30 multiple-choice + 2 free-response = 32 questions · valence electrons · bonds & bond order · the 4-step method · polyatomic ions · formal charge & resonance
How many valence electrons does a nitrogen atom have?
B) 5. N is in group 15, so it has 5 valence electrons (last digit of the group number).
Answer: B) 5. N is in group 15, so it has 5 valence electrons (last digit of the group number).
What is the total number of valence electrons in CH₂O (formaldehyde)?
D) 12. C 4 + H 2 × 1 + O 6 = 12.
Answer: D) 12. C 4 + H 2 × 1 + O 6 = 12.
Which atom can NEVER be the central atom in a Lewis structure?
A) H. Hydrogen can form only one bond, so it is always on the outside (F is the same).
Answer: A) H. Hydrogen can form only one bond, so it is always on the outside (F is the same).
According to the octet rule, how many electrons does a hydrogen atom need around it?
C) 2. Hydrogen follows the duet rule: 2 electrons, like helium.
Answer: C) 2. Hydrogen follows the duet rule: 2 electrons, like helium.
In a neutral molecule, how many bonds does an oxygen atom usually form?
D) 2. O has 6 valence electrons and is 2 short of an octet, so it forms 2 bonds (and keeps 2 lone pairs).
Answer: D) 2. O has 6 valence electrons and is 2 short of an octet, so it forms 2 bonds (and keeps 2 lone pairs).
What is a lone pair?
B) A pair of electrons that belongs to one atom and is not involved in bonding. A shared pair is a bonding pair; a pair held by only one atom is a lone pair.
Answer: B) A pair of electrons that belongs to one atom and is not involved in bonding. A shared pair is a bonding pair; a pair held by only one atom is a lone pair.
How many electrons are shared in a triple bond?
A) 6. 3 shared pairs × 2 electrons = 6 electrons.
Answer: A) 6. 3 shared pairs × 2 electrons = 6 electrons.
What is the bond order of the bond in N₂?
D) 3. N₂ is N≡N, a triple bond, so the bond order is 3.
Answer: D) 3. N₂ is N≡N, a triple bond, so the bond order is 3.
Which carbon–carbon bond is the SHORTEST?
C) C≡C. Higher bond order means a shorter bond: C≡C 1.203 Å < C=C 1.339 Å < C–C 1.535 Å.
Answer: C) C≡C. Higher bond order means a shorter bond: C≡C 1.203 Å < C=C 1.339 Å < C–C 1.535 Å.
Which bond needs the MOST energy to break?
A) N≡N. More shared pairs make the bond stronger, so the triple bond is strongest.
Answer: A) N≡N. More shared pairs make the bond stronger, so the triple bond is strongest.
In the Lewis structure of CO₂, which atom is central?
B) C. C is less electronegative than O (2.5 vs 3.5) and forms the most bonds, so it goes in the middle: O=C=O.
Answer: B) C. C is less electronegative than O (2.5 vs 3.5) and forms the most bonds, so it goes in the middle: O=C=O.
Drawing CO₂: after step 3, each O has an octet but C has only 4 electrons and there are none left. What is the next step?
D) Move one lone pair from each O into a double bond with C. Step 4: move lone pairs into multiple bonds. This never adds electrons; it just shares them, giving O=C=O.
Answer: D) Move one lone pair from each O into a double bond with C. Step 4: move lone pairs into multiple bonds. This never adds electrons; it just shares them, giving O=C=O.
How many lone pairs are on the central N atom in NH₃?
B) 1. 8 electrons total: 3 N–H bonds use 6, and the last 2 form one lone pair on N.
Answer: B) 1. 8 electrons total: 3 N–H bonds use 6, and the last 2 form one lone pair on N.
How many lone pairs are on the O atom in H₂O?
C) 2. 8 electrons: 2 O–H bonds use 4, and the remaining 4 are 2 lone pairs on O.
Answer: C) 2. 8 electrons: 2 O–H bonds use 4, and the remaining 4 are 2 lone pairs on O.
Which is the correct Lewis structure of HCN?
A) A. A has 10 electrons and every atom has an octet (H has 2). B uses 12 electrons (too many). C leaves C with only 6. D has the wrong skeleton: C, the less electronegative atom, should be central.
Answer: A) A. A has 10 electrons and every atom has an octet (H has 2). B uses 12 electrons (too many). C leaves C with only 6. D has the wrong skeleton: C, the less electronegative atom, should be central.
In formaldehyde (CH₂O), how many electrons are shared between C and O?
C) 4. CH₂O has a C=O double bond: 2 shared pairs = 4 electrons.
Answer: C) 4. CH₂O has a C=O double bond: 2 shared pairs = 4 electrons.
What is the total number of valence electrons in NH₄⁺?
D) 8. 5 + 4(1) = 9, then subtract 1 for the + charge = 8.
Answer: D) 8. 5 + 4(1) = 9, then subtract 1 for the + charge = 8.
What is the total number of valence electrons in CO₃²⁻?
B) 24. 4 + 3(6) = 22, then add 2 for the 2− charge = 24.
Answer: B) 24. 4 + 3(6) = 22, then add 2 for the 2− charge = 24.
What is the total number of valence electrons in SO₄²⁻?
C) 32. 6 + 4(6) = 30, then add 2 for the 2− charge = 32.
Answer: C) 32. 6 + 4(6) = 30, then add 2 for the 2− charge = 32.
How is the charge of a polyatomic ion shown on its Lewis structure?
D) Square brackets around the whole structure, with the charge outside at the top right. The charge belongs to the whole ion, so brackets go around everything and the charge is written outside.
Answer: D) Square brackets around the whole structure, with the charge outside at the top right. The charge belongs to the whole ion, so brackets go around everything and the charge is written outside.
Which is the correct Lewis structure of the hydroxide ion, OH⁻?
B) B. OH⁻ has 6 + 1 + 1 = 8 electrons: one O–H bond and 3 lone pairs on O, in brackets with a − charge. A is missing an electron pair and the charge; C gives H a double bond; D puts 10 electrons around O.
Answer: B) B. OH⁻ has 6 + 1 + 1 = 8 electrons: one O–H bond and 3 lone pairs on O, in brackets with a − charge. A is missing an electron pair and the charge; C gives H a double bond; D puts 10 electrons around O.
In the nitrite ion, NO₂⁻, how many lone pairs are on the central N atom?
B) 1. 18 electrons: after the skeleton and filling both O, 2 electrons are left over and go on N as one lone pair. Then one O lone pair makes N=O.
Answer: B) 1. 18 electrons: after the skeleton and filling both O, 2 electrons are left over and go on N as one lone pair. Then one O lone pair makes N=O.
In ONE Lewis structure of nitrate (NO₃⁻), how many N=O double bonds are there?
A) 1. 24 electrons give one N=O and two N–O bonds. (There are three such structures: resonance.)
Answer: A) 1. 24 electrons give one N=O and two N–O bonds. (There are three such structures: resonance.)
What is the formal charge on N in NH₄⁺?
C) +1. FC = V − L − (bonds) = 5 − 0 − 4 = +1, which matches the ion's charge.
Answer: C) +1. FC = V − L − (bonds) = 5 − 0 − 4 = +1, which matches the ion's charge.
In a nitrate (NO₃⁻) Lewis structure, what is the formal charge on each single-bonded O?
A) −1. O has 3 lone pairs (6 electrons) and 1 bond: 6 − 6 − 1 = −1.
Answer: A) −1. O has 3 lone pairs (6 electrons) and 1 bond: 6 − 6 − 1 = −1.
What are resonance structures?
C) Valid Lewis structures that differ only in where the electrons (double bonds) are placed. The atoms stay in the same places; only the position of the double bond (electrons) changes. They are joined by ↔.
Answer: C) Valid Lewis structures that differ only in where the electrons (double bonds) are placed. The atoms stay in the same places; only the position of the double bond (electrons) changes. They are joined by ↔.
Both O=C=O and O≡C–O give every atom an octet. Why is O=C=O the better structure?
B) Its formal charges are all zero. O≡C–O puts +1 and −1 on the O atoms. The best structure keeps formal charges closest to zero.
Answer: B) Its formal charges are all zero. O≡C–O puts +1 and −1 on the O atoms. The best structure keeps formal charges closest to zero.
Experiments show that in the real nitrate ion, the three N–O bonds are…
D) all identical, in between a single and a double bond. The real ion is a blend of the 3 resonance structures, so all three bonds are equal (bond order 1⅓).
Answer: D) all identical, in between a single and a double bond. The real ion is a blend of the 3 resonance structures, so all three bonds are equal (bond order 1⅓).
The formal charges on all the atoms in a Lewis structure must add up to…
A) the overall charge of the molecule or ion. For a neutral molecule the sum is 0; for SO₄²⁻ it is −2, and so on.
Answer: A) the overall charge of the molecule or ion. For a neutral molecule the sum is 0; for SO₄²⁻ it is −2, and so on.
Which molecule has an INCOMPLETE octet on its central atom?
B) BF₃. B has only 3 valence electrons; with 3 bonds it has just 6 electrons around it.
Answer: B) BF₃. B has only 3 valence electrons; with 3 bonds it has just 6 electrons around it.
Draw the Lewis structure of the sulfite ion, SO₃²⁻. Show all 4 steps, and give the formal charge on every atom.
Draw ALL the resonance structures of the carbonate ion, CO₃²⁻, and explain what the real ion looks like.