Chemistry Unit 8 Lesson 2

Types of Radiation

-Alpha emission: a helium nucleus (2 protons, 2 neutrons), often released from a large atom -Beta emission: a high-speed electron or positron (antielectron) from the nucleus (not from the orbitals) -Gamma emission: electromagnetic wave

Changes in the nucleus give off radiation that leaves the atom.Changes in the nucleus give off radiation that leaves the atom.

-In many nuclei, an imbalance exists between the forces holding the nucleus together and those that would tear it apart. These nuclei are unstable and undergo a spontaneous change where particles and energy are lost. This is called radioactivity. -types of radioactivity, nuclear changes that accompany radioactive emissions, and radioactive half-life.

Radioactive decay changes the nucleus and forms new isotopes.

-Large unstable nuclei (atomic number, Z > 83) tend to decay by alpha emission. The atomic number decreases by 2 and atomic mass decreases by 4 -In high N/Z nuclei, neutrons decay into a proton with the release of a beta particle (beta emissions). The atomic number increases by 1, but mass remains the same -In nuclei with many protons (low N/Z), protons can decay into neutrons and positrons (these are also called antielectrons). The atomic number decreases by 1, but mass remains the same -Alternatively, low N/Z nuclei can capture an electron from the electron cloud (electron capture)

Unstable nuclei decay by giving off radiation.

-Nuclei are unstable when there are a relatively high number of protons per neutron. Unstable nuclei decay to stable ones by giving off radiation (particles, energy, and/or heat). --alpha emission - nucleus --beta emission - high-speed electron or positron --gamma emission - electromagnetic waves -Radioactive emissions can change the composition of the nucleus. The time of radioactive decay is measured in half-lives. Heavy isotopes change to stable ones in a series of radioactive decays.

A balance of forces holds the nucleus together.

-The nucleus is made of protons (+1 charge) and neutrons (0 charge), both of which are made of fundamental particles called quarks. -Quarks attract one another to hold protons and neutrons together via the strong nuclear force. -Quarks from one proton also attract quarks from another nucleon with a residual strong force. The strong nuclear force—or, simply, the strong force—acts over short distances to hold the nucleus together. -In contrast, an electromagnetic force tends to repel the protons and tends to make the nucleus fly apart. -Neutrons help stabilize the nucleus. In stable atoms, these opposing forces are balanced. -However, when there are too many protons relative to the number of neutrons, these forces are out of balance, and the nucleus is unstable.

Unstable nuclei decay to become stable.

-Within this region is a line of stable nuclei, a band of stability. -Elements above the band of stability have too many neutrons, decreasing the strong force and destabilizing the nucleus. -Elements below this band of stability have too many protons, increasing the repulsive force and destabilizing the nucleus. -All nuclei with an atomic number above 83 (Z > 83) are unstable, some more so than others.

What is radioactivity?

-is the decay of an unstable atomic nucleus, followed by the release of radiation. The radiation can take the form of a particle or energy.

These types of radiation have different amounts of energy and are capable of penetrating materials in different ways.

For example, alpha particles can be stopped by paper or skin; beta particles, by aluminum foil or Plexiglas; and gamma emissions, by concrete or lead.

You can calculate the amount of material present if you know the half-life.

If you are given the half-life for the decay of a particular radioisotope, then you can calculate how much of that radioisotope remains after several half-lives: fraction remaining (FR) = 1/2n where n = the number of half-lives.

The time it takes for a specific amount of radioactive material to decay is measured in a unit called half-life

The half-life (t1/2) of a radioisotope is the amount of time required for one-half of the radioactive nuclei of the parent substance to decay into the daughter substance(s). So if you start with 100% parent substance, you will have the following over time: After one t1/2 - 50% parent remaining After two t1/2 - 25% parent remaining After three t1/2 - 12.5% parent remaining After four t1/2 - 6.25% parent remaining The half-life varies with different radioisotopes.

So what makes a nucleus stable or unstable?

The stability or instability of a nucleus is related to the structure of the nucleus and the balance of forces that hold it together versus those that tend to make it fly apart.

An unstable nucleus will spontaneously change—scientists use the word decay—to become a more stable nucleus. It does so by emitting a particle or particles and/or energy, which are collectively called radiation.

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How long does it take for a sample of an isotope to decay?

When a radioisotope (a single atom) decays, the original isotope, called the parent isotope, changes into another isotope called the daughter isotope. In a single atom, this change takes place almost instantaneously. However, scientists are often interested in how long it takes for a sample of the isotope to decay.

Radioactively decaying atoms release particles or energy.

When an unstable nucleus of an atom decays, it releases energy in the form of moving particles and/or electromagnetic energy. This radiation is of three types

Unstable nuclei may undergo a series of decays before becoming stable.

You may be under the impression that all unstable nuclei or radioisotopes become stable after one instance of decay. This may be true for isotopes of low-mass or low-atomic-number elements (Z < 20), but it is not usually true for high-mass or high-atomic-number isotopes. Often these radioisotopes undergo a series of decays over time. Each step usually has a different half-life.