
Nuclei exist because protons and neutrons have lower energy when bound together. The binding energy is stored in the nucleus and can be released if the nucleus changes, such as through a nuclear reaction. Nuclear reactions can be represented by nuclear reaction equations. Nuclear reactions may split nuclei (fission) or combine nuclei (fusion). Both types of reactions can release energy. Commercial nuclear reactors generate energy by nuclear fission whereas the Sun generates energy with nuclear fusion reactions.

nuclear reaction, fission, chain reaction, fusion


$$\begin{array}{ccc}\underset{\text{Reactants}}{\underbrace{{}_{0}{}^{1}n+{}_{7}{}^{14}N}}& \to & \underset{\text{Products}}{\underbrace{{}_{6}{}^{14}C+{}_{1}{}^{1}p}}\end{array}$$


Review problems and questions 


How is mass–energy equivalence related to nuclear fission and fusion?

When a nucleus undergoes a nuclear fission or fusion reaction, energy is released. The energy comes from the change in the rest mass of the nuclei in the reaction. In other words, the rest mass of a nucleus (and the neutron that starts the reaction) before a fission reaction is greater than the sum of the rest masses of the two nuclei (and the two or three resultant neutrons) after the reaction. The loss in rest mass is converted into energy through mass–energy equivalence.


Distinguish between atomic physics and nuclear physics.

Atomic physics is the study of physics at the atomic level, which can include the properties of neutral atoms, such as bonding, covalence, conduction, etc. Nuclear physics studies properties of the nucleus itself, which has high potential energies, and is related to radioactivity, fission, and fusion.


Distinguish between chemical reactions and nuclear reactions.

Chemical reactions combine atoms to form chemical compounds by sharing and exchanging electrons. Nuclear reactions change the nucleus, which means that the result is a new element or a new isotope of an element.


Balance the following reaction equation by providing the missing product:
$${}_{1}{}^{2}\text{H}+{}_{2}{}^{3}\text{H}\text{e}\to {}_{Z}{}^{A}\text{X}+{}_{1}{}^{1}\text{H}$$

Answer: ${}_{2}{}^{4}\text{H}\text{e}$—an alpha particle
First we balance the the charge, which will give us the unknown atomic number Z:1 + 2 = Z + 1 ⇒ Z = 2Therefore the unknown element X is helium (He). Now we can find A, which will give us the specific isotope of helium by balancing mass:2 + 3 = A + 1 ⇒ A = 4The unknown element ${}_{Z}{}^{A}\text{X}$ is ${}_{2}{}^{4}\text{H}\text{e}$.


For the reaction
${}_{Z}{}^{A}\text{X}\to {}_{10}{}^{22}\text{N}\text{e}+{}_{1}{}^{0}\text{e}$
what is the unknown ${}_{Z}{}^{A}\text{X}$?
 ${}_{10}{}^{21}\text{N}\text{e}$
 ${}_{11}{}^{22}\text{N}\text{a}$
 ${}_{10}{}^{23}\text{N}\text{e}$
 ${}_{12}{}^{23}\text{M}\text{g}$

The answer is b. To find the answer, first equate the atomic numbers on both sides of the reaction to find that Z = 11, which corresponds to sodium. Then equate the mass on both sides of the reaction, which gives A = 22.


Determine the name of the unknown element, the atomic number Z, and the atomic mass number A in this nuclear reaction equation:
$${}_{0}{}^{1}\text{n}+{}_{92}{}^{235}\text{U}\to {}_{56}{}^{141}\text{B}\text{a}+{}_{Z}{}^{A}\text{?}\text{}+3{}_{0}{}^{1}\text{n}$$


Answer: krypton, Z = 36, A = 92 (${}_{36}{}^{92}\text{K}\text{r}$)
The number of protons must be the same on both sides of the reaction equation, which means that the sum of the atomic numbers must be equal on both sides: $$\begin{array}{ccc}0+92=56+Z+0& \Rightarrow & Z=9256=36\end{array}$$Atomic number of 36 is the element krypton (Kr).
The total number of nucleons (protons plus neutrons) must also be equal on both sides of the reaction equation: $$\begin{array}{ccc}1+235=141+A+3\times 1& \Rightarrow & A=1+2351413=92\end{array}$$The missing element is therefore krypton92, which is represented as ${}_{36}{}^{92}\text{K}\text{r}$.


In this nuclear reaction equation:
$${}_{0}{}^{1}\text{n}+{}_{92}{}^{235}\text{U}\to {}_{Z}{}^{A}?+{}_{35}{}^{87}\text{B}\text{r}+3{}_{0}{}^{1}\text{n}$$
 determine the atomic mass number A, atomic number Z, and the name of the unknown element.

Answer: lanthanum, Z = 57, A = 146 (${}_{57}{}^{146}\text{L}\text{a}$)
The number of protons must be the same on both sides of the reaction equation, which means that the sum of the atomic numbers must be equal on both sides: $$\begin{array}{ccc}0+92=Z+35+3\times 0& \Rightarrow & Z=9235=57\end{array}$$Atomic number of 57 is the element lanthanum (La).
The total number of nucleons (protons plus neutrons) must also be equal on both sides of the reaction equation: $$\begin{array}{ccc}1+235=A+87+3\times 1& \Rightarrow & A=1+235873=\end{array}146$$The missing element is therefore lanthanum146, which is represented as ${}_{57}{}^{146}\text{L}\text{a}$.

 Smoke detectors contain radioactive isotope americium235 and emit alpha particles, yet manufacturers claim that the products are safe. Evaluate the validity of this claim by researching natural exposure levels, medicallyrecommended limits, or exposure from medical imaging.

Answers will vary but the claim is reasonable. Smoke detectors produce lowenergy alpha particles, which can be blocked by a single sheet of paper. Exposure from smoke detectors is roughly 0.01 mrem per year, while the average annual exposure due to natural sources of radiation is 300 millirems. Health experts generally agree that a person's exposure to radiation above these natural background levels should be limited to about 100 mrem per year. Comparison with these guidelines confirms that radiation from smoke detectors is within safety limits.

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