Chemical Bonding — Practice

Question 1

Which one of the following four elements is more electronegative?

Accepted Answer

Oxygen

Question 2

An element with mass number 23 and atomic number 11 forms an ionic compound with another element Y. The charge on the ion formed by it is

Accepted Answer

+1

Question 3

An element A forms a chloride ACl4. The number of electrons in the valence shell of A is

Accepted Answer

4

Question 4

The inert gas element which does not have octet electronic configuration in its outermost orbit is

Accepted Answer

Helium

Question 5

Number of covalent bonds in a methane molecule

Accepted Answer

4

Question 6

The concept of hybridisation of orbitals of an atom was introduced by

Accepted Answer

Linus Pauling

Question 7

The value of the bond angle in the beryllium chloride molecule is

Accepted Answer

180 degrees

Question 8

Explain the difference between the valence electrons and the covalency of an element.

Accepted Answer

Valence electrons are the electrons present in the outermost shell of an atom. Covalency is the total number of covalent bonds that an atom of that element actually forms.

Question 9

Draw simple diagrams to show the arrangement of valence electrons in calcium oxide (CaO), water (H2O) and chlorine (Cl2).

Accepted Answer

CaO is ionic: Ca loses two electrons to give Ca2+ with 2, 8, 8 and O gains them to give O2- with 2, 8. H2O is covalent: oxygen shares one pair with each of two hydrogens and keeps two lone pairs. Cl2 is covalent: each chlorine shares one electron, and each keeps three lone pairs.

Question 10

A, B and C are three elements with atomic numbers 6, 11 and 17 respectively. (i) Which of these cannot form an ionic bond, and why? (ii) Which cannot form a covalent bond, and why? (iii) Which can form both?

Accepted Answer

(i) A, carbon, cannot form an ionic bond. (ii) B, sodium, cannot form a covalent bond. (iii) C, chlorine, can form both.

Question 11

How does the Lewis dot structure help in understanding bond formation between atoms?

Accepted Answer

It shows only the valence electrons, which are the only electrons that take part in bonding, and so makes visible how many an atom must lose, gain or share to reach eight.

Question 12

Nitrogen and hydrogen react to form ammonia, and carbon and hydrogen bond to form methane. For each reaction give the valence of each atom involved and describe the Lewis structure of the product.

Accepted Answer

For ammonia: nitrogen has valence 3 and hydrogen 1. Nitrogen has five valence electrons, shares three of them with three hydrogens, and keeps one lone pair - three N-H bonds. For methane: carbon has valence 4 and hydrogen 1. Carbon shares all four valence electrons with four hydrogens and keeps no lone pair - four C-H bonds.

Question 13

Explain the formation of the N2 and O2 molecules using valence bond theory.

Accepted Answer

Nitrogen is 1s2 2s2 2px1 2py1 2pz1 with three unpaired p electrons. The px orbitals of the two atoms overlap head-on along the internuclear axis to give a sigma px-px bond, and the py and pz orbitals overlap laterally to give pi py-py and pi pz-pz bonds - a triple bond in all. Oxygen is 1s2 2s2 2px2 2py1 2pz1 with two unpaired p electrons. The py orbitals overlap along the axis to give a sigma py-py bond and the pz orbitals overlap laterally to give a pi pz-pz bond - a double bond.

Question 14

How do bond energies and bond lengths of a molecule help us in predicting their chemical properties? Explain with examples.

Accepted Answer

A short bond with a high dissociation energy is a strong bond, so the molecule is stable and unreactive; a long bond with low energy breaks easily, so the molecule is reactive. H-H is 0.74 angstrom at 436 kJ per mol and hydrogen is stable, while I-I is 2.68 angstrom at only 151 kJ per mol and iodine is easily split. Across the hydrogen halides the bond lengthens from H-F at 0.918 to H-I at 1.61 angstrom and the energy falls from 570 to 298 kJ per mol, which is the order of their thermal stability.

Question 15

Explain the formation of sodium chloride and calcium oxide on the basis of electron transfer from one atom to another.

Accepted Answer

Sodium, 2, 8, 1, loses one electron to become Na+ with the neon configuration 2, 8. Chlorine, 2, 8, 7, gains that electron to become Cl- with the argon configuration 2, 8, 8. The oppositely charged ions attract electrostatically to give NaCl. Calcium, 2, 8, 8, 2, loses two electrons to become Ca2+ with 2, 8, 8. Oxygen, 2, 6, gains both to become O2- with the neon configuration 2, 8. Ca2+ and O2- attract to give CaO.

Question 16

Why does the H-N-H angle in ammonia come out as 107 degrees 48 minutes rather than 90 degrees or 109 degrees 28 minutes?

Accepted Answer

If the three hydrogens simply overlapped the three mutually perpendicular p orbitals of nitrogen, the angle would be 90 degrees. It is not, so nitrogen undergoes sp3 hybridisation, which would give a regular tetrahedral 109 degrees 28 minutes. But one of the four sp3 orbitals holds a lone pair, and lone pair-bond pair repulsion exceeds bond pair-bond pair repulsion, so the three bonds are pushed closer together, to 107 degrees 48 minutes.

Practice — Chemical Bonding

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