An iron atom has the ground-state configuration [Ar]4s2 3d6. What is the electron configuration of the Fe2+ ion?
[Ar]3d6
✓
[Ar]4s2 3d4
[Ar]3d5
[Ar]4s1 3d5
Correct Answer
[Ar]3d6
When a transition metal forms a cation, it loses its outermost ns electrons before any (n-1)d electrons, so Fe2+ loses both 4s electrons and keeps all six 3d electrons. [Ar]4s2 3d4 wrongly removes the two electrons from the 3d subshell. [Ar]3d5 is the configuration of Fe3+, which has lost one more electron. [Ar]4s1 3d5 has the right electron count but wrongly keeps a 4s electron while removing 3d electrons.
Category: Ionic Bonding
Question 2
Which of these hydrogen-halogen bonds is the most polar?
H-Cl
H-F
✓
H-Br
H-I
Correct Answer
H-F
Bond polarity grows with the electronegativity difference between the bonded atoms, and fluorine is the most electronegative halogen, so H-F has the largest difference. H-Cl is polar but chlorine is less electronegative than fluorine. H-Br has an even smaller difference. H-I is the least polar of the four because iodine is the least electronegative halogen listed.
Category: Bond Polarity
Question 3
A bond in which two identical atoms share electrons equally, as in Cl2, is classified as which type?
Polar covalent
Ionic
Nonpolar covalent
✓
Metallic
Correct Answer
Nonpolar covalent
When the two bonded atoms have the same electronegativity, neither pulls the shared pair more strongly, so the bond is nonpolar covalent. Polar covalent bonds form between atoms with different electronegativities. Ionic bonding involves transfer of electrons to form oppositely charged ions. Metallic bonding describes electrons shared among many metal atoms in a solid.
Category: Bond Polarity
Question 4
How many valence electrons must be shown in the Lewis structure of CO2?
12
18
22
16
✓
Correct Answer
16
Carbon contributes 4 valence electrons and each oxygen contributes 6, giving 4 + 2(6) = 16. 12 comes from counting only 4 electrons for each oxygen. 18 adds an extra pair that no atom supplies. 22 results from counting total electrons in carbon (6) and oxygen (8 each) instead of valence electrons.
Category: Lewis Structures
Question 5
In the Lewis structure of N2, the two nitrogen atoms are joined by a triple bond. How many lone pairs does each nitrogen atom carry?
1
✓
2
3
Correct Answer
1
Each nitrogen brings 5 valence electrons for a total of 10; the triple bond uses 6, leaving 4 electrons as one lone pair on each atom, which completes each octet. 2 lone pairs on each atom would require 14 electrons. 0 lone pairs would leave each nitrogen with only six electrons. 3 lone pairs on each atom would need 18 electrons.
Category: Lewis Structures
Question 6
Which molecule has an odd total number of valence electrons, so its Lewis structure cannot give every atom an octet?
CO
NO
✓
N2
HCN
Correct Answer
NO
NO has 5 + 6 = 11 valence electrons, an odd number, so one atom must carry an unpaired electron and fall short of an octet. CO has 4 + 6 = 10 valence electrons. N2 has 5 + 5 = 10. HCN has 1 + 4 + 5 = 10. All three of these have even counts and can satisfy the octet rule.
Category: Lewis Structures
Question 7
In the Lewis structure of BF3, with three B-F single bonds, how many electrons surround the boron atom?
8
4
6
✓
10
Correct Answer
6
Boron has only three valence electrons and forms three single bonds, so it is surrounded by three shared pairs, or 6 electrons, making it electron deficient. 8 would be a full octet, which boron does not reach in this structure. 4 would mean only two bonds. 10 would be an expanded octet, which boron, a second-period element, cannot form.
Category: Lewis Structures
Question 8
Which molecule has a central atom surrounded by more than eight valence electrons?
CF4
NF3
OF2
SF6
✓
Correct Answer
SF6
Sulfur in SF6 forms six bonds and holds 12 electrons, an expanded octet possible for third-period and heavier elements. CF4 has carbon with exactly eight electrons in four bonds. NF3 has nitrogen with three bonds and one lone pair, eight electrons. OF2 has oxygen with two bonds and two lone pairs, also eight electrons; second-period atoms cannot exceed an octet.
Category: Lewis Structures
Question 9
In the Lewis structure of carbon monoxide, C and O share a triple bond and each atom has one lone pair. What is the formal charge on the carbon atom?
-1
✓
+1
-2
Correct Answer
-1
Formal charge equals valence electrons minus lone pair electrons minus half the bonding electrons; for carbon this is 4 - 2 - 3 = -1. A value of 0 would require carbon to own four electrons, but it owns five. A value of +1 is the formal charge on the oxygen atom (6 - 2 - 3). A value of -2 would require carbon to own six electrons.
Category: Formal Charge and Resonance
Question 10
When several Lewis structures can be drawn for a molecule, which one is generally preferred?
The one with the most double bonds
The one with formal charges closest to zero
✓
The one with the most lone pairs on the central atom
The one with a positive charge on the most electronegative atom
Correct Answer
The one with formal charges closest to zero
The most likely structure keeps formal charges as small as possible and, when charges are needed, places negative formal charge on the more electronegative atoms. The one with the most double bonds is not favored for that reason alone. The one with the most lone pairs on the central atom is not a selection rule. The one with a positive charge on the most electronegative atom is the opposite of the guideline.
Category: Formal Charge and Resonance
Question 11
The nitrate ion, NO3-, can be drawn as three resonance structures, each with one N=O double bond and two N-O single bonds. How do the three measured nitrogen-oxygen bond lengths compare?
One is shorter than the other two
One is longer than the other two
All three are equal
✓
All three are different
Correct Answer
All three are equal
The real ion is an average of its resonance structures, so the extra bonding is spread evenly and all three bonds are identical, intermediate between a single and a double bond. One is shorter than the other two would be true only if a single structure were real. One is longer than the other two describes no measured form of the ion. All three are different contradicts the symmetry of the averaged structure.
Category: Formal Charge and Resonance
Question 12
Use these bond energies to estimate the enthalpy change for H2 + Cl2 -> 2HCl: H-H = 436 kJ/mol, Cl-Cl = 243 kJ/mol, H-Cl = 432 kJ/mol.
+185 kJ
+247 kJ
+679 kJ
-185 kJ
✓
Correct Answer
-185 kJ
Enthalpy change is approximately the energy of bonds broken minus the energy of bonds formed: (436 + 243) - 2(432) = 679 - 864 = -185 kJ. The value +185 kJ reverses the subtraction. The value +247 kJ forgets that two H-Cl bonds form, giving (436 + 243) - 432. The value +679 kJ counts only the bonds broken and ignores the energy released when bonds form.
Category: Bond and Lattice Energy
Question 13
Which of these carbon-halogen bonds has the greatest bond energy?
C-F
✓
C-Cl
C-Br
C-I
Correct Answer
C-F
Bond strength generally decreases down a group because larger atoms form longer bonds, so the bond to the smallest halogen, fluorine, is strongest. C-Cl is weaker because chlorine is larger and the bond is longer. C-Br is weaker still. C-I is the weakest and longest of the four.
Category: Bond and Lattice Energy
Question 14
Which ionic compound requires the most energy to separate one mole of the solid into gaseous ions?
NaCl
MgO
✓
NaF
KBr
Correct Answer
MgO
This energy, the lattice energy, increases with ion charge and decreases with distance between ions; MgO combines 2+ and 2- ions of small size, so its value is far larger. NaCl contains only singly charged ions. NaF also contains only singly charged ions, though they are small. KBr has singly charged ions that are relatively large, so its value is the smallest of the four.
Category: Bond and Lattice Energy
Question 15
The Born-Haber cycle applies Hess's law mainly to calculate which quantity that cannot be measured directly?
Ionization energy
Enthalpy of sublimation
Lattice energy
✓
Bond dissociation energy
Correct Answer
Lattice energy
The cycle breaks the formation of an ionic solid into measurable steps, and the one step that cannot be measured directly, the lattice energy, is found from the others. Ionization energy is measured directly and is an input to the cycle. Enthalpy of sublimation of the metal is a measured input as well. Bond dissociation energy of the nonmetal is also a measured input.
Category: Bond and Lattice Energy
Question 16
A central atom with five regions of electron density has which electron-pair geometry?
Octahedral
Square pyramidal
Tetrahedral
Trigonal bipyramidal
✓
Correct Answer
Trigonal bipyramidal
Five electron regions spread out as far as possible in a trigonal bipyramidal arrangement, with three equatorial and two axial positions. Octahedral is the geometry for six regions. Square pyramidal is a molecular structure for six regions with one lone pair. Tetrahedral is the geometry for four regions.
Category: Molecular Geometry
Question 17
SF4 has four bonding pairs and one lone pair on the sulfur atom. What is its molecular structure?
Seesaw
✓
Tetrahedral
Square planar
Trigonal pyramidal
Correct Answer
Seesaw
With five electron regions, the lone pair takes an equatorial position, and the four remaining atoms form a seesaw shape. Tetrahedral would require four regions and no lone pairs. Square planar arises from six regions with two lone pairs. Trigonal pyramidal arises from four regions with one lone pair.
Category: Molecular Geometry
Question 18
The chlorine atom in ClF3 is bonded to three fluorine atoms and holds two lone pairs. Which shape does the molecule take?
Trigonal planar
T-shaped
✓
Trigonal pyramidal
Seesaw
Correct Answer
T-shaped
Five electron regions form a trigonal bipyramid; the two lone pairs occupy equatorial positions, leaving the three fluorine atoms in a T shape. Trigonal planar requires three regions and no lone pairs. Trigonal pyramidal comes from four regions with one lone pair. Seesaw is the shape when only one of the five regions is a lone pair.
Category: Molecular Geometry
Question 19
How does the H-N-H bond angle in NH3 compare with the ideal tetrahedral angle?
Exactly 109.5°
Slightly larger than 109.5°
Slightly smaller than 109.5°
✓
Exactly 120°
Correct Answer
Slightly smaller than 109.5°
The lone pair on nitrogen repels the bonding pairs more strongly than they repel each other, squeezing the angle to about 107°. Exactly 109.5° applies only when all four regions are bonding pairs, as in CH4. Slightly larger than 109.5° has the lone pair effect in the wrong direction. Exactly 120° is the angle for a trigonal planar arrangement.
Category: Molecular Geometry
Question 20
Which of these molecules is polar?
CO2
BF3
CH4
NH3
✓
Correct Answer
NH3
NH3 has polar N-H bonds arranged in a trigonal pyramid, so the bond dipoles do not cancel and the molecule has a net dipole moment. CO2 is linear, so its two bond dipoles cancel. BF3 is trigonal planar and symmetric, so its bond dipoles cancel. CH4 is tetrahedral and symmetric, giving no net dipole.
Category: Molecular Polarity