Nuclear Chem

Question 1

Define:

nuclear force

Answer 1

The nuclear force holds protons and neutrons together in the nucleus.

In very large molecules, or ones with an over-abundance of neutrons, the nuclei can become unstable and decay.

Question 2

Define:

alpha particle and alpha decay

Answer 2

An alpha particle is a helium nucleus, consisting of 2 protons and 2 neutrons (without the electrons).

Alpha decay is a nuclear decay, and occurs when a nucleus emits an alpha particle. The daughter nucleus of alpha decay will have an atomic number 2 less, and mass number 4 less than the parent nucleus.

Question 3

What will the daughter atom be when uranium 238 undergoes a single alpha decay?

Answer 3

²³⁴₉₀Th

In single alpha decay, an alpha particle is emitted. To identify the daughter nucleus, therefore, simply subtract 2 from the parent nucleus’ atomic number, and 4 from its mass.

Question 4

Under what conditions would alpha decay be an isotope’s preferred form of radioactive decay?

Answer 4

Alpha decay is typical only in large nuclei, atomic number 60 or greater.

Many of the most famous radioactive elements, such as radium, uranium, and plutonium, decay primarily via alpha decay.

Question 5

Define:

beta particle and beta decay

Answer 5

A beta particle is simply an electron, a massless negatively-charged particle.

Beta decay is a nuclear decay, and occurs when a nucleus emits a beta particle. The daughter nucleus of beta decay will have an atomic number 1 more, and mass number identical to the parent nucleus.

Question 6

What will the daughter atom be when carbon 14 undergoes a single beta decay?

Answer 6

¹⁴₇N

In single beta decay, a beta particle (electron) is emitted. To identify the daughter nucleus, therefore, simply add 1 to the parent nucleus’ atomic number, without changing its mass. The element with an atomic number one larger than carbon is nitrogen.

Question 7

Under what conditions would beta decay be an isotope’s preferred form of radioactive decay?

Answer 7

Beta decay is typical in smaller nuclei, when the isotope’s mass number is greater than the element’s atomic weight.

Ex: The carbon 16 isotope, which is heavier than carbon’s atomic weight of 12.011, decays via beta decay.

Question 8

Define:

positron and positron decay

Answer 8

A positron is an anti-electron, a massless positively-charged particle.

Positron decay is a nuclear decay, and occurs when a nucleus emits a positron. The daughter nucleus of positron decay will have an atomic number 1 less, and mass number identical to the parent nucleus.

Question 9

What will the daughter atom be when carbon 11 undergoes positron decay?

Answer 9

¹¹₅B

In positron decay, a positron is emitted. To identify the daughter nucleus, therefore, simply subtract 1 from the parent nucleus’ atomic number, without changing its mass. The element with an atomic number one less than carbon is boron.

Question 10

Define:

electron capture

Answer 10

Electron capture is a nuclear decay, and occurs when a nucleus captures an electron, which merges with a proton to form a neutron. The daughter nucleus of electron capture will have an atomic number 1 less, and mass number identical to the parent nucleus.

Question 11

What will the daughter atom be when beryllium 7 decays via electron capture?

Answer 11

⁷₃Li

In electron capture, an electron merges with a proton, forming a neutron. To identify the daughter nucleus, therefore, simply subtract 1 from the parent nucleus’ atomic number, without changing its mass. The element with atomic number one less than beryllium is lithium.

Question 12

Under what conditions would positron emission or electron capture be an isotope’s preferred form of radioactive decay?

Answer 12

Positron emission and electron capture are typical in smaller nuclei, when the isotope’s mass number is less than the element’s atomic weight.

Ex: The carbon 11 isotope, which is lighter than carbon’s atomic weight of 12.011, decays via positron emission.

It is difficult to predict whether positron emission or electron capture will be preferred under these conditions.

Question 13

Define:

**gamma ray **and gamma emission

Answer 13

A gamma ray is a high energy photon, a massless and uncharged particle.

Gamma emission is a nuclear decay, and occurs when a nucleus emits a gamma ray. The daughter nucleus of gamma emission will have identical atomic and mass numbers to the parent nucleus, it is simply lower in energy.

Question 14

Define:

half-life of a radioactive isotope

Answer 14

A radioactive isotope’s half-life is the time it takes for exactly one-half of the individual atoms in a population of the isotope to undergo radioactive decay.

After one half-life elapses, exactly one-half of the original amount of the parent nuclei will still be present; the remainder will have decayed into the daughter nuclei.

Question 15

If radioactive isotope X has a half-life of 1 hour, and 3 hours elapse, what proportion of the original population of X will remain?

Answer 15

One eighth of the original population will remain after 3 hours.

For each half-life that elapses, one half of the remaining isotopes will decay. So after one hour, 1/2 the original population remains. During the second hour, 1/2 of the remaining half (1/4 more) decays, leaving behind one quarter of the original population. In the third hour, 1/2 of the remaining quarter (1/8 more) decays, leaving behind one eighth of the original population.

Question 16

If radioactive isotope Z has a half-life of 3 days, how long will it take a 120 g sample of Z to decay until only 30 g of Z remain?

Answer 16

6 days

For each half-life that elapses, one half of the remaining isotopes will decay. After 3 days, 60 g of Z will decay, and 60 g will remain. After 3 more days, half of the 60 g will decay, leaving behind 30 g.

Question 17

Define:

mass defect of an atom

Answer 17

An atom’s mass defect is the difference between the mass of the individual protons and neutrons in the atom’s nucleus and the nucleus’s overall mass.

The mass defect exists because some of the neutrons’ and protons’ mass is converted to energy, which is used to hold the nucleus together, so the nucleus weighs less than the protons and neutrons that make it up.

Question 18

Define:

**binding energy **of a nucleus

Answer 18

A nucleus’ binding energy is the energy needed to cause the nucleus to decompose into protons and neutrons.

The binding energy is exactly the same magnitude as the mass defect; the energy released when a nucleus is formed is exactly the same as the energy needed to break it apart.

Question 19

What is the equation relating a nucleus’ mass defect and its binding energy?

Answer 19

E = mc²

where:

E = the nucleus’ binding energy
m = the nucleus’ mass defect
c = the speed of light, 3x10⁸ m/s

Since the value of c² is so large, a very small mass defect accounts for a very large binding energy.