Chemistry 102 – Chapter 7 Summary and Review Answers

1. What is the relationship between the energy of light, the speed of light, the wavelength of light and the frequency of light? Which can vary and which are constant?

Answer: Energy = E; frequency = ν; wavelength = λ; speed of light = c; constant = h E = hν; c = ν λ

Energy, wavelength and frequency can vary (as frequency increases, energy increases and wavelength decrease). Speed of light is a constant (2.998×108 m∙s–1) as is h (Plank’s constant) (6.626×10–34 J∙s)

2. Consider a ground state atom of selenium. What is the electron configuration, the abbreviate configuration and the full orbital diagram for this atom? How many unpaired electrons are in one ground state atom of selenium?

Answer: The electron configuration for Se is 1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p4 The abbreviated configuration for Se is [Ar] 4s2 3d10 4p4 The full orbital diagram is shown to the right: There are two unpaired electrons in one ground state atom of Se.

3. According to the explanations of the photoelectric effect, light (energy packets) can be treated as particles. Therefore, above the threshold frequency, what must be increased to increase the number of electrons ejected? To increase the energy of the electrons ejected?

Answer: Above the threshold frequency, to increase the number of electrons ejected the intensity of the light (the number of photons) must be increased. Above the threshold frequency, to increase the energy of the ejected electrons, the frequency (energy of the photons) must be increased.

4. What is Pauli’s exclusion principle? Within the two properties of electrons discussed in Chapter 7 (charge and spin) – does this make sense? (Hint: same spins, like same charges, experience repulsion)

Answer: Pauli’s exclusion principle states that no two electrons will have the same set of quantum numbers within one atom. Yes, this makes sense as one electron in an atomic orbital will have a negative charge and spin up. A second electron will have a negative charge and spin down. Adding a third electron will have both charge and spin repulsion and will occupy a higher energy (empty) atomic orbital.

5. Which transitions in a hydrogen atom are visible? Which are absorptions? Which transition results in the emission of a photon with the longest wavelength, shortest wavelength, largest frequency, and smallest frequency?

n = 1 Æ n = 2 n = 2 Æ n = 1 n = 2 Æ n = 3 n = 3 Æ n = 2

n = 4 Æ n = 2 n = 2 Æ n = 4 n = 3 Æ n = 4 n = 4 Æ n = 3

Pictorially show Bohr’s model of a hydrogen atom and show the first two transitions. Also show a transition associated with the ionization energy of a single hydrogen atom.

Answer: Transitions from a higher energy to a lower energy are emissions, nf > ni (in the reverse, these are absorptions, nf < ni). Transitions to nf = 1 are in the UV range, nf = 2 are in the visible for ni = 3, 4, 5 and 6; and transitions to nf = 3 are in the IR.

Absorptions: n = 1 Æ n = 2 n = 2 Æ n = 3 n = 2 Æ n = 4 n = 3 Æ n = 4

Emissions: n = 2 Æ n = 1 n = 3 Æ n = 2 n = 4 Æ n = 2 n = 4 Æ n = 3 (UV) (Visible, red) (Visible, green) (IR) Highest frequency lowest frequency Shortest wavelength longest wavelength

6. In a ground state atom of lead, when n = 4, how many atomic orbitals contain electrons with ml = 0?

Answer: For n = 4, l = 0, 1, 2 or 3. In a ground state atom of lead all of these are occupied. Therefore, there are four atomic orbitals which correspond to ml = 0. With 2 electrons per atomic orbital, this corresponds to 8 sets of quantum numbers: (4, 0, 0, ½) (4, 0, 0, –½) (4, 1, 0, ½) (4, 1, 0, –½) (4, 2, 0, ½) (4, 2, 0, –½) (4, 3, 0, ½) (4, 3, 0, –½)