Materials Science Questions and Answers for Freshers on “Atomic Bondings in Solids”.
1. Which of the following is a secondary bond?
a) Metallic bond
b) Hydrogen bond
c) Covalent bond
d) Ionic bond
Answer: b
Clarification: Metallic, covalent, and ionic bonds are strong in nature, hence primary; hydrogen bond has comparatively lower bond energies; hence secondary.
2. VSEPR stands for Valence Shell Electron Pair Repulsion. This theory helps us predict:
a) Bond angles
b) Bond length
c) Electron energy
d) Shell radius
Answer: a
Clarification: VSEPR theory implies that electrons in valence shell arrange themselves in a manner so as to experience minimum repulsion from other electrons. For e.g., in a CH4 molecule, all the bonds are symmetrically arranged at approximately 109°.
3. In water, the bond angle between the H-O H-O bond is less than 109° Why?
a) The H atoms attract each other
b) Oxygen atom is much larger than Hydrogen atoms
c) Lone pair-bond pair repulsion is greater than bond pair-bond pair repulsion
d) VSEPR theory does not apply for planar molecules like water
Answer: c
Clarification: Bond angle is 109° for symmetric molecules like methane. However, it is known that repulsion between a lone pair and a bond pair is more than that between two bond pairs. Hence, bond angle is reduced from 109° so as to experience minimum repulsion from lone pairs.
4. For covalent molecules, van der Waals radius is always greater than a covalent radius.
a) True
b) False
Answer: a
Clarification: Van der Waals radius is half the internuclear distance between two non-bonded atoms on their closest approach whereas covalent radius is half the bond length of a homo-nuclear molecule. Since the distance between two bonded atoms is less than non-bonded atoms, hence the answer.
5. If E1 is the bond enthalpy of a sigma-bond and E2 is that of a pi-bond, then:
a) E1 > E2
b) E1 = E2
c) E1 < E2
d) Can’t be predicted
Answer: a
Clarification: Since the overlap of orbitals in a sigma-bond is along the internuclear axis contrary to sideways overlap in pi-bonds, the bonding is more effective in a sigma-bond. Hence, E1 > E2. Though, the total energy of a double bond is more than a single bond)
6. Which of the following molecules is non-linear?
a) Hbr
b) CO2
c) C2H2
d) H2S
Answer: d
Clarification: H2S is non-linear due to presence of lone pairs on S atom. Hbr being a binary molecule is linear. Double bonds in CO2 and C2H2 also leads to linear geometry.
7. With an increase in bond length, bond energy:
a) increases
b) decreases
c) may either increase or decrease
d) does not change
Answer: b
Clarification: The coulombic force of attraction is inversely proportional to the square of a distance between the charges. As the internuclear distance in a bond increases, the force of attraction between nuclei and electrons decreases.
8. sp3 hybridization is present in which of the following molecules?
a) BeCl2
b) H2O
c) C2H4
d) BCl3
Answer: b
Clarification: Hybridization takes place between one 2s and three 2p orbitals of Oxygen so as to minimize electronic repulsion. BeCl2, C2H4, and BCl3 have sp, sp2, and sp2 hybridization respectively.
9. Which of the following is the correct order for bonding energies of molecular orbitals?
a) σ1s < σ2s < σ*2s < σ*1s
b) σ1s < σ2s < σ*1s < σ*2s
c) σ1s < σ*1s < σ2s < σ*2s
d) none of the mentioned
Answer: c
Clarification: Energy of bonding MO is less than that of anti-bonding MO formed from same atomic orbitals. Also, MO formed from atomic orbitals of higher energy possess higher energy.
10. Which of the following molecules is diamagnetic?
a) O2
b) CO
c) (N_2^+)
d) NO
Answer: b
Clarification: Out of the above species, only CO does not have unpaired electrons. Others have partially filled molecular orbitals and hence are attracted by external magnetic fields.
Materials Science for Freshers,