Nucleons are the particles found in the nucleus, protons and neutrons, and their total is the mass number. Protons and electrons is wrong because electrons are found outside the nucleus. Neutrons and electrons is wrong for the same reason. Electrons and positrons are light particles and are not found in a stable nucleus.
Category: Nuclear Stability
Question 2
The mass of a nucleus is less than the total mass of its separate nucleons. What is the energy equivalent of this missing mass called?
Ionization energy
Nuclear binding energy
✓
Activation energy
Lattice energy
Correct Answer
Nuclear binding energy
Using E = mc2, the mass defect corresponds to the nuclear binding energy, the energy that would be needed to separate a nucleus into its individual nucleons. Ionization energy is needed to remove an electron from an atom. Activation energy is the barrier a chemical reaction must overcome. Lattice energy relates to separating an ionic solid into its ions.
Category: Nuclear Stability
Question 3
Lead-208 is a stable nuclide with 82 protons. How does its number of neutrons compare with its number of protons?
Protons outnumber neutrons
The two numbers are equal
Neutrons outnumber protons
✓
Neutrons are exactly double
Correct Answer
Neutrons outnumber protons
Lead-208 has 208 - 82 = 126 neutrons, a neutron-to-proton ratio of about 1.5; heavy stable nuclei need extra neutrons to offset the repulsion among many protons. Protons outnumber neutrons does not describe any heavy stable nuclide. The two numbers are equal is typical only of light stable nuclei. Neutrons are exactly double would mean 164 neutrons, far more than lead-208 has.
Category: Nuclear Stability
Question 4
Uranium-238 (atomic number 92) undergoes alpha decay. Which nuclide is produced?
Thorium-238
Protactinium-234
Plutonium-242
Thorium-234
✓
Correct Answer
Thorium-234
Emitting an alpha particle removes 2 protons and 2 neutrons, so the mass number falls from 238 to 234 and the atomic number from 92 to 90, which is thorium. Thorium-238 has the right element but keeps the old mass number. Protactinium-234 has atomic number 91, which would follow beta decay of thorium-234. Plutonium-242 adds the alpha particle instead of removing it.
Category: Nuclear Equations
Question 5
A carbon-14 nucleus, with atomic number 6, emits a beta particle. What nucleus is left behind?
Nitrogen-14
✓
Boron-14
Carbon-13
Nitrogen-15
Correct Answer
Nitrogen-14
In beta decay a neutron becomes a proton and an electron is emitted, so the atomic number rises by one to 7 while the mass number stays at 14, giving nitrogen-14. Boron-14 lowers the atomic number, which is what positron emission would do. Carbon-13 changes the mass number without changing the element. Nitrogen-15 changes both numbers.
Category: Nuclear Equations
Question 6
Which change takes place in a nucleus during positron emission?
A neutron becomes a proton
A proton becomes a neutron
✓
Two protons and two neutrons leave
An excited nucleus emits a photon
Correct Answer
A proton becomes a neutron
In positron emission a proton converts into a neutron and a positron, so the atomic number decreases by one and the mass number is unchanged. A neutron becomes a proton describes beta decay. Two protons and two neutrons leave describes alpha decay. An excited nucleus emits a photon describes gamma emission.
Category: Radioactive Decay
Question 7
A light nuclide has too many neutrons to be stable and lies above the band of stability. By which process is it most likely to decay?
Positron emission
Electron capture
Beta emission
✓
Alpha emission
Correct Answer
Beta emission
Beta emission converts a neutron into a proton, lowering the neutron-to-proton ratio and moving the nuclide toward the band of stability. Positron emission and electron capture both convert a proton into a neutron, which suits nuclides with too few neutrons. Alpha emission is typical of very heavy nuclides with atomic numbers above 83.
Category: Radioactive Decay
Question 8
Which type of nuclear radiation passes most easily through dense materials and requires thick lead or concrete for shielding?
Alpha particles
Beta particles
Positrons
Gamma rays
✓
Correct Answer
Gamma rays
Gamma rays are high-energy photons with no mass or charge, so they penetrate far into matter and need dense shielding. Alpha particles are stopped by a sheet of paper or the outer layer of skin. Beta particles are stopped by a thin sheet of metal such as aluminum. Positrons are charged particles with the mass of an electron and are about as penetrating as beta particles.
Category: Radioactive Decay
Question 9
A radioisotope has a half-life of 5.0 days. Starting with 80.0 g, how much of the isotope remains after 15.0 days?
10.0 g
✓
20.0 g
26.7 g
40.0 g
Correct Answer
10.0 g
Fifteen days is 15.0 / 5.0 = 3 half-lives, so the amount is halved three times: 80.0 to 40.0 to 20.0 to 10.0 g. An answer of 20.0 g stops after two half-lives. An answer of 26.7 g divides by 3 instead of halving three times. An answer of 40.0 g is the amount after only one half-life.
Category: Radioactive Decay
Question 10
Carbon-14 has a half-life of 5730 years. A wooden artifact contains 25 percent of the carbon-14 found in living wood. About how old is the artifact?
5,730 years
11,460 years
✓
17,190 years
22,920 years
Correct Answer
11,460 years
Going from 100 percent to 25 percent takes two half-lives (100 to 50 to 25), so the age is 2 x 5730 = 11,460 years. An answer of 5,730 years is one half-life, which would leave 50 percent. An answer of 17,190 years is three half-lives, leaving 12.5 percent. An answer of 22,920 years is four half-lives, leaving about 6 percent.
Category: Radioactive Decay
Question 11
The natural radioactive decay series that begins with uranium-238 ends with which stable nuclide?
Lead-208
Bismuth-209
Lead-206
✓
Radon-222
Correct Answer
Lead-206
Through a series of alpha and beta decays, uranium-238 is eventually converted into stable lead-206. Lead-208 is the end of the thorium-232 series. Bismuth-209 is not the endpoint of the uranium-238 series. Radon-222 is a radioactive gas formed partway through the uranium-238 series, not its end.
Category: Radioactive Decay
Question 12
A uranium-235 nucleus absorbs a neutron and splits into two smaller nuclei, releasing more neutrons and a large amount of energy. What is this process called?
Nuclear fusion
Alpha decay
Electron capture
Nuclear fission
✓
Correct Answer
Nuclear fission
Fission is the splitting of a heavy nucleus into lighter nuclei, and the neutrons released can trigger further fissions in a chain reaction. Nuclear fusion joins light nuclei into a heavier one. Alpha decay ejects only a small helium nucleus. Electron capture draws an inner electron into the nucleus and does not split it.
Category: Fission and Fusion
Question 13
What is the term for the minimum amount of a fissionable material that can sustain a self-sustaining chain reaction?
Critical mass
✓
Molar mass
Mass defect
Atomic mass
Correct Answer
Critical mass
Below the critical mass, too many neutrons escape from the material without causing further fission, so the chain reaction dies out. Molar mass is the mass of one mole of a substance. Mass defect is the difference between a nucleus's mass and the mass of its separate nucleons. Atomic mass is the average mass of an element's atoms.
Category: Fission and Fusion
Question 14
In a nuclear power reactor, what is the job of the control rods?
They slow neutrons down without absorbing them
They absorb neutrons to regulate the chain reaction
✓
They carry heat from the core to the turbines
They contain the uranium fuel that undergoes fission
Correct Answer
They absorb neutrons to regulate the chain reaction
Control rods made of neutron-absorbing elements such as boron or cadmium are raised or lowered to control how many neutrons cause fission. Slowing neutrons down without absorbing them is the job of the moderator. Carrying heat from the core is the job of the coolant. The uranium fuel is held in the fuel rods, not the control rods.
Category: Fission and Fusion
Question 15
Changing one element into another by bombarding its nuclei with particles such as neutrons or helium nuclei is called what?
Spontaneous fission
Beta decay
Nuclear transmutation
✓
Gamma emission
Correct Answer
Nuclear transmutation
Nuclear transmutation converts one element into another; in 1919 Rutherford changed nitrogen into oxygen by bombarding it with alpha particles. Spontaneous fission happens without bombardment. Beta decay is a spontaneous change of a neutron into a proton. Gamma emission releases energy without changing the element.
Category: Fission and Fusion
Question 16
Which radioisotope, with a half-life of about 6 hours, is widely used in medical imaging to locate damaged tissue in organs such as the heart, liver and lungs?
Carbon-14
Uranium-238
Plutonium-239
Technetium-99m
✓
Correct Answer
Technetium-99m
Technetium-99m emits easily detected high-energy photons and decays quickly, so patients receive little exposure; hospitals generate it on site from molybdenum-99. Carbon-14 has a half-life of thousands of years and is used for dating and tracing. Uranium-238 has a half-life of billions of years. Plutonium-239 is a reactor and weapons material, not an imaging agent.
Category: Uses of Radioisotopes
Question 17
Which SI unit of radioactivity equals exactly one nuclear decay per second?
Becquerel
✓
Curie
Gray
Roentgen
Correct Answer
Becquerel
The becquerel (Bq) measures activity as one decay per second. The curie is a much larger activity unit equal to 3.7 x 10^10 decays per second. The gray measures absorbed dose, not activity. The roentgen measures exposure through the ionization radiation produces in air.
Category: Radiation Effects
Question 18
Which unit measures absorbed dose as 1 joule of radiation energy per kilogram of tissue?
Curie
Gray
✓
Becquerel
Roentgen
Correct Answer
Gray
One gray (Gy) is the absorption of 1 J of radiation energy per kg of tissue. The curie measures activity, the number of decays per unit time. The becquerel also measures activity, one decay per second. The roentgen measures exposure in air rather than energy absorbed by tissue.
Category: Radiation Effects
Question 19
Which type of radiation is the most hazardous when its source is inhaled or swallowed, even though it is easily stopped outside the body?
Gamma rays
Beta particles
Alpha particles
✓
X-rays
Correct Answer
Alpha particles
Alpha particles are heavy and highly charged, so they deposit their energy in a very short distance and cause intense ionization in nearby tissue once inside the body. Gamma rays are more penetrating but ionize far less along their path. Beta particles ionize less than alpha particles. X-rays, like gamma rays, are photons with low ionizing power per distance traveled.
Category: Radiation Effects
Question 20
When an aluminum-27 nucleus (atomic number 13) absorbs a helium-4 nucleus, it emits one neutron. What is the other product?
Phosphorus-31
Silicon-30
Sulfur-31
Phosphorus-30
✓
Correct Answer
Phosphorus-30
Mass numbers must balance: 27 + 4 = 1 + A, so A = 30; atomic numbers must balance: 13 + 2 = 0 + Z, so Z = 15, which is phosphorus. Phosphorus-31 forgets to subtract the neutron's mass number. Silicon-30 has atomic number 14 and does not balance the charge. Sulfur-31 has atomic number 16 and the wrong mass number.
Category: Nuclear Equations