Useful Values:

A. Multiple Choice Questions

1. Which of the following compounds do not belong with the others (odd one out):

A great resource for this question is the website ptable.com. It enables you to see trends in many different properties of the elements, and allows you to build compounds and explore them in Wikipedia, the best reference for chemicals.

If you form compounds with silicon (Si) and chlorine (Cl), near the top is SiCl₄, silicon tetrachloride. If you click on the link, you'll go to the Wikipedia page for SiCl₄. There you'll see that this compound has covalent bonds, and the structure is tetrahedral. Also, you'll find that this compound is a liquid at room temperature, with a melting point of −68.74 °C and a boiling point of 57.65 °C.

You'll find that the other compounds listed – ferric chloride (FeCl₃), calcium chloride (CaCl₂) and cesium chloride (CsCl; there is a typo on the test) – are all ionic compounds that are solids at room temperature, have high melting points, and have crystal structures instead of molecular structures.

Therefore, the correct answer is a. SiCl₄. I would recommend exploring the ptable.com website to get more familiar with periodic trends, electron configurations, and isotopes, all things that are on the test. Though your teacher may not let you use ptable on tests, it can be a great resource to use on homework and to study for the final. There are many other periodic table resources, both on-line and printed, but this one is the best I've come across, except maybe for some beautiful books.

2. Which of the following statements are TRUE?

Not sure why you got this question about the photoelectric effect. It could be part of the typical introduction to quantum numbers for the electron configurations in atoms, but the particle nature of light that the photoelectric effect experiments ascertained is not really a chemistry thing, especially an introductory chemistry thing. Nevertheless, this question is a little misleading, since the question asks which statements are true, not which statement is true, hinting that maybe more than one of the statements is true. It turns out that none of them as written are true, so I think most people got credit for this question.

For a pretty good explanation of the photoelectric effect from a physics perspective, with some incorporation of chemistry when talking about how different metals have different "work functions" and therefore have different frequencies of light that produce the effect, see the Khan Academy article on the Photoelectric Effect at https://www.khanacademy.org/science/modern-physics-essentials/x1bb01bdec712d446:why-light-and-matter-are-two-faced/x1bb01bdec712d446:einstein-s-elegant-equation-to-decode-the-photoelectric-effect/a/photoelectric-effect.

The energy of the emitted "photoelectron" is proportional to the frequency of the light hitting the metal surface. Frequency and wavelength have an inverse relationship. The higher the frequency of light, the shorter the wavelength. I think you might be able to develop an intuition for this reading this BYJU'S page (https://byjus.com/physics/frequency-and-wavelength/). It is therefore inversely proportional to the wavelength of light.

The choices here also have Kinetic Energy with capital letters, which may throw some students off. There are no correct answers in the choices.

3. An object with a mass of 0.255 kg and density of 2.89 g/mm³ measures 3.4 mm in length and 4.6 mm in width. What is the height of the object?

I created an image of a hypothetical block (or box) with the width and length given, and the height h that you are solving for to give you an idea of what was being asked for in this question.

The main concepts being tested are the definition of density as the mass per unit volume \(d= \frac{m}{V}\), manipulation of this formula to solve for V, the idea that the volume of a rectangular prism is the length x width x height (given the statement of the problem, you have to assume that the object is a rectangular prism, even though it doesn't state this explicitl), and maybe most important, the idea of significant figures. When you do the math with a calculator with all the numbers given, you get that the height h is 5.64...., with as many decimal places as the calculator shows, but because you're doing multiplication and division in this problem, the amount of significant figures – because you're measuring – is the lowest amount of significant figures in the measurements done in the problem, which is 2. Therefore, the answer is b. 5.6 mm, which has two significant figures, not a. The other answers are off by orders of magnitude, or use scientific notation incorrectly.

4. An experiment requires you to make several measurements. Based on the values in the table below, what would be the best statement that represents the data set if the true value is 0.2135?

Accuracy is being close to the known answer, precision is how close your measurements are to each other. Given this, d. The measurements in Trial 1 are accurate and precise, Trial 2 is precise, not accurate, Trial 3 is neither. is the correct answer.

5. Isotopes differ with respect to the number of what?

Isotopes differ in neutrons, so b. Neutrons is the correct answer. I recommend looking at the Isotopes section of the Ptable website to explore the isotopes for all the elements. The physics of why some elements have more isotopes than others, and which isotopes are stable, is really interesting. Check out this Wikipedia article for more info.

6. How many neutrons, electrons and protons are there in an isotope with Z = 26, A = 58 and Charge = 2+?

Since Z is the atomic number, or the number of protons in the nucleus of an element, the only possible answer is d. 32, 24, 26 (since the last number is the number of protons). Fortunately, there is not a typo in this problem, since A, the mass number, or number of protons plus neutrons, is 58, giving the number of neutrons as 58–26=32, and since the charge is 2+, it means that there are 2 less electrons than protons in this ion (it is an ion because it has a charge), giving 26–2=24 electrons.

7. Which set of the following Quantum Numbers are allowed?

Two of the choices can be eliminated immediately, because m_s cannot be 3/2 or −3/2. Another choice is eliminated because m_l can only be between −l and l. So the only possible answer is c. n=25, l=24, m_l = −20, m_s= −1/2;, which is non-sensical, but theoretically possible. This electron would be in a highly energetic state, in an orbital very far from the nucleus, but quantum theory does not rule it out.

8. The Intensity of Light is given by the formula \( I = \frac{nh\nu}{At} \)

where n is the number of photoelectrons, \( \nu \) is the frequency of light, h is Planck's Constant, A is the incident area and t is the time.

Based solely on the above information, which of the following proportionalities are false?

This one is about putting the variable in question on the left of each proportion to the left side of the equation, and seeing if the variable on the right of the proportion is in the numerator or denominator on the right side of the equation. Doing this with A and t (I is already on the left, and is proportional to \(\nu \)), we get that answers b and c are correct, but answer d. \( A \propto \frac{1}{n} \) is false, so it is the correct answer. \( A \propto n \).

9. Electrons are lost from neutral atoms to form cations. In which of these elements are the electrons lost from the n–2 shell, where n is the highest occupied level?

The Main Group Elements include columns 1-2 and 13-18 in the main block of the periodic table in the Electrons section of the Ptable website. They successively fill the s and p suborbitals of each shell, and the metals lose electrons from the n shell to form cations. The d-block elements and the transition metals are the same, and they fill the d suborbital of the n-1 shell successively, so lose electrons from the n-1 shell to form cations. The landthanides (or f-block elements) are often pictured outside the main group of elements, and they fill the f suborbital of the n-2 shell successively (by successively, I mean as you go from left to right in the periodic table). So the answer to the question is d. Lanthanides.

10. Which of the following statements best describes an electron?

The geometry of atomic orbitals is a fascinating topic, but usually not taught in beginning chemistry, even in college. Take a look at The Orbitron, a cool site where you can look at the orbitals. You also have some videos in your Canvas files that show the orbitals. Remember that orbitals are basically defining the area where there is a good chance (typically over 90%) of finding an electron inside of them at any given time. The nodes are the areas where you wouldn't expect an electron to be, and hence answer d is wrong. The Heisenberg uncertainty principle says that you can't measure the exact location AND exact momentum (speed and direction) of an electron simultaneously, so answer a is wrong. The de Broglie wavelength is discussed here. It appears you can measure this wavelength from line spectra of several elements pretty precisely. Quantum theory says that each atomic orbital has a different energy, and these energies come in integer multiples of each other. So b. Electrons in non-degenerate orbitals have different energies is the correct answer. Degnerate orbitals are different lobes of the same suborbital set (like the 3 p orbitals or the 5 d orbitals of any shell). These do have the same energy.

11. How many paired and unpaired valence electrons are present in the ground state configuration of iron?

In iron, the 3d subshell or suborbital where the valence electrons are, since in the transition metals, the 3d subshell is at higher energy than the 4s subshell. If you look on the Ptable site, you can see the electron configuration has one pair of electrons in one of the 3d subshells, and one electron each in the remaining four subshells. You can also figure this out by writing out the electron configuration by hand.

12. What is the total number of electrons in n=7 level of an atom?

There is a formula you can use for this, and you can discover this formula either by yourself by noticing the pattern, or watch this video. In it, the guy goes over many cases of determining the maximum quantity of electrons allowed given several combinations of quantum numbers. Given the formula of max_electrons = 2n², where n is the principal quantum number, for this question, the answer is 2(7²)=98, so d. 98 is the correct answer.

13. Convert 196.15K to a temperature in °F

The formula is °F = (K × (9/5)) - 459.67, which gives -106.6°F, but because the fraction is used for multiplication, the answer needs to be rounded to the nearest whole number, which is -107°F, so the answer is b. -107°F. See here for one of the many on-line calculators and formulas associated with these temperature conversions. There is also some good info on this page about the different temperature scales and why they came to be used. If you want to have the formulas derived, please ask me at a tutoring session.

14. Convert 75 pm wavelength into frequency

See this page from the on-line chemistry LibreText for these types of conversions. The teacher is wanting two things from this question – one is to be able to convert from picometers (pm) to meters (m), then seeing if the frequency is really high or low. 1 pm = 1.0 x 10^-12 m, so 75 pm = 7.5 x 10^-11 m. Then use the formula \(\nu=\frac{c}{\lambda}\) to get c. 3.99 x 10¹⁸ Hz, or you can use one of the many on-line converters available, like this one.

15. Which of the following would be considered a physical change?

Answers a, c, and d are obviously chemical changes (a is an oxidation forming mostly CO2 and water from wax, a hydrocarbon; c involves first physically then chemically breaking down food into raw materials that the body uses, mostly by bacteria; and d is a classic chemical reaction that can produce all sorts of bells and whistles). Evaporating milk is answer the teacher was looking for, but I could see this process producing chemical changes as well, if the evaporation was done at elevated temperuature, or if there was a biological organism in the milk. But that is splitting hairs... the answer is b. Evaporating milk.

B. Free Response Section

1. How does the number of photoelectrons change with respect to the intensity of the incident light? Explain your answer.

See the notes for question 2 above to find the answer for this question.

2. Why is it common for a phosphorus atom to form P^3- anion rather than a P³⁺ cation? Based on the above, classify phosphorus a metal, non-metal or metalloid.

Phosphorus has 5 electrons in its valence shell, and if it gained 3 electrons, it would fill its valence shell, which is a more stable state than if it lost 2 electrons to become a 3+ cation, since it would still have 3 electrons in its valence shell, which wouldn't fill it. Sometimes P shares its 5 valence electrons to be formally a 5+ oxidation state atom, when it bonds to atoms close to it in the periodic table. But it would never lose 2 electrons and become a 3+ cation. It is a non-metal, which you can see on the Properties tab of the Ptable website, and also it is obvious from its appearance.

3. For an electron in a Bohr-like atom that transitions from the n=7 level to the n=4 level, calculate the wavelength in nm and the energy in eV of the light that is emitted/absorbed.

We can go over 3 and 4 on Monday in our tutoring session if anyone would like.

4. Draw the complete orbital diagrams for the following species: