CHM110 Exam 4 Review

What is the electron configuration of Co³⁺?

• (d) [Ar]4d⁶

• (c) [Ar]4s²3d⁴

• (b) [Ar]

• (a) [Ar]4s²3d⁷

• (e) [Ar]3d⁶

Answer explanation

Cobalt (Co) has an atomic number of 27. The electron configuration for Co is [Ar]4s²3d⁷. For Co³⁺, we remove 3 electrons, typically from the 4s and 3d orbitals, resulting in [Ar]3d⁶, making (e) the correct choice.

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Which set ranks the species from smallest to largest radius?

• (a) F⁻ < Na⁺ < Cl⁻

• (b) F⁻ < Cl⁻ < Na⁺

• (c) Na⁺ < F⁻ < Cl⁻

• (d) Na⁺ < Cl⁻ < F⁻

Answer explanation

The ionic radii trend shows that Na⁺ (smallest) has lost an electron, F⁻ (larger) has gained an electron, and Cl⁻ (largest) has gained an electron as well. Thus, the correct order is Na⁺ < F⁻ < Cl⁻.

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An element has a small increase between its first and second ionization energy, a large increase between its second and third ionization energy, and small increases between all subsequent ionization energies. The element is likely a(n)

• (a) alkaline earth metal

• (b) halogen

• (c) noble gas

• (d) alkali metal

Answer explanation

The small increase between the first and second ionization energies indicates the removal of an electron from a similar energy level, typical for alkaline earth metals. The large increase after the second ionization suggests a stable noble gas configuration is reached.

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NGSS.HS-PS1-1

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How many unpaired electrons are present in a ground state atom of sulfur?

• (c) 3

• (d) 4

• (a) 1

• (e) 5

• (b) 2

Answer explanation

Sulfur has an atomic number of 16, with the electron configuration of 1s² 2s² 2p⁶ 3s² 3p⁴. The 3p subshell has 4 electrons, resulting in 2 unpaired electrons. Therefore, the correct answer is (b) 2.

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NGSS.HS-PS1-1

Which bond is most polar?

• C-F

• C-H

• C-N

• C-C

• C-Cl

Answer explanation

The C-F bond is the most polar due to the large electronegativity difference between carbon and fluorine, making it highly polar compared to C-H, C-N, C-C, and C-Cl bonds.

The carbon-carbon bond in HCCH is _____ than the carbon-carbon bond in H2CCH2.

• shorter and weaker

• shorter and stronger

• longer and weaker

• longer and stronger

Answer explanation

The carbon-carbon bond in HCCH (acetylene) is a triple bond, making it shorter and stronger than the double bond in H2CCH2 (ethylene).

Choose the best Lewis structure for OCl2.

• (a)

• (b)

• (c)

• (d)

• (e)

Answer explanation

The best Lewis structure for OCl2 (dichlorine monoxide) is (e), which shows the correct arrangement of electrons. Oxygen is the central atom with two single bonds to chlorine and two lone pairs, satisfying the octet rule.

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Which of the following atoms can have an expanded octet?

• C

• O

• F

• Cl

• both F and Cl

Answer explanation

Chlorine (Cl) can have an expanded octet because it has available d orbitals to accommodate more than eight electrons. In contrast, carbon (C), oxygen (O), and fluorine (F) cannot expand their octets.

What is the formal charge of the phosphorus atom in the Lewis structure at right?

• +1

• -1

• -2

• -3

• 0

Answer explanation

To calculate the formal charge of phosphorus, use the formula: Formal Charge = Valence Electrons - electrons assigned in structure. If phosphorus has 5 valence electrons and 4 electrons in the structure, the charge is +1.

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• 574 kJ

• 176 kJ

• 88 kJ

• 419 kJ

• -419 kJ

Answer explanation

To find ΔH°, calculate the bond energies of reactants minus products. Breaking P-Cl and Cl-Cl bonds requires energy, while forming P-Cl bonds releases energy. The net result is -419 kJ, indicating an exothermic reaction.

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