Energy Unit Learning Goal 3: Examine how changes in the nucleus of an atom result in emissions of radioactivity.

REVIEW: Nucleus  Very Small The typical nucleus is about cm.   Very Large Density The typical density is about 1.6 x g/cm 3.  160,000,000,000,000  Large amounts of Energy The typical nuclear process produces millions of times more energy than those associated with normal chemical processes.

Nuclear Symbols Element symbol Mass number (p + + n o ) Atomic number (number of p + )

Radioactive Decay  Radioactivity when a nucleus spontaneously decomposes, forming a different nucleus and producing one or more particles.  Nuclear Equation Both the atomic number and the mass number must be conserved.  That is the sums of both on each side must be equal.

When an unstable nucleus decays, there are three particles it may produce: an alpha particle (symbol ) a beta particle (symbol ) a gamma ray (symbol )

Types of Radioactive Decay  Alpha-particle Production Alpha particle is a helium nucleus  Atomic number of helium is 2; mass number is 4.

Alpha particles are made of 2 protons with 2 neutrons. This means that when a nucleus emits an alpha particle, it loses 2 protons and so its atomic number decreases by 2. Also, when a nucleus emits an alpha particle, its atomic mass decreases by 4 (that's 2 protons plus 2 neutrons)

Alpha Radiation Limited to VERY large nuclei.

Types of Radioactive Decay  Beta-particle production A beta-particle is an electron.  The net effect of beta-particle production is to change a neutron to a proton. No change in mass number, but an increase in one in atomic number.

A Beta particle is the same as an electron. It has a charge of -1, and a mass of around 1/2000th of a proton. But wait a minute! If a nucleus contains protons and neutrons, what's an electron doing coming out of a nucleus? To answer this, we need to know more about protons and neutrons: Protons & neutrons are made of combinations of even smaller particles, called "quarks". Under certain conditions, a neutron can decay, to produce a proton plus an electron. The proton stays in the nucleus, whilst the electron flies off at high speed.

Beta Radiation Converts a neutron into a proton.

Types of Radioactive Decay  Gamma Ray A high-energy photon of light. A nuclide in an excited nuclear energy state can release excess energy by producing gamma rays. Gamma rays are photons of light so they have zero charge and zero mass number.

Gamma rays Gamma rays are waves, not particles. This means that they have no mass and no charge. Gamma rays have a high penetrating power - it takes a thick sheet of metal such as lead, or concrete to reduce them significantly. We don't find pure gamma sources - gamma rays are emitted alongside alpha or beta particles. Strictly speaking, gamma emission isn't 'radioactive decay' because it doesn't change the state of the nucleus, it just carries away some energy.

Types of Radioactive Decay  Positron Production The positron is a particle with the same mass as an electron but opposite charge. Does not occur often enough to make it practical to change mercury to gold. Results in no change of mass, but a decrease of 1 in atomic number.

Types of Radioactive Decay  Electron Capture A process in which one of the inner-orbital electrons is captured by the nucleus. Atomic number decreases by one. Mass remains the same.

Types of Radiation

Alpha particles are easy to stop, gamma rays are hard to stop.

Decay Series  Often radioactive nucleus cannot achieve stable state through a single decay process.  A decay series will occur until a stable nuclide is formed.

A Decay Series A radioactive nucleus reaches a stable state by a series of steps

Nuclear Fission   The process of splitting an atom.  Releases an enormous amount of energy. 1 mol of uranium-238 can produce about 26 million times as much energy as 1 mol of methane.

Nuclear Fusion  The process of combining two light nuclei. Produces more energy than fission.