1. Atoms, Molecules & Ions Isotopes and % Abundance
....and the historical development of the human understanding of matter, atoms, and subatomic particles.
Bonus points may be available for suggesting good wrong answers for future use. So if you were thinking that the answer you really wanted wasn’t among the choices, let me know.

2. A compound made of carbon and oxygen was analyzed and the follow data was obtained. Which law do these data illustrate?
mass of C (g)
mass of O(g) 12 16
2.5
3.2
0.625
0.80 1
1.33
The law of conservation of matter
The law of multiple proportions
The law of constant composition
The law of conservation of mass 2

3. The law of conservation of matter The law of multiple proportions
A compound made of carbon and oxygen was analyzed and the follow Data was obtained. Which law do these data illustrate?
mass of C (g)
mass of O(g) 12 16
2.5
3.2
0.625
0.80 1
1.33
The law of conservation of matter
The law of multiple proportions
The law of constant composition
aka law of definite proportions
The law of conservation of mass 3

4. The law of conservation of matter The law of multiple proportions
One compound of carbon and oxygen contains g of oxygen per gram of carbon, whereas a second compound contains g of oxygen per gram of carbon. What chemical law do these data illustrate?
The law of conservation of matter
The law of multiple proportions
The law of constant composition
The law of conservation of mass

5. The law of conservation of matter The law of multiple proportions
One compound of carbon and oxygen contains g of oxygen per gram of carbon, whereas a second compound contains g of oxygen per gram of carbon. What chemical law do these data illustrate?
The law of conservation of matter
The law of multiple proportions
this law can be demonstrated with molecular compounds and transition metal compounds because of the many oxidation state possibilities
CO and CO2
Fe2O3 and FeO
The law of constant composition
The law of conservation of grams

6. One compound of carbon and oxygen contains 1
One compound of carbon and oxygen contains g of oxygen per gram of carbon, whereas a second compound contains g of oxygen per gram of carbon. If the first compound has an equal number of oxygen and carbon atoms, what can we conclude about the composition of the second compound?
The second compound must have twice as many oxygen atoms per carbon atom.
The second compound must have half as many oxygen atoms per carbon atom as the first compound.
The second compound contains half as many carbon atoms as the first compound.
The first compound contains half as many carbon atoms as the second compound.

7. One compound of carbon and oxygen contains 1
One compound of carbon and oxygen contains g of oxygen per gram of carbon, whereas a second compound contains g of oxygen per gram of carbon. If the first compound has an equal number of oxygen and carbon atoms, what can we conclude about the composition of the second compound?
The second compound must have twice as many oxygen atoms per carbon atom.
The second compound must have half as many oxygen atoms per carbon atom as the first compound.
The second compound contains half as many carbon atoms as the first compound.
The first compound contains half as many carbon atoms as the second compound.

8. What happens to most of the α particles that strike the gold foil in Rutherford’s experiment, and why do they behave that way?
α particles are scattered across a range of deflection angles due to the high density of the foil nuclei.
Most α particles pass through the foil without being deflected because most of the volume of the atoms that comprise the foil is empty space.
Most α are scattered at acute angles as they pass close to the foil nuclei.
Most α particles are deflected in a backwards direction from the foil due to the high density of the foil atom nuclei.

9. What happens to most of the α particles that strike the gold foil in Rutherford’s experiment, and why do they behave that way?
α particles are scattered across a range of deflection angles due to the high density of the foil nuclei.
Most α particles pass through the foil without being deflected because most of the volume of the atoms that comprise the foil is empty space.
Most α are scattered at acute angles as they pass close to the foil nuclei.
Most α particles are deflected in a backwards direction from the foil due to the high density of the foil atom nuclei.

10. Radioactive substances emit “rays.” These rays are particles or energy. In the image below, which ray represents beta particles?
orange
red
green
any color could represent a beta particles, since we don’t know if the charge of the plates are the same magnitude.

11. Radioactive substances emit “rays.” These rays are particles or energy. In the image below, which ray represents beta particles?
orange
red
green
any color could represent a beta particles, since we don’t know if the charge of the plates are the same magnitude.
Let’s assume the plates are the same magnitude of charge, why do the alpha particles deflect less than the beta particles? 11

12. Radioactive substances emit “rays.” These rays are particles or energy. In the image below, which ray represents beta particles?
Let’s assume the plates are the same magnitude of charge, why do the alpha particles deflect less than the beta particles?
Beta particles have a 1− charge, and alpha particles have 2+ charge, thus one might think alpha might deflect more, however, the mass of alpha particle is ~8000 times more massive and thus does not bend as much. 12

13. Emissions from Radioactive Substances
Alpha particles, essentially a helium nucleus
made of 2 protons & 2 neutrons, +2 charge
represented as or
fairly heavy, slower moving
clothing is enough to stop their progress
Beta particles are essentially electrons
represented as or
very little mass, fast moving
requires a thin metal foil to stop their progress
Gamma rays is essentially energy
represented as
more energetic than x-rays
requires thick lead shield to stop their progress 13

14. How many nucleons are there in a nuclide (aka isotope) of 14C?6 8 12 14

15. How many nucleons are there in a nuclide (aka isotope) of 14C (carbon-14)?6 8 12 14
Nucleons are protons and neutrons
Remember the mass number minus the atomic number ( = 8) always equals the number of neutrons. Mass number = number of nucleons. 14

16. How many electrons would an isotope of Cr3+ ion contain?21 24 28
not enough information to determine

17. How many electrons would an isotope of Cr3+ ion contain?21 24 28
not enough information to determine
24+
24e-
21e-
Cr3+ Cr
loses three
electrons

18. A particular atom of chromium has a mass of 52
A particular atom of chromium has a mass of amu, whereas the atomic weight of chromium is amu. Explain the difference in the two masses.
The amu value and the amu value represent two different isotopes of chromium.
The atomic weight of chromium is less than the mass of the specific chromium atom since atomic weights are used to describe neutral atoms.
The atomic weight of chromium (51.99 amu) is an average atomic mass of all the naturally occurring isotopes of chromium, whereas the is the mass of just one of those isotopes.

19. A particular atom of chromium has a mass of 52
A particular atom of chromium has a mass of amu, whereas the atomic weight of chromium is amu. Explain the difference in the two masses.
The amu value and the amu value represent two different isotopes of chromium.
The atomic weight of chromium is less than the mass of the specific chromium atom since atomic weights are used to describe neutral atoms.
The atomic weight of chromium (51.99 amu) is an average atomic mass of all the naturally occurring isotopes of chromium, whereas the is the mass of just one of those isotopes.

20. Both chlorine and bromine exist as two naturally-occurring isotopes, distributed as shown to the right. % natural occurrence is based on the distribution of isotopes. Chlorine reacts with bromine to form ClBr. How many different possible molar masses are there for ClBr?
No Calculator
isotope
% natural occurrence
chlorine-35
76%
chlorine-37
24%
bromine-79
51%
bromine-81
49%

21. Three possible molar masses:
Both chlorine and bromine exist as two naturally-occurring isotopes, distributed as shown to the right. % natural occurrence is based on the distribution of isotopes. Chlorine reacts with bromine to form ClBr. How many different possible molar masses are there for ClBr?
isotope
% natural occurrence
chlorine-35
76%
chlorine-37
24%
bromine-79
51%
bromine-81
49%
Three possible molar masses:
35+79=114
35+81=116, (37+79=116, repeat)
37+81=118

22. No Calculator ¼ ½ ⅓ ⅜ ⅛ Molar Masses chlorine-35 76% chlorine-37 24%
Both chlorine and bromine exist as two naturally-occurring isotopes, distributed as shown to the right. % natural occurrence is based on the distribution of isotopes. Chlorine reacts with bromine to form ClBr. Given the three possible molar masses: 114, 116, 118, select the choice from the right for the approximate fractional distribution in natural occurring compounds of these three molar masses?
isotope
% natural occurrence
chlorine-35
76%
chlorine-37
24%
bromine-79
51%
bromine-81
49%
Molar Masses
114
116
118 1 2 ⅓ 3 ⅜ ⅛ 4 5
No Calculator

23. ｝ No Calculator ¼ ½ ⅓ ⅜ ⅛ Molar Masses chlorine-35 76% chlorine-37 24%
Both chlorine and bromine exist as two naturally-occurring isotopes, distributed as shown to the right. % natural occurrence is based on the distribution of isotopes. Chlorine reacts with bromine to form ClBr. Given the three possible molar masses: 114, 116, 118, select the choice from the right for the approximate fractional distribution in natural occurring compounds of these three molar masses? Consider the % occurrence of each isotope to be ¼, ¾, ½. To get frequency of molecule, multiply the two frequencies of the isotopes together. From previous problem: the three possible molar masses: =114 ¾ * ½ = ⅜ =116 ¾ * ½ = ⅜ =116 ¼ * ½ = ⅛ =118 ¼ * ½ = ⅛
isotope
% natural occurrence
chlorine-35
76%
chlorine-37
24%
bromine-79
51%
bromine-81
49%
Molar Masses
114
116
118 1 2 ⅓ 3 ⅜ ⅛ 4 5 ｝
No Calculator

24. The element X has three naturally occurring isotopes
The element X has three naturally occurring isotopes. The isotopic masses and % abundance are given in the table to the right. Estimate the value that the average atomic mass of the element would be closest to.
isotope
% abundance
mass
48X 25
47.98
50X 50
49.96
52X
51.99 48 49 50 51 52
No Calculator

25. isotope
% abundance
mass
48X 25
47.98
50X 50
49.96
52X
51.99
The element X has three naturally occurring isotopes. The isotopic masses and % abundance are given in the table to the right. Calculate the value that the average atomic mass of the element would be closest to. 48 49 50 51 52
No calculator necessary. Look at the %abundance to realize that an equal % of 52 and an equal % of 48 would make the average fall exactly in the middle at mass 50 25

26. Graphical Readout from a Mass Spectrometer
Chlorine has two isotopes, 35Cl (75.78% abundant) and 37Cl (24.22% abundant). How many different molar masses are possible for a Cl2 molecule? 1 2 3 4 5
Not enough information to determine
Graphical Readout from a Mass Spectrometer

27. Graphical Readout from a Mass Spectrometer
Chlorine has two isotopes, 35Cl (75.78% abundant) and 37Cl (24.22% abundant). How many different molar masses are possible for a Cl2 molecule? 1 2 3
35+35, 35+37, 37+37
the percentages in this problem are distractors 4 5
Not enough information to determine
Graphical Readout from a Mass Spectrometer

28. Mass Spectrometry; Determining masses and percentages of isotopes.
Mass Spectrometer
Percent Abundance
determined from signal intensity
Graphical Readout from a Mass Spectrometer

29. Zn has 5 oxidation states.
A sample of zinc was run through a mass spectrometer. Which of the following statements can be concluded from the spectrograph shown below.
Signal Intensity
Zn has 5 oxidation states.
Zinc does not have an isotope with 35 neutrons.
There are 4 naturally occurring isotopes.
The most common isotope is 65Zn.
Zinc most commonly forms Zn2+.

30. Not enough information to determine
Chlorine has two isotopes, 35Cl (75.78% abundant) and 37Cl (24.22% abundant). How many different molar masses are possible for a Cl2 molecule? Given the three possible molar masses: 70, 72, 74, select the choice from the right for the approximate fractional distribution in natural occurring elemental molecules of these three molar masses?
Molar Masses 70 72 74 1 ⅜ 2
⁹/₁₆
³/₁₆
⅟₁₆ 3 4 5 6 1 2 3 4 5
Not enough information to determine

31. ⅜ ¼ ⁹/₁₆ ³/₁₆ ⅟₁₆ ¾ ½ Molar Masses 70 72 74 1 2 3 4 5 6 1 2 3 4 5
Chlorine has two isotopes, 35Cl (75.78% abundant) and 37Cl (24.22% abundant). How many different molar masses are possible for a Cl2 molecule? Given the three possible molar masses: 70, 72, 74, select the choice from the right for the approximate fractional distribution in natural occurring elemental molecules of these three molar masses? 1 2 3 4
¾ * ¾ = 9/₁₆
and
¾*¼=³/₁₆ + ¼*¾=³/₁₆ = ⅜
¼ * ¼ = ⅟₁₆ 5
Not enough information to determine
Molar Masses 70 72 74 1 ⅜ 2
⁹/₁₆
³/₁₆
⅟₁₆ 3 4 5 6