# Nuclear Physics Test

What is a *proton*?
A subatomic particle that is positively charged.
What is an *electron*?
A subatomic particle that is negatively charged.
What is a *neutron*?
A subatomic particle that is neutrally charged.
What is a *nucleus*?
The central, dense part of an atom that contains most of the mass.
Which subatomic particles make up the nucleus?
Protons and/or neutrons.
What is an *isotope*?
An atom that has the same number of protons but a different number of neutrons.
Which subatomic particle determines the *identity* of an element?
Protons.
Which subatomic particle determines the *isotope* of an element?
Neutrons.
Which subatomic particle determines the *ion* of an element?
Electrons.
What is the *atomic number* (Z)?
The number of protons in an atom.
What is the *mass number* (A)?
The number of protons and neutrons in an atom.

(Electrons don’t count because they have so little mass compared to protons and neutrons.)

What is an *atomic mass unit* (u)?
An easy-to-use unit for measuring very small masses, like those on an atomic scale.
Which subatomic particle has the *least mass*?
Electrons.

(A proton is about 1800X more massive than an electron.)

What is the mass of a *proton* in atomic mass units (amu)?
1.
What is the mass of a *neutron* in atomic mas units (amu)?
1.
What is the mass of an *electron* in atomic mass units (amu)?
0.

(This doesn’t mean that electrons have zero mass! Just that, compared to protons and neutrons, electrons have negligible mass.) How are the mass number and the atomic number represented in chemical symbol form?
Top = mass number (protons + neutrons)
Bottom = atomic number (protons) What is the symbol for a *proton*? What is an alternate symbol for a *proton*?
Because the (common) hydrogen atom has 1 proton. What is the symbol for a *neutron*? What is the symbol for a *electron*?
Given the mass number and the atomic number, how do you calculate the number of neutrons?
Find the difference.

# neutrons = mass number − atomic number

*Einstein’s famous formula*
E = mc²
What is the significance of *E = mc²*?
It says that mass and energy can be converted into one another.
How do protons stay together in the nucleus if like charges repel? Why don’t they fly apart?
The *strong nuclear force* holds the protons together. It’s stronger than the repulsive force that protons feel from being like charges.
Where does the *strong nuclear force* come from?
Nuclear binding energy.
Where does *nuclear binding energy* come from?
From the mass of protons and neutrons.

Protons and neutrons give up a little bit of their mass when they form a nucleus. This little bit of mass is converted into the binding energy that holds the nucleus together.

What happens to the nuclear binding energy when a nucleus is split?
The energy is released as heat.
How is binding energy related to the stability of a nucleus?
More stable nuclei have higher binding energies – it requires more energy to break apart stable nuclei.
An unstable nucleus that breaks down and emits particles and energy as a result.
How is the ratio of protons to neutrons related to the stability of a nucleus?
Ratios that are closer to 1:1 are more stable.

There needs to be at least as many neutrons as protons for a stable nucleus.

Why does the number of neutrons need to be greater than or equal to the number of protons in order to have a stable nucleus?
Because the strong nuclear force that attracts protons and neutrons to each other has a much shorter range than the repulsive force that the protons experience.

Protons are repelled by all of the other protons in the nucleus because they’re like charges. Compared to this repulsive force, the strong nuclear force has a very short range – it only attracts things that are next to each other. So as the number of protons increases, the number of neutrons needs to increase even more to make up for the short range of the strong nuclear force.

Why are elements with high atomic numbers (above 83) radioactive (spontaneously decay)?
As the number of protons increases, there is a tipping point at which the additional strong nuclear force that comes from adding more neutrons isn’t enough to balance the repulsive force that protons exert on each other. This is mostly because the strong nuclear force has such a short range.
What are the 3 main types of *radioactive decay*?
1. alpha decay
2. beta decay (and positron emission)
3. gamma decay

Sometimes positron emission is categorized as its own thing. What is an *alpha particle*?
A helium nucleus (2 protons and 2 neutrons). What is a *beta particle*?
A high-speed electron.
What is a *gamma ray*?
A very high-energy electromagnetic wave that has no charge and no mass. What is a *positron*?
Like an electron, except it has a positive charge.

(It’s NOT a proton because it has negligible mass.))

If a nucleus has only protons and neutrons, then where does the electron come from during beta decay?
From a neutron. A neutron is a proton and an electron bound together.
Which kind of decay has the *least* penetrating power?
Alpha decay.
Which kind of decay has the *most* penetrating power?
Gamma decay.
What is a *daughter element*?
What an element turns into after undergoing radioactive decay. How do you figure out what the missing piece is in a nuclear reaction?
Make sure the tops (mass numbers) and the bottoms (atomic numbers) on the left equal those on the right.

The coefficients in the reaction matter.

What is *half-life*?
The time it takes for half of the atoms in a radioactive sample to decay. What does a graph of radioactive decay look like (amount vs time)?
Exponential decay curve.
In a half-life problem, what do you almost always need to do?
Figure out how many half-lives passed (how many times the substance halved itself). What is *fusion*?
A reaction in which nuclei combine and a lot of energy is released. What is *fission*?
A reaction in which a nucleus splits and releases neutrons and a lot of energy.
Which one is a “clean” reaction: fission or fusion?
Fusion because no harmful radioactive substances are produced. What is the general process in an artificial fission reaction?
A neutron is shot at a large atom, which absorbs the neutron, which makes it unstable, and so it splits into smaller atoms, releasing energy and more neutrons, which go on to split more atoms. It’s a chain reaction.
Nuclear power plants conduct which kind of reaction: fission or fusion?
Fission.
Where does *fusion* commonly take place?
In stars (like the sun).
Which reaction generally releases more energy: fission or fusion?
Fusion.
How would protons, neutrons, and electrons be arranged from *least to most massive*?
Electrons, protons, and neutrons.
Who discovered the *neutron*?