances from the nucleus.

(ii)The electronic probability distribution is affected by neighboring atoms. For this reason, the size of an atom may change from one compound to another.

Q.26. Why E.A of Fluorine is less than the expected value?

Ans: Since the size of fluorine is very small when electron is added in the fluorine it is strongly repelled by the already existing electrons. An extra amount of energy is provided to add an electron therefore its electron affinity is less than the expected value.

Q.27. Why sigma bond is stronger than pi bond? Or Why pi bond is more diffused than sigma bond?

Ans:  Sigma bond is more diffused than pi bond due to the linear overlapping of orbitals. Moreover electron density is present between two nuclei which is strongly attracted by two nuclei. While in pi bond electron density is not strongly attracted by two nuclei therefore it is weak than pi bond.

Q.28.Define bond length.

Ans.The distance between the nuclei of two atoms forming a covalent bond is called bond length. In general it is the sum of the covalent radii of the combined atoms.

Q.29.What is dipole moment? What are its units?

Ans.The dipole moment may be defined as the product of electric charge (q) and distance (r) between the two oppositely charged centers. It is vector quantity as it has magnitude and direction. It plays a major role in determining the % age ionic character of a covalent bond and the shapes of molecules. The dipole moment is measured in Debye units (D). It is denoted by symbol µ.

Q.30.Define bond energy. Give its units.

Ans.The bond energy is defined as the average amount of energy required to break all bonds of particular type in one mole of substance. It is determined by measuring the heat involved in a chemical reaction.

It may also defined as the energy required to break Avogadro’s number

(6.02 x 1023) of bonds or the energy released when an Avogadro number of bonds are formed. It is a measure of strength of bonds. The bond energy is measured in KJ mol^–1.

Q.31.In many cases, the distinction between a coordinate covalent and

a covalent bond vanishes after bond formation. Explain with the help of

an example.

Ans. A coordinate covalent bond is not essentially different from other covalent bonds, it involves the sharing of pair of electrons between two atoms. An example is formation of NH4+ ion in which all bonds are identical, so the d st nction between a coordinate covalent bond and covale t bond vanishes after bond formation.     

Q.32.NaCl is a harder substance at room temperature than glucose explain.

Ans. The hardness of substance depends on the strength of the forces between the particles forming a substance. NaCl is an ionic compound and consists of Na⁺ and C^l– ions which are held together by strong electrostatic forces of attraction while glucose consists of molecules which are held together by weak intermolecular forces. Therefore, NaCl is a harder substance at room temperature than glucose.

Q.33. Why CO2 is non–polar molecule although C–O bond is polar?

Ans. Each C–O bond in CO2 is polar. The two bond dipoles in CO2 are equal in magnitude and are exactly opposite in direction. The bond dipoles cancel each other. Therefore, the overall dipole moment of CO2 is zero. Thus CO2 is a non–polar molecule.    O= C  = O

Q.34.The abnormality of the bond length and bond strength in HI is less prominent than that of HCl, give reason?

Ans:Chlorine has higher electronegative than iodine. So, the polarities of HCI and HI bonds are unequal. Therefore, abnormality of bond length and bond strength of HCI is more prominent than HI.

Q.35. A lone pair of electrons occupi s more space than a bond pair?

Ans:A lone pair of electrons occupies more space than bond pair because lone pair is attracted by only one nucleus while bond pair is attracted by two nuclei. Due to less nuclear attraction to lone pair its electronic charge is spread out more in space than that of bond pair.

Q.36.Why H2O is a polar molecule?

Ans.H₂O is a bent molecule with two polar bonds. Both the bonds are identical, so the bond dipoles are equal in magnitude. Because the molecule is bent, however the bond dip les do not directly oppose to each other and therefore, do not cancel each other. Hence the H₂O molecule has an overall dipole moment (µ= 1.85D), because H2O has dipole moment, it is polar molecule.

Q.37.What is molecular orbital theory?

Ans. According to molecular orbital theory, atomic orbitals overlap to form molecular orbitals n atomic orbitals combine to form n molecular orbitals. Half of them are bonding molecular orbitals and half antibonding molecular orbitals. In this combination, the individual atomic orbital character is lost in order to form an entirely new orbital that belongs to whole molecule. The theory successfully explains bond order and paramagnetic property of O₂.