101. Use the graph description below.
A bar graph compares electrical conductivity of an ionic compound in three states: solid, molten, and aqueous solution. The solid bar is nearly zero, while the molten and aqueous bars are high.
The pattern is best explained by
ⓐ. formation of neutral atoms only in molten and aqueous states
ⓑ. cations and anions become fixed in aqueous solution
ⓒ. free movement of ions in molten and aqueous states
ⓓ. covalent bond formation in the solid state
Correct Answer: free movement of ions in molten and aqueous states
Explanation: Ionic compounds have charged particles, but conductivity depends on whether those particles can move. In the solid state, ions are locked in fixed lattice positions, so the conductivity is nearly zero. In the molten state, ions can move through the liquid. In aqueous solution, ions separate and move through water. The graph therefore reflects ion mobility rather than the mere presence or absence of charge.
102. A sodium chloride crystal is struck sharply and breaks along planes. This brittleness is commonly explained by
ⓐ. ion-layer displacement brings like charges together
ⓑ. conversion of \(\mathrm{Na^+}\) into neutral sodium atoms
ⓒ. formation of covalent bonds between all neighbouring ions
ⓓ. like charges move apart after every layer shift
Correct Answer: ion-layer displacement brings like charges together
Explanation: Ionic crystals are hard but often brittle. When a layer of ions is displaced by force, ions of the same charge may come next to each other. Like charges repel strongly, so the crystal can split along planes. This is different from a metal, where layers can slide more easily because metallic bonding is non-directional and delocalized. Brittleness in ionic solids is therefore connected with the ordered arrangement of alternating charges.
103. A table of properties for a substance is shown below.
| Property | Observation |
|---|
| P | high melting point |
| Q | conducts electricity when molten |
| R | brittle crystalline solid |
| S | does not conduct in solid state |
The observations together most strongly suggest that the substance is
ⓐ. a non-polar covalent gas
ⓑ. an ionic compound
ⓒ. a monoatomic noble gas
ⓓ. a mixture of uncharged molecules only
Correct Answer: an ionic compound
Explanation: The listed properties fit an ionic compound well. High melting point indicates strong forces in the solid. Conductivity when molten shows that charged particles are present and become mobile after melting. Brittleness and crystallinity are also common features of ionic lattices. Lack of conduction in the solid state is expected because ions are fixed in place, not because charges are absent.
104. Ionic compounds often dissolve better in polar solvents such as water because
ⓐ. non-polar solvents always form stronger ion-dipole attractions
ⓑ. water removes all charges from ions permanently
ⓒ. polar solvent molecules can interact with separated ions
ⓓ. ionic compounds are made of neutral covalent molecules only
Correct Answer: polar solvent molecules can interact with separated ions
Explanation: Polar solvents have partial positive and partial negative ends. These ends can attract anions and cations respectively, helping to separate ions from the lattice. Water is a common polar solvent, so many ionic compounds dissolve in it. Non-polar solvents cannot stabilize separated ions as effectively. Dissolving does not mean the ions lose their charges permanently; it means ions become solvated and dispersed in the solvent.
105. The best reason an aqueous solution of \(\mathrm{NaCl}\) conducts electricity is that
ⓐ. fixed \(\mathrm{Na^+}\) and \(\mathrm{Cl^-}\) ions in the crystal lattice
ⓑ. hydrated electrons produced from \(\mathrm{Na^+}\) ions
ⓒ. mobile \(\mathrm{Na^+}\) and \(\mathrm{Cl^-}\) ions
ⓓ. neutral \(\mathrm{NaCl}\) molecules moving through water
Correct Answer: mobile \(\mathrm{Na^+}\) and \(\mathrm{Cl^-}\) ions
Explanation: When \(\mathrm{NaCl}\) dissolves in water, its ions become separated and hydrated. The solution contains mobile \(\mathrm{Na^+}\) and \(\mathrm{Cl^-}\) ions. These mobile charged particles carry electric current through the solution. The solid crystal itself would not conduct well because its ions are fixed in lattice positions. Conductivity in aqueous ionic solutions is therefore due to ion movement, not conversion of ions into free electrons.
106. Consider these statements about ionic compounds.
I. They generally have strong electrostatic forces in the solid state.
II. They commonly conduct electricity in solid state because ions are mobile.
III. They may conduct in molten or aqueous state because ions can move.
The valid set is
ⓐ. I only
ⓑ. I and III only
ⓒ. I and II only
ⓓ. II and III only
Correct Answer: I and III only
Explanation: Ionic compounds have strong electrostatic attraction between cations and anions in the solid lattice. However, the ions in the solid are fixed in position and cannot move freely, so solid ionic compounds usually do not conduct electricity well. When molten or dissolved in water, ions become mobile. Mobile ions can carry charge through the liquid or solution. Statement II is the one that confuses the presence of ions with their ability to move.
107. A solid has a high boiling point and dissolves in water to give a conducting solution. The most reasonable microscopic picture is
ⓐ. fixed neutral atoms with no charged particles at any stage
ⓑ. isolated covalent molecules that conduct without charges
ⓒ. electrons only moving through a lattice of neutral molecules
ⓓ. strong lattice ions that become mobile in water
Correct Answer: strong lattice ions that become mobile in water
Explanation: A high boiling point suggests strong attractive forces in the substance. A conducting aqueous solution suggests the presence of mobile charged particles after dissolving. Ionic solids fit both observations because their ions are strongly held in a crystal lattice but can become mobile in solution. Neutral covalent molecules generally do not conduct unless they produce ions by reaction or dissociation. The combined property pattern is more informative than any single observation alone.
108. A claim says, "Solid \(\mathrm{KBr}\) does not conduct electricity, so it cannot contain ions." The best correction is that
ⓐ. solid \(\mathrm{KBr}\) contains only neutral molecules
ⓑ. \(\mathrm{KBr}\) contains ions fixed in the solid lattice
ⓒ. \(\mathrm{KBr}\) becomes ionic only after electricity is passed through it
ⓓ. ions conduct electricity only when they are fixed in place
Correct Answer: \(\mathrm{KBr}\) contains ions fixed in the solid lattice
Explanation: Lack of electrical conduction in the solid state does not prove that ions are absent. In solid \(\mathrm{KBr}\), \(\mathrm{K^+}\) and \(\mathrm{Br^-}\) ions are present in a regular lattice. The ions cannot move freely because they are held at fixed lattice positions. When the compound is molten or dissolved, the ions can move and conduction becomes possible. The key condition for conduction is mobility of charged particles, not merely their existence.
109. Match the property of an ionic compound with its microscopic reason.
| Property | Microscopic reason |
|---|
| P. High melting point | 1. strong electrostatic attraction in lattice |
| Q. Brittleness | 2. like-charge repulsion after layer displacement |
| R. Conducts when molten | 3. mobile ions are present |
| S. Poor conductor as solid | 4. ions fixed in lattice positions |
The proper matching is
ⓐ. P-2, Q-1, R-4, S-3
ⓑ. P-3, Q-4, R-1, S-2
ⓒ. P-1, Q-2, R-3, S-4
ⓓ. P-4, Q-3, R-2, S-1
Correct Answer: P-1, Q-2, R-3, S-4
Explanation: High melting point follows from strong electrostatic attraction in the ionic lattice. Brittleness occurs when displacement brings like charges near each other and causes repulsion. Molten ionic compounds conduct because their ions become mobile. Solid ionic compounds conduct poorly because the ions remain fixed in their lattice positions. Each property is best explained by connecting the macroscopic observation with ion arrangement and ion mobility.
110. A sample is tested in three forms: solid, molten, and aqueous solution. It conducts only in the molten and aqueous forms. This result supports the presence of
ⓐ. mobile ions after melting or solvation
ⓑ. neutral atoms that conduct only when cold
ⓒ. electrons trapped permanently in fixed bonds
ⓓ. molecules with no charged particles at all
Correct Answer: mobile ions after melting or solvation
Explanation: The result is typical of an ionic compound. In the solid state, ions are present but fixed in a lattice, so they cannot carry charge through the solid. Melting breaks the rigid lattice enough for ions to move. Dissolving in water separates and solvates ions, allowing them to migrate through the solution. This test separates ionic conduction by mobile ions from metallic conduction by mobile electrons.
111. Covalent bond formation is most directly described as
ⓐ. sharing of one or more electron pairs between atoms
ⓑ. complete transfer of electrons from every atom to another atom
ⓒ. attraction between only gaseous cations and gaseous anions
ⓓ. complete separation of all valence electrons from both atoms
Correct Answer: sharing of one or more electron pairs between atoms
Explanation: A covalent bond forms when atoms share one or more electron pairs. The shared electrons help the bonded atoms attain a more stable valence-shell arrangement. This is different from ionic bonding, where electron transfer produces cations and anions. Covalent bonding is common between non-metal atoms with comparable tendencies to attract electrons. In simple Lewis structures, a shared pair is represented as a bond pair between the two atoms.
112. In the formation of \(\mathrm{H_2}\), each hydrogen atom becomes stable because it
ⓐ. completes an octet with \(8\) electrons
ⓑ. loses its only electron to form \(\mathrm{H^+}\) in every molecule
ⓒ. gains \(7\) electrons from the other hydrogen atom
ⓓ. gains access to \(2\) electrons through one shared pair
Correct Answer: gains access to \(2\) electrons through one shared pair
Explanation: Each hydrogen atom has one valence electron. When two hydrogen atoms form \(\mathrm{H_2}\), they share one pair of electrons. Each hydrogen atom then counts two electrons in its valence shell, satisfying the duplet rule. Hydrogen cannot follow an octet rule because its first shell can hold only \(2\) electrons. The bond in \(\mathrm{H_2}\) is therefore a simple covalent bond based on one shared pair.
113. A chlorine molecule, \(\mathrm{Cl_2}\), forms when two chlorine atoms
ⓐ. each share one electron to form one shared pair
ⓑ. each lose seven electrons to form two cations
ⓒ. both atoms keep separate valence shells with no sharing
ⓓ. form a lattice of \(\mathrm{Cl^+}\) and \(\mathrm{Cl^-}\) ions
Correct Answer: each share one electron to form one shared pair
Explanation: Each chlorine atom has \(7\) valence electrons. By contributing one electron each, two chlorine atoms form one shared electron pair. This shared pair gives each chlorine atom access to an octet in the simple Lewis picture. Since the two atoms are identical, complete electron transfer is not a suitable description. \(\mathrm{Cl_2}\) is best treated as a covalent molecule with one single bond.
114. The bond pair in a covalent molecule means
ⓐ. an electron pair localized on one atom only
ⓑ. an electron pair not involved in bonding
ⓒ. a pair of ions arranged in a crystal lattice
ⓓ. an electron pair shared by bonded atoms
Correct Answer: an electron pair shared by bonded atoms
Explanation: A bond pair is the pair of electrons shared between two atoms in a covalent bond. It lies in the bonding region and is counted by both bonded atoms in simple Lewis structures. A lone pair, in contrast, remains on one atom and is not shared as a bond. This distinction helps in drawing Lewis structures and predicting molecular shapes. Calling every pair a bond pair would incorrectly include non-bonding electron pairs as bonds.
115. Use the arrangement described below. Two fluorine atoms approach each other. Each fluorine atom has \(7\) valence electrons and one unpaired electron available for sharing. The molecule formed is best represented as
ⓐ. \(\mathrm{F^+F^-}\)
ⓑ. \(\mathrm{F-F}\)
ⓒ. \(\mathrm{F=F}\)
ⓓ. \(\mathrm{F\equiv F}\)
Correct Answer: \(\mathrm{F-F}\)
Explanation: Each fluorine atom needs one more electron to complete its octet. Two fluorine atoms share one electron pair, so a single covalent bond forms. A single line between atoms represents one shared pair. A double or triple bond would show too many shared pairs for ordinary \(\mathrm{F_2}\). The molecule is neutral because the electrons are shared rather than fully transferred from one fluorine atom to the other.
116. Consider these statements about covalent bonding.
I. Covalent bonding involves sharing of electron pairs.
II. Covalent bonding commonly occurs between non-metal atoms.
III. Covalent bonding always produces cations and anions before bond formation.
The valid set is
ⓐ. I only
ⓑ. II and III only
ⓒ. I and II only
ⓓ. I, II, and III
Correct Answer: I and II only
Explanation: Covalent bonding is based on sharing of electron pairs between atoms. It is common between non-metals because they often have similar tendencies to attract electrons and can complete stable arrangements by sharing. Statement III describes ionic bonding, where electron transfer forms ions. Covalent molecules such as \(\mathrm{H_2}\), \(\mathrm{Cl_2}\), and \(\mathrm{H_2O}\) are not formed by first making a lattice of cations and anions. The key difference is shared electron density between atoms rather than full charge separation.
117. A Lewis description of \(\mathrm{H_2O}\) says oxygen forms two \(\mathrm{O-H}\) bonds and has two lone pairs. The number of bond pairs around oxygen is
ⓐ. \(2\)
ⓑ. \(1\)
ⓒ. \(3\)
ⓓ. \(4\)
Correct Answer: \(2\)
Explanation: \( \textbf{Given structure:} \) Oxygen forms two \(\mathrm{O-H}\) bonds.
\( \textbf{Meaning of one bond:} \) One single covalent bond contains one shared electron pair.
\( \textbf{Bond-pair count:} \) Two \(\mathrm{O-H}\) bonds therefore give two bond pairs around oxygen.
\( \textbf{Lone-pair information:} \) Oxygen also has two lone pairs, but lone pairs are not shared as bonds.
\( \textbf{Total electron-pair count around oxygen:} \) There are two bond pairs and two lone pairs.
\( \textbf{Required quantity:} \) The question asks only for bond pairs.
\( \textbf{Final answer:} \) Oxygen has \(2\) bond pairs in \(\mathrm{H_2O}\). Counting lone pairs as bond pairs would double-count non-bonding electrons.
118. The bonding in \(\mathrm{N_2}\) is stronger than in \(\mathrm{H_2}\) mainly because \(\mathrm{N_2}\) contains
ⓐ. no shared electron pairs
ⓑ. one shared electron pair only
ⓒ. complete electron transfer from one nitrogen atom to another
ⓓ. three shared electron pairs between the nitrogen atoms
Correct Answer: three shared electron pairs between the nitrogen atoms
Explanation: In \(\mathrm{N_2}\), the two nitrogen atoms share three electron pairs, forming a triple bond. A triple bond has more shared electron pairs than a single bond such as the bond in \(\mathrm{H_2}\). More shared pairs usually make the bond stronger and shorter, although exact values depend on the atoms involved. Nitrogen atoms do not form \(\mathrm{N^+}\) and \(\mathrm{N^-}\) in ordinary \(\mathrm{N_2}\). The notation \(\mathrm{N\equiv N}\) represents three shared electron pairs between the atoms.
119. Match each molecule with the simplest shared-pair description.
| Molecule | Shared-pair description |
|---|
| P. \(\mathrm{H_2}\) | 1. one shared pair |
| Q. \(\mathrm{O_2}\) | 2. two shared pairs |
| R. \(\mathrm{N_2}\) | 3. three shared pairs |
| S. \(\mathrm{Cl_2}\) | 4. one shared pair |
The proper matching is
ⓐ. P-2, Q-1, R-3, S-4
ⓑ. P-1, Q-3, R-2, S-4
ⓒ. P-4, Q-3, R-1, S-2
ⓓ. P-1, Q-2, R-3, S-4
Correct Answer: P-1, Q-2, R-3, S-4
Explanation: \(\mathrm{H_2}\) has one shared pair and is represented as \(\mathrm{H-H}\). \(\mathrm{O_2}\) has two shared pairs and is commonly represented as \(\mathrm{O=O}\). \(\mathrm{N_2}\) has three shared pairs and is represented as \(\mathrm{N\equiv N}\). \(\mathrm{Cl_2}\) has one shared pair because each chlorine atom needs one electron to complete an octet. The matching follows the number of shared electron pairs, not the atomic size of the element.
120. A graph is described below.
For two approaching hydrogen atoms, potential energy is plotted on the y-axis and internuclear distance on the x-axis. The curve falls as attraction develops, reaches a minimum at a certain distance, and then rises sharply at very small distance.
The minimum point of the curve represents
ⓐ. separated \(\mathrm{H}\) atoms with nearly zero interaction
ⓑ. equilibrium distance for the \(\mathrm{H-H}\) bond
ⓒ. a distance shorter than equilibrium with strong repulsion
ⓓ. distance where \(\mathrm{H-H}\) electron sharing stops
Correct Answer: equilibrium distance for the \(\mathrm{H-H}\) bond
Explanation: As two hydrogen atoms approach, attractive interactions can lower the potential energy. At a certain distance, the balance between attraction and repulsion gives minimum energy. This distance corresponds to the stable bond length for the \(\mathrm{H-H}\) bond. If the atoms come too close, nucleus-nucleus and electron-electron repulsions increase sharply. The stable covalent bond is associated with the energy minimum, not with zero distance between nuclei.
