Radioactivity Nuclear Chemistry

Radiation Radiation: The process of emitting energy in the form of waves or particles. Where does radiation come from? Radiation is generally produced when particles interact or decay. A large contribution of the radiation on earth is from the sun (solar) or from radioactive isotopes of the elements (terrestrial). Radiation is going through you at this very moment!

Background Radiation 13% are man-made Radon gas Food Cosmic rays Gamma rays Medical Nuclear power

Review Atomic Terms Nucleons – particles found in the nucleus of an atom ex. Neutrons and Protons Nuclear Forces – strong nuclear force holds neutrons and protons together to form a nucleus Isotopes – atoms with identical atomic numbers but different mass numbers

Writing Isotopes Mass number = number of protons + number of neutrons A X Element symbol Z Atomic number = number of protons Number of neutrons = Mass Number – Atomic Number

Writing Isotopes An isotope is an atom with a different number of neutrons: Notice that the mass number is different. How many neutrons does each isotope have? O 8 16 O 8 17 O 8 18 Each isotope has 8 protons – if it didn’t then it just wouldn’t be oxygen any more.

Most of the isotopes which occur naturally are stable. A few naturally occurring isotopes and all of the man-made isotopes are unstable. Radioactive Decay– the spontaneous decomposition of a nucleus Unstable isotopes can become stable by releasing different types of particles. Radioactive - a substance that is capable of ALWAYS emitting radiation under any conditions Radioisotope - simply an isotope that is radioactive or that undergoes radioactive decay.

Radioactivity Alpha particles (a) Beta particles (b) Gamma-rays (g) Three types of radiation (or radioactivity): Alpha particles (a) Beta particles (b) Gamma-rays (g) Nothing will actually stop gamma but there are some materials that can greatly slow it down

Alpha Decay (a) 88 protons 138 neutrons 86 protons 136 neutrons Unstable nucleus New nucleus Alpha particle Alpha Decay (a) Mass # Radium 226Ra Radon Rn222 + p n n p a (4He) 2 88 protons 138 neutrons 86 protons 136 neutrons 2 protons 2 neutrons Note: The alpha particle is really a cation (+2) but shown with no charge cause it very rapidly picks up two electrons and becomes a neutral He atom instead of an ion. Isotopes with a high number of protons tend to do this like #83 and up The alpha-particle (a) is a Helium nucleus. It’s the same as the element Helium, with the electrons stripped off !

Alpha Decay Example 1 Example 2 These are examples of Radioactive Equations Result: 2 less protons and 2 less neutrons loss of 4 in mass number and loss of 2 in atomic number.

Rn + Y He Rn He + Po Alpha Decay, Practice #1 222 86 A Z 4 2 222 86 4 218 84

X + Th He He U + Th Alpha Decay, Practice #2 A Z 230 90 4 2 4 2 234 92

Th + Y He He + Ra Th Alpha Decay, Practice #3 230 90 A Z 4 2 4 2 226 88 Th 230 90

X + Pb He He + Pb Po Alpha Decay, Practice #4 A Z 214 82 4 2 4 2 214 218 84

Beta Decay (b) Carbon C14 Nitrogen N14 + e- 6 protons electron Unstable nucleus New nucleus Beta particle Beta Decay (b) Carbon C14 Nitrogen N14 + e- 6 protons 8 neutrons Note that in beta decay, the atomic mass does not change, since the neutron and proton have nearly the same mass… 7 protons 7 neutrons electron (beta-particle)

Beta Decay A beta particle is a fast moving electron which is emitted from the nucleus of an atom undergoing radioactive decay. Beta decay occurs when a neutron changes into a proton and an electron. Atoms with a high neutron to proton ratio tend to do this to level out the N to P ratio (Like Iodine) Result: the nucleus has one less neutron, but one extra proton.

Beta Decay, Practice #1 Th 234 90 Y A Z + b -1 Th 234 90 Pa 91 + b -1

Beta Decay, Practice #2 X A Z Pb 210 82 + b -1 Tl 210 81 Pb 82 + b -1

Beta Decay, Practice #3 Bi 210 83 Y A Z + b -1 Bi 210 83 Po 84 + b -1

Beta Decay, Practice #4 X A Z Bi 214 83 + b -1 Pb 214 82 Bi 83 + b -1

Unstable nucleus New nucleus Gamma radiation Gamma particles (g) In much the same way that electrons in atoms can be in an excited state, so can a nucleus. Neon Ne20 Neon Ne20 + gamma 10 protons 10 neutrons (in excited state) 10 protons 10 neutrons (lowest energy state) A gamma is a high energy light particle. After  or  decay, surplus energy is sometimes emitted. This is called gamma radiation and has a very high frequency with short wavelength. The atom is not changed.

Gamma Decay Gamma rays are not charged particles like a and b particles. Gamma rays are electromagnetic radiation with high frequency. When atoms decay by emitting a or b particles to form a new atom, the nuclei of the new atom formed may still have too much energy to be completely stable. This excess energy is emitted as gamma rays (gamma ray photons have energies of ~ 1 x 10-12 J).