1 question, one for each idea where we can. Answer them, then see which ideas to fix.
Question 1 of 1
The Lewis diagram of phosgene, COCl₂, is shown. How many valence electrons does the diagram represent in total?
Answer and reasoning
A20 A student who counts each line as one electron picks this: 4 + 16 = 20. Each line is a shared pair of two electrons, so the bonds hold 8 electrons.
B24Correct The diagram has four lines (one double bond and two single bonds), which are four shared pairs, or 8 electrons, and eight lone pairs, or 16 electrons: 24 in all. This matches the valence electrons of the atoms, 4 + 6 + 2(7) = 24.
C32 A student who counts each shared pair once for each atom it joins picks this: 2 × 8 + 16 = 32. A shared pair counts toward both atoms' octets, but it is only two electrons in the molecule.
D48 A student who thinks a Lewis diagram accounts for every electron of every atom picks this, adding the atomic numbers: 6 + 8 + 2(17) = 48. The diagram shows only valence electrons: two for each line and two for each pair of dots.
Working Lines: one double bond (2 lines) and two single bonds = 4 bonding pairs = 8 electrons. Dots: 2 lone pairs on O and 3 on each Cl = 8 lone pairs = 16 electrons. Total = 8 + 16 = 24, which matches 4 + 6 + 2(7) = 24.
In preparation: 0 of 1 sections compiled and reviewed. The rest show key terms and common mistakes from our question bank until they are.
2.5.A.1 Lewis diagram Fix
Lewis diagram
A representation of a molecule or ion that shows its valence electrons: each line is a bonding (shared) pair of electrons between two atoms, and each pair of dots is a lone (nonbonding) pair on one atom.
Total valence electrons of a species
The number of electrons a Lewis diagram must show: the sum of the valence electrons of all the atoms, plus one for each unit of negative charge or minus one for each unit of positive charge on an ion. SO₄²⁻ has 6 + 4(6) + 2 = 32.
Bonding pair and lone pair
A bonding pair is a pair of electrons shared between two atoms and counted toward the valence shell of both. A lone pair belongs to one atom only.
Octet rule
In most Lewis diagrams of molecules made of second-period atoms, each atom other than hydrogen is surrounded by eight electrons, counting both its lone pairs and every pair it shares. Hydrogen is surrounded by two electrons and forms one bond.
Multiple bonds
A double bond (two shared pairs, four electrons) or a triple bond (three shared pairs, six electrons). Multiple bonds are drawn when single bonds and lone pairs cannot give the atoms octets with the available number of valence electrons.
Central atom
The atom to which the other atoms in a small molecule or ion are bonded. It is usually the least electronegative atom other than hydrogen; a hydrogen atom forms one bond, so it is not a central atom.
Constructing a Lewis diagram
Count the total valence electrons; connect the atoms with single bonds around the central atom; complete the octets of the outer atoms with lone pairs; place any remaining electrons on the central atom; if an atom still lacks an octet, convert lone pairs on a neighboring atom into multiple bonds.
Students often think Each line in a Lewis diagram represents one electron, as each dot does. In fact No. Each line stands for a shared pair of two electrons. A double bond is four electrons and a triple bond is six.
Students often think Because a shared pair counts for both atoms, it must be counted twice when adding up the electrons in the molecule. In fact No. A shared pair counts toward the octet of both atoms it joins, but it is only two electrons in the molecule. In the total, each shared pair is counted once.
Four diagrams proposed for hydrogen cyanide, HCN, are shown. Which diagram is the Lewis diagram of HCN that follows the established principles for constructing Lewis diagrams?
Answer and reasoning
ADiagram 1Correct H forms one bond. With 1 + 4 + 5 = 10 valence electrons, the arrangement H–C≡N with one lone pair on N gives H two electrons and both C and N octets (C: 2 + 6; N: 6 + 2), using exactly 10 electrons.
BDiagram 2 A student who stops once all the valence electrons are placed picks this. It uses 10 electrons, but C is surrounded by only four; two of N's lone pairs must become bonding pairs, giving a triple bond.
CDiagram 3 A student who thinks H can form two bonds picks this. A hydrogen atom is surrounded by only two electrons in a Lewis diagram, so it forms one bond and is not placed between two other atoms; C is the central atom in HCN.
DDiagram 4 A student who adds lone pairs until every atom has an octet, without checking the total, picks this. C and N each have eight electrons around them, but the diagram shows 14 electrons, and HCN has only 10.
In formaldehyde, CH₂O, the C atom is bonded to two H atoms and to the O atom. Which description of the Lewis diagram of CH₂O follows the established principles for constructing Lewis diagrams?
Answer and reasoning
AA C–O single bond, with three lone pairs on O and none on C A student who stops once all the valence electrons are placed picks this. It uses 12 electrons, but C has only six around it; a lone pair on O must become a second C–O bonding pair.
BA C=O double bond, with two lone pairs on O and none on CCorrect CH₂O has 4 + 2(1) + 6 = 12 valence electrons. Two C–H bonds (4), a C=O double bond (4) and two lone pairs on O (4) use all 12 and give C and O octets and each H two electrons.
CA C–O single bond, with three lone pairs on O and one pair on C A student who adds lone pairs until every atom has an octet, without checking the total, picks this. Every atom has an octet, but the diagram shows 14 electrons, two more than CH₂O has.
DA C=O double bond, with four lone pairs on O and none on C A student who thinks only lone pairs count toward an atom's octet picks this. The four electrons of the double bond count toward O's octet, so O needs only two lone pairs; this diagram shows 16 electrons.
Which claim about the Lewis diagram of nitrogen, N₂, is correct, and why?
Answer and reasoning
AN₂ has five bonds, because each N atom has five valence electrons to share A student who thinks an atom forms one bond per valence electron picks this. Each N needs only three more electrons, so three shared pairs complete both octets; the other two electrons on each atom stay as a lone pair.
BN₂ has a single bond, because the remaining valence electrons stay as lone pairs A student who stops once all the electrons are placed picks this. A single bond with two lone pairs on each N uses all 10 electrons, but each N then has only six around it; two lone pairs must become shared pairs.
CN₂ has a double bond, because two lone pairs on each N atom complete octets A student who adds lone pairs until every atom has an octet, without checking the total, picks this. Each N would have eight electrons, but the diagram would show 12 electrons, and N₂ has only 10.
DN₂ has a triple bond, because sharing three pairs gives each N atom an octetCorrect N₂ has 10 valence electrons. A triple bond (6 shared electrons) plus one lone pair on each N gives each N atom 6 + 2 = 8 electrons and uses all 10.
Compiled from the AP Chemistry Course and Exam Description (effective Fall 2024) and our question bank · Specialist review in progress. How these pages are made · Free, no account
