Atomic Detectives
A scientist performs an experiment in which a beam produced from atoms bends toward a positively charged plate.
Another scientist claims, “This experiment proves that atoms contain a small, dense, positively charged nucleus.”
Is that conclusion supported by the experiment? Explain.
No. The experiment supports the existence of negatively charged particles in atoms, electrons. It does not provide evidence for a small, dense nucleus.
A mass spectrum has three peaks:
90 amu: 5.00%
91 amu: 80.00%
92 amu: 15.00%
A student calculates an average atomic mass of 90.2 amu.
Without doing the full calculation, explain why the answer cannot be reasonable.
Most of the atoms have a mass of 91 amu, so the average should be close to 91 amu. An average of 90.2 amu is much too close to the least abundant isotope.
Three radioactive emissions are observed:
Identify X, Y, and Z and explain why Y bends less than X.
X = beta
Y = alpha
Z = gamma
A student says:
“Stars build heavier elements by repeatedly adding one hydrogen nucleus at a time until they eventually reach iron.”
What is misleading about this explanation?
Stellar nucleosynthesis involves different stages of fusion in which larger nuclei can fuse with other nuclei. The path to heavier elements is not simply the repeated addition of individual hydrogen nuclei.
A radioactive sample contains 37.5 mg after three half-lives.
What was the original mass of the radioactive isotope?
37.5 × 2 × 2 × 2 = 300. mg
A particle has 28 protons, 31 neutrons, and 26 electrons. A second particle has 28 protons, 33 neutrons, and 28 electrons.
Identify two ways these particles differ and one way they are related.
Different isotopes and ions.
Same element.
A student says:
“The atomic mass on the periodic table is the mass of the most common isotope.”
Explain the student's error.
The periodic-table atomic mass is a weighted average of the masses of the naturally occurring isotopes. It is not simply the mass of the most abundant isotope.
A radioactive nucleus changes from:
A = 137, Z = 55
to:
A = 137, Z = 56
A student says this must be alpha decay because the element changed.
Is the student correct? Identify the decay and justify your answer.
No. It is beta decay. The mass number remains 137 while the atomic number increases from 55 to 56.
A neutron strikes a large nucleus. The nucleus splits into smaller nuclei and releases additional neutrons.
Explain why this process can lead to a chain reaction.
This is fission. The neutrons released by one fission event can strike other nuclei, causing additional fission events that release still more neutrons.
A student starts with 100. g of a radioactive isotope with a half-life of 5.00 years and says:
“After one half-life I lose 50 g. Therefore, after three half-lives I will have lost 150 g.”
Explain the error and determine how much remains after three half-lives.
Half-life removes half of the amount currently present, not the same mass each time.
12.5 g
In an experiment, almost all positively charged particles pass straight through a thin sheet of metal, but a very small number are deflected at large angles. What conclusion about atomic structure is supported by these observations?
The atom is mostly empty space, and its positive charge and most of its mass are concentrated in a very small, dense nucleus.
An element has two isotopes at approximately 63 amu and 65 amu. The 63 amu isotope is about twice as abundant as the 65 amu isotope.
Without calculating, where should the average atomic mass fall?
A. Below 63 amu
B. Between 63 and 64 amu
C. Exactly 64 amu
D. Between 64 and 65 amu
B. Between 63 and 64 amu
An unknown form of radiation:
Identify the radiation and justify your answer.
Its attraction toward the positive plate shows that it is negatively charged, and its penetrating ability is consistent with beta radiation.
Why do nuclei in stars require extremely high temperatures for fusion to occur?
Positively charged nuclei repel one another. High temperatures give the nuclei enough kinetic energy for sufficiently energetic collisions to overcome this repulsion and allow fusion to occur.
A radioactive sample contains 18.0 mg after three half-lives.
A second sample of the same isotope originally contained 96.0 mg.
How much radioactive isotope remains in the second sample after the same amount of time?
Three half-lives means 1/8 remains.
96.0 mg ÷ 8 = 12.0 mg
Determine the number of protons, neutrons, and electrons in:
⁶⁵₂₉Cu²⁺
29 protons
36 neutrons
27 electrons
A mass spectrum shows:
24.00 amu: 79.00%
25.00 amu: 10.00%
26.00 amu: 11.00%
Calculate the average atomic mass.
24.32 amu
Complete the nuclear equation and identify the type of decay:
²²⁶₈₈Ra → ²²²₈₆Rn + _____
⁴₂He, alpha decay
Complete the nuclear equation:
²⁰₁₀Ne + ⁴₂He → _____
Then identify the process.
²⁴₁₂Mg; fusion
A radioactive isotope has a half-life of 1.50 × 10³ years. A sample contains 6.25% of the radioactive isotope that was originally present.
How old is the sample?
6.00 × 10³ years
6,000 years
Two particles each contain 18 electrons. Particle A has 17 protons and Particle B has 18 protons. Are they the same element? Explain.
No. Element identity is determined by the number of protons. Particle A and Particle B are different elements even though they have the same number of electrons.
An element has two naturally occurring isotopes:
Q-10: 10.01 amu
Q-11: 11.01 amu
The average atomic mass is 10.31 amu.
What is the percent abundance of Q-11?
30.00%
A radioactive nucleus decays. Its mass number remains unchanged, but its atomic number increases by 1.
What type of decay occurred, and what changed inside the nucleus?
A neutron changed into a proton, increasing the atomic number by 1 while leaving the mass number unchanged.
A nuclear reaction is represented by:
²H + ³H → ⁴He + ¹n
Is this fusion or fission? Explain using the equation.
Fusion. Two small nuclei combine, producing a larger nucleus.
A 160. mg sample of a radioactive isotope decreases to 80.0 mg after 12 minutes, 40.0 mg after 24 minutes, and 20.0 mg after 36 minutes.
What is the half-life of the isotope, and how much of the sample will remain after 60 minutes?
Half life: 12 min.
5.00 mg