Assignment 08 A

1- How many unpaired electrons are in the Lewis electron-dot symbol for an oxygen atom? a) 2 b) 4 c) 1 d) 0 (There are two unpaired electrons in different 2p orbitals.)

2- Which of the following bonds is polar? a) O—O b) F—F c) O—S d) S—S (Oxygen is more electronegative than sulfur. Therefore, there will be an unequal sharing of electrons and the bond will be polar.)

3- Which ionic compound is not expected to form from a combination of the following pairs of elements? a) Ca3N2 b) MgS c) SrO d) GaS e) Li2S (This compound would not form in these proportions, since Ga develops a 3+ charge and S a 2− charge. Charges would not be balanced in this formula.)

4- Which combination of atoms would most likely produce a covalent bond? a) Cd, Cl b) Fe, S c) Cu, Se d) K, O e) N, F (Covalent bonds tend to form between nonmetal atoms.)

5- How many valence electrons are involved in writing the Lewis structure of AsF3? a) 5 b) 21 c) 32 d) 26 (There are seven valence electrons for each fluorine atom and five for the arsenic atom.)

6- Which of the following molecules has two resonance structures? − a) CH3CO2 b) CH4 c) CO2 − d) NO3 − e) ClO4 (A double bond resonates between the carbon and each of the two oxygen atoms in this molecule.)

7- Use bond enthalpies to estimate the enthalpy change for the following gas-phase reaction:

CH4 + Cl2  CH2Cl2 + H2 a) 170 kJ/mol b) −24 kJ/mol c) 390 kJ/mol d) 717 kJ/mol e) 24 kJ/mol f) −1263 kJ/mol (There are two C—H bonds and one Cl—Cl bond broken; sum the bond enthalpies for this and subtract the sum of the bond enthalpies from the formation of two C—Cl bonds and one H—H bond.)

− − 8- What are the formal charges on the chlorine atom in the chlorite ion ClO2 and the chlorate ion ClO3 , respectively? a) 2+ in both b) 0, 1+ c) 3+, 5+ d) 1+ in both e) 1+, 2+ (Chlorine has eight electrons around it in each case. Four are shared in the chlorite ion; there are two nonbonding (lone pair) electron pairs. Substitute these numbers into the equation for calculating the formal charge. The formal charge on the chlorate ion is calculated in the same manner except that for the chlorate ion, the chlorine has six shared electrons and only one lone pair.)

9- The Lewis structures of which of the following compounds obey the octet rule? 2− (i) SO3 (ii) BH3 − (iii) I3 − (iv) AsF6 − (v) O2 a) iii b) V c) ii d) Iv e) i and v f) i (Sulfur has a lone pair as well as three other shared pairs of electrons around it.)

10- Which of the following shows the incorrect number of valence electrons in parentheses? a) Cl (7) b) As (5) c) Ca (2) d) Li (1) e) Se (4) (Selenium is in group 6A and should have six valence electrons, not four.)

11- By referring only to the periodic table, arrange the following in order of increasing electronegativity: Ga, P, As, S, O, F a) Ga < P < As < S < O < F b) As < Ga < P < S < O < F c) Ga < As < P < S < O < F d) Ga < As < P < S < F < O e) Ga < As < S < P < O < F f) Ga < As < P < O < S < F (Electronegativity increases to the right on the periodic table. It also increases up the periodic table.)

12- Which one of the following ions has a noble−gas electron configuration? a) Cr3+ b) Zn2+ c) Cu2+ d) Se2− e) Fe3+ (There are eight valence electrons on the selenium ion.)

+ − − 13- Predict the order of the N—O bond lengths in NO , NO2 , and NO3 . + − a) NO shortest, NO3 longest + − b) NO longest, NO2 shortest + − c) NO shortest, NO2 longest − − d) NO3 shortest, NO2 longest − (In the NO3 ion, the bonds could be described as "one-and-a-third" bonds (three bonds with 4 + − shared electron while in NO there would be a triple bond and two double bonds in NO2 .)

14- How many pairs of electrons are there around the central atom in the Lewis structure of SeF4? a) 5 b) 3 c) 2 d) 6 e) 4 (Selenium has an expanded octet. There are 34 valence electrons, which leaves one extra electron pair after fulfilling the octets of the fluorine atoms.)

− 15- Write a single Lewis structure that satisfies the octet rule for ClO2 . What are the formal charges? a) Cl 0; O 0; O 1− b) Cl 0; O 0 c) Cl 1−; O 1+ d) Cl 1−; O 0 e) Cl 1+; O 1− (Chlorine has two unshared and two shared pairs of electrons. This gives it a formal charge of 1+. Oxygen has three unshared pairs and one shared pair, so it has a formal charge of 1−.)

16- Arrange the following in order of increasing bond polarity: C—O, P—O, Cu—O, Mn—O a) C—O < P—O < Mn—O < Cu—O b) P—O < C—O < Cu—O < Mn—O c) Mn—O < Cu—O < P—O < C—O d) C—O < P—O < Cu—O < Mn—O (The polarities of the bonds will increase with the electronegativity difference between oxygen and the other atom. The largest difference exists when the other metal is as far as possible away from oxygen on the periodic table. Remember the trends for electronegativity.)

17- The Lewis electron-dot structure of IF5 gives the central I a) five nonbonding pairs and one bonding pair. b) five bonding pairs only. c) one nonbonding pair and five bonding pairs. d) a completed octet. e) one nonbonding pair and four bonding pairs. (There are 42 valence electrons that are used to first fulfill the octet of each of the fluorine atoms. This leaves one pair left over that would be around I and not involved in the bonding.)

18- What kind of covalent bond exists between the carbon and the oxygen atoms in carbon monoxide? a) Double covalent bond b) Single covalent bond c) Triple covalent bond d) It is not a covalent bond. It is an ionic bond because the atoms are different. (A triple covalent bond is the only way 10 valence electrons can be distributed and ensure that both the carbon and the oxygen atoms have completed octets.)

19- In the same sense that we describe the O—O bonds in O3 as “one-and-a-half” bonds, or simply say that the “bond order” is 1.5, what would the bond order for the S—O bond in sulfur trioxide be assuming that an octet is attained by all atoms? a) 3 b) 1 and 2 c) 1 d) 1.33 (There are three resonance forms of sulfur trioxide, each with one double bond and two single bonds. There are four pairs of electrons in three domains, 4/3 = 1.33.)

20- In which of the following are there two bonding pairs and two nonbonding pairs on the central atom? a) BeCl2 b) CH4 c) NCl3 d) OF2 e) BF3 f) PH3 (There is a bonding pair between oxygen and each fluorine. Oxygen has two nonbonding pairs of electrons.)

21- Consider the molecule H2N—CH2—CH2—CO2H. Examine the Lewis structure for this molecule and determine which of the following statements is incorrect. a) There are 11 single bonds in this molecule. b) There is one oxygen-oxygen bond in this molecule. c) There are three single bonds and one lone pair of electrons around the N atom. d) There is one double bond in this molecule. e) There are five lone pairs of electrons in this molecule. (One oxygen atom forms a double bond with the carbon atom, and the other forms single bonds with carbon and hydrogen atoms.)

22- What is the number of bonding and nonbonding electron pairs around the central atom in each of the following? NH3, H2S, CO2 a) NH3: (3 bonding, 1 nonbonding); H2S: (2 bonding, 1 nonbonding); CO2: (4 bonding, 0 nonbonding) b) NH3: (3 bonding, 0 nonbonding); H2S: (2 bonding, 1 nonbonding); CO2: (4 bonding, 0 nonbonding) c) NH3: (3 bonding, 1 nonbonding); H2S: (2 bonding, 2 nonbonding); CO2: (2 bonding, 0 nonbonding) d) NH3: (3 bonding, 0 nonbonding); H2S: (2 bonding, 2 nonbonding); CO2: (4 bonding, 0 nonbonding) e) NH3: (3 bonding, 1 nonbonding); H2S: (2 bonding, 2 nonbonding); CO2: (4 bonding, 0 nonbonding) (The central atom of each has its octet filled. Given the position of each of the central atoms, CO2 has two double bonds and thus all of the electrons around it are bonding.)

23- Which of the following orders of electronegativity is incorrect? a) C < N < O b) Si < P < N c) I < Br < Cl d) Se < S < O e) C < Si < P (Carbon should be more electronegative than silicon, since it is above silicon in the periodic table.)

24- Write Lewis structures for the following: (i) HOBr (ii) H2O2 (the O atoms are bonded to one another) (iii) H2CO (both H atoms are bonded to C) Pick the incorrect response below. a) There are three nonbonding pairs of electrons around the bromine atom in structure i. b) There is a double bond in structure iii. c) Oxygen in structure i has two nonbonded pairs of electrons. d) The total number of bonds in structure ii is three. e) There is a double bond between the oxygen atoms in structure ii. (There is a single bonds between the oxygen atoms, making this an incorrect statement.)

25- Is the C—O bond in CH3OH longer or shorter than the C—O bond in CH3COCH3? Why? a) longer; CH3COCH3 has a double bond b) they are the same length c) shorter; CH3COCH3 has a double bond d) longer; CH3COCH3 is a bigger molecule e) shorter; CH3COCH3 has a triple bond (CH3COCH3 has a double bond, while CH3OH has a single bond. Single bonds are longer than double bonds.)