Nuclear Chemistry

Other than radioactive decay, how can an animal or plant lower its natural percentage of carbon-14?

It can't. Carbon-14 levels in a living organism stay stable over its lifetime as long it is alive exchanging carbon with the environment (eating, breathing, pooping, whatever). After death, an animal or plant stops taking up and giving off carbon. After this the carbon-14 levels decay at a consistent known rate. We use this rate, expressed in half-lives, to calculate the age of a long-dead animal or plant.

A beta particle is emitted from the nucleus. Where within the nucleus does it come from?

A neutron breaks down into a proton and an electron. The proton stays put while the electron is kicked out of the nucleus as a beta particle.

How much more mass does a proton contain compared to an electron?

A proton is a little over 1800 times as massive as an electron. Here is an example I came up with that might be useful: This is roughly the difference in mass between an adult human and 53 full-grown Asian elephants.

What happens to the atomic number of an element when it undergoes beta decay? What does this do to the element? Why does this happen?

*The atomic number goes up by one, which by definition means that it gains a proton and becomes a different element with a different atomic number. *A neutron in the nucleus decays into a proton and an electron, the latter of which is kicked out as a beta particle.

What is the answer to Life, the Universe, and Everything?

42

The half-life of carbon-14 is 5730 years. Suppose a wooden cup is found in the Egyptian desert. The wood the cup is made of has 50% of the amount of carbon-14 that would be found in a living tree. How long ago was it that the tree this cup is made of was alive and exchanging carbon with the environment?

5730 years x 1 half-life (time to decay to 50% of its original value) ~5730 years

The half-life of carbon-14 is 5730 years. Suppose a paleontologist finds a frozen wooly mammoth that has 25% of the carbon-14 that would be found in a living animal. How long ago was it that the mammoth was alive, exchanging carbon-14 with the environment?

5730 years x 2 half-lives (time to decay to 25% of its original value) = roughly 17190 years

The half-life of tritium is 12 years. The level of radioactivity of a sample is often measured in disintegrations per minute (dpm). Suppose I accidentally spilled 800 dpm of tritium from a sample I'm working with onto the lab bench. How long before it goes down to a safer, near-background level of 50 dpm?

800 dpm/2 = 400 dpm (1 half life) 400 dpm/2 = 200 dpm (2 half-lives) 200 dpm/2 = 100 dpm (3 half-lives) 100 dpm/2 = 50 dpm (4 half-lives) 4 half-lives x 12 years/half life = 48 years! (This is one of the many reasons scientists have to be very careful with radioactive samples.)

atomic mass

All elements have several isotopes that exist in nature. Atomic mass is the weighted average of all the isotopes of an element.(Be careful not to get this mixed up with mass number, which is the total number of protons and neutrons in a given atom.)

Suppose a radioisotope is unstable due to an abundance of neutrons compared to protons. How does the atom naturally decrease the number of neutrons?

Beta decay. Beta decay consists of a neutron breaking down into a proton and a beta particle, so beta decay decreases the number of neutrons and increases the number of protons.

Every living organism on Earth contains a small, consistent, predictable fraction of which radioactive isotope? This isotope is used to estimate the age of living things years after they die. What is this dating technique called?

Carbon-14, which has a half-life of 5730 years. The technique of dating once-living objects with carbon-14 is called (surprise) carbon dating. (BTW, You don't have to remember the lengths of any half-lives. They will be given to you for any math problems on the test.)

Which of the five senses can detect radioactivity?

None of them. Although the slang term for a highly radioactive area or sample is "hot," radioactivity cannot be felt, seen, or otherwise sensed without the use of radiation-detecting instruments.

What happens to the mass number of an element when it undergoes beta decay? Why?

Nothing. The mass number is the number of protons and neutrons. Since a neutron decays into a proton during beta decay, the decayed neutron is replaced by a proton. Therefore the mass number doesn't change because the total number of neutrons and protons stays the same. (If this isn't clear to you at first, see the examples in the following couple of cards.)

Uranium can be dug out of the ground and is the only thing that limits bad guys from making an atomic bomb. Why doesn't every terrorist and/or every Curly, Moe, and Larry on Earth have an atomic bomb?

Only uranium-235 can be used to make bombs. Nearly all uranium on Earth is U-238 instead of U-235. Uranium-235 is only 0.7% of all uranium, and is chemically identical to U-238. It is very hard to separate. (You will have to know which isotope is which, but not the exact 0.7% figure.)

How does radiation kill cells?

Radiation forms ions in cells, which knock electrons off atoms. This weakens large molecules such as DNA, RNA, and proteins and causes them to break or become tangled and nonfunctional.

What type of cells does radiation preferentially kill?

Radiation preferentially kills rapidly-dividing cells, since their DNA constantly doubling and RNA and protein levels are high, providing more targets for radiation damage. This is why radiation sometimes used to treat cancer. Normal cells are also damaged, but cancer cells are damaged more.

What subatomic particle sustains the nuclear chain reaction in nuclear reactors and atomic bombs?

The neutron

Radioactive half-life (Note: this and the next two cards contain important "big picture" concepts. You'll want to make sure you understand these concepts in order to solve some of the math problems.)

The radioactive half-life for a given radioisotope is the time for half the radioactive nuclei in any sample to undergo radioactive decay. For example, if a radioisotope has a half-life of 5 years, then 50% of it will be gone in 5 years, 50% of the remaining isotope will be gone 5 years after that (leaving a total of 25%), 12.5% will be left 5 years after that, 6.25% 5 years after that, etc. For time periods that do not conveniently fall exactly on a half-life, an equation involving logarithms is used, but that will not be covered in this unit.

Geiger counter

a handheld device used to detect radioactivity. Almost always used for safety purposes instead of measuring radioactivity in samples.

alpha particle

a helium nucleus that is emitted from a radioactive atom. Alpha particles are a form of radiation called (you guessed it) alpha radiation.

gamma particle

a high-energy photon (particle of light) that is emitted from a radioactive nucleus. Gamma particles have zero mass and so do not affect the math in the word problems in this unit. Since light can have either wavelike or particle-like properties, you will often see this written as "gamma ray" rather than "gamma particle."

transmutation

a radioactive nucleus breaks down by emitting alpha or beta particles. The loss of these particles changes the number of protons, and therefore converts it into another element. It can also occur artificially when an atom is split or is bombarded with smaller particles.

Arrange alpha, beta, and gamma particles in order of increasing ability to penetrate matter.

alpha < beta< gamma. (Alpha can't go through paper. Beta can go through paper but not plywood. Gamma can go through moderately-thick layers of lead, but is stopped by thick layers of lead.)

atomic mass unit

also called an amu. Equal to 1/12 the mass of a carbon-12 nucleus. This is approximately equal to the mass of a proton or a neutron, but since protons and neutrons have slightly different mass, an amu is defined as this particular fraction.

stable isotope

an "ordinary" element: an isotope that does not break down and does not emit particles or rays. Stable isotopes are the eventual breakdown products of radioisotopes.

beta particle

an electron that is emitted from a radioactive atom. Beta particles are a form of radiation called beta radiation.

radioisotope

an isotope that emits alpha particles, beta particles, and/or gamma rays.

isotope

atoms that have the same number of protons (and are therefore the same element) but different numbers of neutrons. Isotopes are chemically identical, but some may be radioactive while others are not.

fission

breaking a large nucleus into smaller nuclei. This is what releases the energy in nuclear reactors and atomic bombs.

fusion

squashing smaller nuclei together to form larger nuclei. This releases even more energy than nuclear fission.

strong nuclear force

the strongest of the four fundamental forces in nature. It only acts at very, very close range (on the size scale of an atomic nucleus) and holds the nucleus together. The attraction of the strong nuclear force is what keeps the nucleus from flying apart. Otherwise, the positive charges of the protons would repel each other when crammed so close together in the nucleus.

nuclear radiation

the emission of alpha particles, beta particles, or gamma rays from the nucleus of an atom

mass number

the number of protons and neutrons in the nucleus of an atom

atomic number

the number of protons in the nucleus of an atom. The number of protons is unique to each element and defines that element. Example: there is one and only one element in the whole universe with 6 protons, and that atom is carbon.