2.1 Types of Chemical Bonds

1. ACa > K and S > O
   A student who thinks more protons always mean a stronger attraction picks this. Ca has more protons than K and is more electronegative, but S, with more protons than O, has its valence shell farther from the nucleus and more shielded, so S is less electronegative than O.
2. BK > Ca and S > O
   A student who thinks larger atoms attract shared electrons more strongly picks this. K is larger than Ca and S is larger than O, but in each pair the smaller atom has the higher electronegativity.
3. CK > Ca and O > S
   A student who equates electronegativity with reactivity picks this, choosing the more reactive element of each pair. K is more reactive than Ca, but it lies to the left of Ca and has the lower electronegativity.
4. DCa > K and O > S Correct
   Electronegativity increases from left to right across a period, so Ca is greater than K, and it decreases down a group, so O is greater than S.

CED 2.1.A.1 · Read this in Fix

1. AThe C–H bonds and the C–F bond are equally polar, since each joins two different elements.
   A student who thinks any bond between different elements is polar, with no degrees, picks this. The difference for C–H is 0.35, small enough for the bond to be treated as effectively nonpolar; for C–F it is 1.43.
2. BThe C–H bonds are effectively nonpolar, and the C–F bond is ionic, with an F⁻ ion.
   A student who treats a polar bond as a transfer of electrons picks this. The C–F electrons are shared unequally, giving F a partial negative charge, but they are still shared: CH₃F is a molecule and contains no ions.
3. CThe C–H bonds and the C–F bond are nonpolar, since each is a pair of shared electrons.
   A student who thinks shared always means shared equally picks this. F is much more electronegative than C, so the C–F pair lies closer to F and the bond is polar.
4. DThe C–H bonds are effectively nonpolar, and the C–F bond is polar covalent, with F as δ−. Correct
   C and H have similar electronegativities (difference 0.35), so the C–H bonds are effectively nonpolar even though C is slightly more electronegative. C and F differ by 1.43, so the C–F electrons are shared unequally in a polar covalent bond.

CED 2.1.A.2 · Read this in Fix

1. AThe shared electrons lie closer to Cl, which gains a partial positive charge.
   A student who marks the atom that gains electron density as positive picks this. Electrons are negative, so the atom with the greater share of them is partially negative.
2. BThe shared electrons have moved onto Cl, leaving H with a full positive charge.
   A student who reads any shift of electrons as a transfer picks this. The shading surrounds both nuclei, so the pair is still shared; H carries a partial positive charge, not the full charge of an ion.
3. CThe shared electrons lie closer to Cl, which has a partial negative charge. Correct
   The shading is strongest around the Cl nucleus, so the shared pair is distributed unequally, closer to Cl. Cl has the higher electronegativity and is partially negative (δ−) relative to H, which is partially positive (δ+).
4. DThe shared electrons lie closer to Cl, but neither atom carries a charge.
   A student who thinks the atoms of a neutral molecule are all uncharged picks this. The molecule is neutral overall, but the unequal distribution of the shared pair gives Cl a partial negative charge and H a partial positive charge.

CED 2.1.A.3.i · Read this in Fix

1. AAdd a sample to water and observe whether the solid dissolves.
   A student who thinks that dissolving in water identifies an ionic compound picks this. Covalent solids such as sugar dissolve in water, and some ionic solids dissolve very little, so the result would not settle the question.
2. BMelt a sample and test whether the liquid conducts electricity. Correct
   An ionic compound conducts when molten, because its ions are then free to move; a covalent compound made of molecules does not. Examining properties such as this is the best way to characterize the bonding.
3. CTest whether a dry crystal of the solid conducts electricity.
   A student who thinks ionic solids conduct because they contain ions picks this. The ions in a solid are held in place, so neither an ionic nor a covalent crystal of this kind conducts, and the test cannot tell them apart.
4. DHold a magnet near the solid and observe whether it is attracted to it.
   A student who thinks charged ions are attracted to a magnet picks this. Charge is not magnetism, so the test gives no evidence about ions.

CED 2.1.A.4 · Read this in Fix

1. AThey are held in pairs between two neighboring atoms as covalent bonds.
   A student who thinks metal atoms are joined by covalent bonds picks this. The dots in the model are scattered singly through the solid, not held in pairs between particular atoms.
2. BThey each stay with the atom they came from and remain part of that atom.
   A student who thinks each electron remains with its own atom picks this. In the model the valence electrons are not attached to particular atoms; they are delocalized through the solid.
3. CThey have been taken by half of the atoms, which become negative ions.
   A student who thinks a metal contains positive and negative ions picks this. All twelve circles in the model are positive; the negative charge is the delocalized valence electrons.
4. DThey are spread through the solid and do not belong to any one atom. Correct
   The model shows the valence electrons scattered throughout the array of positive ions. They are delocalized: not associated with any individual atom.

CED 2.1.A.5 · Read this in Fix

1. AThe bulb lights only after melting, because the ions are then free to move. Correct
   LiCl is a compound of a metal and a nonmetal and is ionic. In the solid the ions cannot move, so the bulb stays dark; in the liquid the Li⁺ and Cl⁻ ions move to the electrodes and charge flows.
2. BThe bulb lights before and after melting, because LiCl is made of charged ions.
   A student who thinks an ionic solid conducts because it contains ions picks this. In solid LiCl the ions are held in fixed positions, so no charge flows until the solid melts.
3. CThe bulb lights only after melting, because electrons are then set free to move.
   A student who thinks every current is carried by free electrons picks this. In molten LiCl the mobile charged particles are the Li⁺ and Cl⁻ ions.
4. DThe bulb stays dark before and after melting, because LiCl molecules are neutral.
   A student who thinks an ionic compound consists of neutral molecules picks this. LiCl consists of Li⁺ and Cl⁻ ions, which carry charge through the liquid.

CED 2.1.A.4 · Read this in Fix

1. AF needs one electron to fill its octet, while C needs four, and an atom closer to an octet pulls harder.
   A student who explains attraction by what an atom needs picks this. The attraction for shared electrons is Coulombic: it comes from the nuclear charge, the shielding and the distance, not from how close the atom is to an octet.
2. BF has nine protons, while C has six, and in both atoms two inner electrons shield the valence shell. Correct
   Both atoms have their valence electrons in the second shell, shielded by the same two inner electrons. F has the greater nuclear charge, so by Coulomb's law shared electrons are attracted to an F atom more strongly.
3. CF has seven valence electrons, while C has four, and these electrons pull on the shared pair.
   A student who thinks the valence electrons do the attracting picks this. Electrons repel one another; the shared pair is attracted by the nucleus, which has a greater charge in F.
4. DF has fewer inner electrons than C, and its valence shell is therefore shielded less from the nucleus.
   A student who thinks shielding decreases across a period picks this. F and C both have two inner electrons, so the shielding is about the same; the nuclear charge is what differs.

CED 2.1.A.1 · Read this in Fix

1. ACl, because its nucleus has more protons, which exert a greater pull on a shared pair.
   A student who considers nuclear charge alone picks this. Ten inner electrons shield the Cl nucleus, compared with two in F, and the valence shell of Cl is farther from the nucleus, so the attraction for a shared pair is weaker in Cl.
2. BF, because its nine protons are divided among fewer electrons, so each is held more tightly.
   A student who thinks nuclear attraction is shared out among the electrons picks this. The force on an electron depends on the charges and the distance, not on how many other electrons there are to share it with.
3. CF, because a shared pair is much closer to its nucleus and fewer inner electrons shield the pair. Correct
   In both atoms the nuclear charge is offset by the inner electrons (9 − 2 and 17 − 10), leaving a similar net attraction on the valence shell. The valence shell of F is much closer to the nucleus, so by Coulomb's law a shared pair is attracted more strongly by F.
4. DCl, because its larger number of shells and electrons gives it a stronger hold on a shared pair.
   A student who thinks larger atoms hold shared electrons more strongly picks this. The extra shell puts a shared pair farther from the nucleus and adds shielding, which weakens the attraction.

CED 2.1.A.1 · Read this in Fix

1. ADiagram 1 Correct
   Cl has the higher electronegativity, so the shared electrons lie closer to Cl: Cl is partially negative and Br partially positive. The difference is small (0.20), so the electrons are still shared and the charges are partial.
2. BDiagram 2
   A student who gives the negative end to the atom with more electrons picks this. Br has more electrons than Cl, but Cl has the higher electronegativity, so Cl is the partially negative atom.
3. CDiagram 3
   A student who treats any difference in electronegativity as a transfer of an electron picks this. With a difference of only 0.20 between two nonmetals, the electrons are shared, slightly unequally.
4. DDiagram 4
   A student who thinks covalent bonds always share electrons equally picks this. The two electronegativities are not equal, so the shared pair lies slightly closer to Cl and the atoms carry small partial charges.

CED 2.1.A.3.i · Read this in Fix

1. AThe bond is purely ionic like that in NaCl, since HCl is made of H⁺ ions and Cl⁻ ions.
   A student who thinks HCl is made of ions because hydrochloric acid contains ions picks this. In a molecule of HCl the electrons are shared; the difference of 0.96 lies well below that of NaCl.
2. BThe bond is polar covalent, a separate type of bond that has no ionic character.
   A student who sees bond types as separate categories with sharp boundaries picks this. Bonding is a continuum: all polar bonds have some ionic character, and the bond in HCl lies between the two extremes.
3. CThe bond is polar covalent, with partial ionic character, between the bonds in Cl₂ and NaCl. Correct
   The difference for HCl lies between the values for Cl₂ and NaCl. The pair is shared unequally, so the bond is polar covalent, and like all polar bonds it has some ionic character: ionic and covalent bonding form a continuum.
4. DThe bond is purely covalent like that in Cl₂, since H and Cl are both nonmetal atoms.
   A student who thinks covalent bonds always share electrons equally picks this. The difference of 0.96 shows that the pair is shared unequally, unlike the pair in Cl₂.

CED 2.1.A.3.iii · Read this in Fix

1. AIts valence electrons are delocalized and can move through the whole solid. Correct
   In a metallic solid the valence electrons are delocalized and not associated with any individual atom, so they can move through the solid and carry charge when a voltage is applied.
2. BIts positive ions are loosely held and can move through the whole solid.
   A student who thinks the ions carry the current in a metal picks this. The positive ions of solid copper stay in their positions; the delocalized electrons move.
3. CIts atoms share electron pairs, which pass from one covalent bond to the next one.
   A student who thinks metal atoms are joined by covalent bonds picks this. The valence electrons of a metal are not localized in pairs between particular atoms.
4. DIts positive and negative ions attract each other and pass charge along the wire.
   A student who thinks a metal contains positive and negative ions picks this. Copper contains positive ions and delocalized electrons, and no negative ions.

CED 2.1.A.5 · Read this in Fix

1. ABF₃ is ionic and LiI is covalent, as the two electronegativity differences show.
   A student who lets the electronegativity difference alone decide the type of bond picks this. The properties show the opposite: liquid LiI conducts and liquid BF₃ does not.
2. BLiI and BF₃ are both ionic, as each electronegativity difference is far from zero.
   A student who treats every strongly polar bond as a transfer of electrons picks this. BF₃ has very polar bonds, but its low melting point and nonconducting liquid show that it consists of molecules, not ions.
3. CLiI and BF₃ are both covalent, as each of the formulas shows a single molecule.
   A student who reads every formula as a molecule picks this. The formula LiI gives the ratio of ions; the high melting point and the conducting liquid show that LiI is ionic.
4. DLiI is ionic and BF₃ is covalent, as the melting points and the conductivities show. Correct
   LiI melts at a high temperature and conducts as a liquid, so it contains ions. BF₃ melts far below room temperature and does not conduct as a liquid, so it consists of molecules with covalent bonds, even though its electronegativity difference is the larger. Properties are the best guide to the type of bonding.

CED 2.1.A.4 · Read this in Fix

1. AThe O–H bond has the greater dipole, with O partially positive in each bond.
   A student who marks the atom that gains electron density as positive picks this. O attracts the shared electrons in both bonds, so it is partially negative in both.
2. BThe O–H bond has the greater dipole, with O partially negative in both bonds. Correct
   The difference in electronegativity is 1.24 for O–H and 0.28 for O–Cl, and in single bonds a greater difference gives a greater bond dipole. O is the more electronegative atom in each bond, so it is partially negative relative to both H and Cl.
3. CThe O–Cl bond has the greater dipole, with both atoms pulling strongly.
   A student who judges polarity by how electronegative the atoms are picks this. The dipole depends on the difference: 0.28 for O–Cl and 1.24 for O–H.
4. DThe O–H bond has the greater dipole, with Cl partially negative in the O–Cl bond.
   A student who thinks a halogen is always the negative end of a bond picks this. O (3.44) is more electronegative than Cl (3.16), so in the O–Cl bond O is partially negative and Cl partially positive.

CED 2.1.A.3.ii · Read this in Fix

1. A0.12
   A student who gives the stronger pull to the atom with the most electrons picks this, choosing the bond to Cl: 3.16 − 3.04 = 0.12. This is the smallest difference of the three bonds.
2. B0.54
   A student who thinks a bond between two highly electronegative atoms is the most polar picks this, choosing O–F: 3.98 − 3.44 = 0.54. The dipole depends on the difference, which is greater for N–H.
3. C0.84 Correct
   The differences are 0.84 for N–H, 0.12 for N–Cl and 0.54 for O–F. In single bonds a greater difference gives a greater bond dipole, so the N–H bond has the greatest dipole.
4. D1.78
   A student who subtracts the lowest value in the table from the highest picks this: 3.98 − 2.20 = 1.78. F and H are not bonded to each other in any of the three bonds, so this is not the difference for one of them.

Working In single bonds a greater difference in electronegativity gives a greater bond dipole. N–H: 3.04 − 2.20 = 0.84. N–Cl: 3.16 − 3.04 = 0.12. O–F: 3.98 − 3.44 = 0.54. The greatest difference is 0.84, for the N–H bond.

CED 2.1.A.3.ii · Read this in Fix

1. AN–P
   A student who thinks elements in the same group have the same electronegativity picks this. The graph shows N about 0.85 above P.
2. BC–S Correct
   The points for C (group 14, period 2) and S (group 16, period 3) are at almost the same height, about 2.55 and 2.6, so the difference in electronegativity is close to zero and the electrons are shared almost equally.
3. CP–S
   A student who thinks atoms with valence electrons in the same shell attract shared electrons equally picks this pair of period 3 neighbors. The graph shows S about 0.4 above P, because S has the greater nuclear charge.
4. DO–F
   A student who thinks two strongly electronegative atoms share equally picks this. The graph shows F about 0.55 above O, so the shared electrons lie closer to F.

Working Electrons are shared most nearly equally when the two electronegativities are most nearly the same, that is, when the two points are at the same height on the graph. Reading the graph: C ≈ 2.55 and S ≈ 2.6 (difference ≈ 0.0); P ≈ 2.2 and S ≈ 2.6 (≈ 0.4); O ≈ 3.45 and F ≈ 4.0 (≈ 0.55); N ≈ 3.05 and P ≈ 2.2 (≈ 0.85). C and S have the smallest difference.

CED 2.1.A.2 · Read this in Fix

1. AThe bond is polar covalent, with the shared electrons closer to the Cl atom. Correct
   Cl lies to the right of S in the same period, so Cl has the higher electronegativity. S and Cl are both nonmetals, so they share electrons, and the shared electrons lie closer to Cl: a polar covalent bond.
2. BThe bond is nonpolar covalent, with electrons shared equally by the two nonmetal atoms.
   A student who thinks covalent bonds always share electrons equally picks this. S and Cl differ in electronegativity, so the sharing is unequal.
3. CThe bond is polar covalent, with the shared electrons closer to the larger S atom.
   A student who thinks the larger atom attracts shared electrons more strongly picks this. Cl has the greater nuclear charge with the same inner-shell shielding, so Cl attracts the shared electrons more strongly.
4. DThe bond is ionic, with electrons transferred so that each atom has an octet.
   A student who thinks every bond forms by one atom taking electrons to complete an octet picks this. Two nonmetal atoms of similar electronegativity share electrons; both S and Cl complete their valence shells by sharing.

CED 2.1.A.3 · Read this in Fix