Module 1
An electron absorbs energy and moves from n=2 to n=5. When it returns to a lower energy level, does it absorb or emit light? Explain.
It emits light. Moving from a higher to a lower energy level releases energy as a photon.
Two atoms approach each other, and their potential energy decreases until reaching a minimum. What does this minimum represent?
The equilibrium bond length, where attractive and repulsive forces balance and the bonded atoms are the most stable.
What is the electron-domain geometry and molecular geometry of BeCl2?
Electron-domain geometry: linear
Molecular geometry: linear
Identify which of the following molecules are polar:
CO2, H2O, CH4, NH3
H2O and NH3
Which requires more energy: removing the first electron from a neutral sodium atom (Na) or removing the first electron from an Na+ ion?
Na+
Removing a valence electron from Na+ would require removing it from the 2p orbital, which is held closer to the nucleus than the 3s orbital.
Why do atoms produce discrete emission lines rather than a continuous spectrum?
Electrons occupy quantized energy levels, so they will only emit photons with the corresponding energy difference.
Write the electron configuration of sulfur. How many valence electrons does it have?
1s22s22p63s23p4
6 valence electrons
Draw the Lewis structure of HCN. State the electron domain geometry and molecular geometry around the central carbon.
H-C=N:
Linear EDG and linear MG
Rank the following bonds from least polar to most polar:
C-O, O-H, C-C, C-F
C-C < C-O < O-H < C-F
What is the molecular geometry of H3O+?
Trigonal pyramidal (lone pair on the O)
Calculate the frequency of electromagnetic radiation with a wavelength of 600 nm.
v = 5.00x10^14 Hz
Write the electron configuration for an excited-state Cl-.
1s22s22p63s23p54s1
What is the difference between structural isomers and resonance structures?
Structural isomers have the same molecular formula but different atom connectivity.
Resonance structures have the same atom connectivity but different electron arrangements.
BF3 and NH3 each contain three bonds to a central atom. Why is BF3 nonpolar while NH3 is polar?
BF3 has a trigonal planar molecular geometry, allowing the B-F dipoles to cancel.
NH3 is trigonal pyramidal due to the lone pair, so the bond dipoles do not cancel.
What is everyone going to get on this exam?
A sample of EM radiation has an energy of 300 kJ/mol of photons. Calculate the wavelength of this radiation in nm.
399 nm
Slides for figure
Identify which element is represented by the following photoelectron spectrum (PES). Which electron would be the easiest to remove?
Sodium
The electron in the 3s orbital (lowest binding energy)
See slide for figure
Identify the functional groups in the following molecule.
Ether, ketone, aldehyde, amide, carboxylic acid
A student argues that if a molecule contains at least one polar bond, the molecule must be polar because electrons are distributed unevenly.
Construct a CER to either support or refute this statement.
Evidence: CO2 contains two polar C=O bonds but is nonpolar overall (example).
Reasoning: Molecular polarity depends on the vector sum of individual dipoles, which is dependent on the geometry of the molecule. Because CO2 is linear and symmetric, the two C=O dipoles point and opposite directions and cancel out. Therefore, the presence of polar bonds does not necessarily produce an overall molecular dipole.
Atomic radius increases down and to the left.
Explanation: Moving down within a group adds more energy levels.=. Moving to the left within a period decreases the amount of positive charge (protons) in the nucleus. Both of these factors allow the electron cloud to occupy more space, increasing the atomic radius.
First ionization energy increases up and to the right.
Explanation: Moving up within a group decreases the amount of energy levels. Moving to the right within a period increases the amount of positive charge (protons) in the nucleus. Both of these factors place electrons in closer proximity to the nucleus, increasing the amount of energy it would take to overcome the electrostatic attraction.
An unknown compound generates an absorbance spectrum with two peaks: 80% absorbance at 450 nm and 60% absorbance at 600 nm. No other wavelengths are absorbed. Draw the transmittance spectrum that corresponds to this data.
Transmittance spec with 20% transmittance at 450 nm and 40% transmittance at 600 nm. All other transmittance at 100%.
See slides for figure
A student argues that this potential energy diagram represents the same pair of bonded atoms, once with a single bond and again with a double bond (ex. C-C and C=C). Provide a CER to either support or refute this student's claim.
Claim: The student's claim is incorrect.
Evidence: The bonds occur at different minimum potential energy, however, their minima occur at approximately the same internuclear distance.
Reasoning: A C=C bond is generally stronger and shorter than a C-C bond due to the greater electron density between the nuclei. Since the two curves have the same equilibrium internuclear distance, they are not consistent with the expected relationship between a C-C and a C=C bond.
Draw the line structure that represents the following structural formula:
CHCCH₂CH₂CHOHCH₂COH
See slides for figure
Draw a wedge-dash structure for CH2Cl2. Identify the partial charges, dipole moments, and net polarity of the molecule.
See slides for figure.
The carbonate ion, CO32-, has three equivalent C-O bonds. Are these bonds expected to be stronger or weaker than a typical C-O bond? Draw structure(s) to support your answer.
Stronger than a typical single bond.
Resonance can delocalize the electrons from the C=O bond (pi bond electrons) across all three C-O bonds, producing an average bond order of 1 1/3.