Radioactivity

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Structure of an Atom Neutron: uncharged particle in the nucleus of an atom

Proton: roughly same mass as neutron; positively charged

Electron: almost no mass; negatively charged

Nuclei Notation

-chemical symbol (e.g. C for Carbon)

-atomic mass is placed on the upper left of the chemical symbol (e.g. 12 for a Carbon atom with 6 protons and 6 neutrons)

-atomic number is placed below the atomic mass (e.g. 6 for a Carbon atom because it has 6 protons)

Atomic mass: number of protons + number of neutrons

Atomic number: number of protons

Isotope: an element with the same proton and electron configurations but with a different number of neutrons

Ionising Radiation Alpha particles

-helium nuclei ejected from unstable nuclei

-heavily ionising

-short range

-stopped by a thin card

Beta particles

-fast moving electrons ejected from unstable nuclei

-less ionising

-travel long distances in air

-stopped by 1-2mm of aluminium

Gamma rays

-photons of high energy electromagnetic waves

-heavily penetrating

-interact with atoms which may then emit ionising radiation

-stopped by tens of cm of lead

Effects on a Nucleus of Radiation Emissions Alpha: deducts 2 protons from a nucleus; deducts 4 from the atomic mass

Beta: neutron splits into a proton and an electron; electron is removed from atom while protons stays; adds 1 proton

Gamma rays: no specific effect

Balancing Nuclear Equations Decay equations must balance.

Sum of atomic masses before decay = sum of atomic masses after decay

Sum of atomic numbers before decay = sum of atomic numbers after decay

Detecting Ionising Radiation Photography film

-fogs up when exposed to ionising radiation

-used in badges worn by workers at continuous risk from exposure

The Geiger–Müller tube

-beeps in the presence of an ionising radiation

-more exposure increases number of beeps per unit time

Sources of Background Radiation From Earth's rocks

-slow decay of isotopes of uranium produce radon and thoron gases

-radon is highly radioactive

Cosmic rays

-explosion of stars create a violent reaction

-reaction produces cosmic rays which shower the Earth

Medical

-some diagnosis/treatment of illnesses use radioactive materials which contribute to background radiation

Nuclear power and weapons

-testing nuclear weapons contributes to a small amount of background radiation

-leaks from nuclear power stations contribute to a small amount of background radiation

Activity of Radioactive Source -radioactive decay is a random process

-cannot predict when an unstable atom will decay or which atoms will decay at any given moment

-can predict the percentage of unstable atoms in a sample that will decay at any given time

-rate of decay is measured in becquerels (1 Bq = 1 decay per second)

-radiation emitted by a radioactive source will decrease over time

-the smaller the number of remaining undecayed nuclei, the slower the process of decay

Half-life Half-life: time taken for half the original number of unstable nuclei to decay

Applications of Radioactivity Medical tracers: radioactive source is put into body, and will build up at a blockage to detect the area of problem.

Sterilisation: kills microorganisms and bacteria on surgical equipment.

Non-medical tracers: radioactive tracers/detectors/gamma cameras to map the flow of liquids and gases through industrial processes (e.g. piping network).

Radioactive dating: aka carbon dating; measuring amount of radiation in an object to figure out its half-life, then deducing its age; used by archaeologists.

Hazards of Radioactivity -damages structure of a cell's DNA; causes mutations; replications of cells are passed on to turn into cancer

-radiation changes atoms; prohibits proper functions of cells and tissue

-radioactive waste emits radiation; poisons ecosystems if placed in water; buried underground in the hopes that when it is dug up to be disposed of, it will be much less harmful

Geiger and Marsden's Experiment -alpha particle source placed in a block of lead with a narrow hole

-beam of alpha particles was fired at a gold leaf

-zinc sulphide screen was used as a detector

-most alpha particles passed through without recoiling (bouncing back) or deflecting off gold atoms

-some deflected as they passed through gold leaf due to electric repulsion of positive charges of the alpha particles and the positive charges of the gold nucleus

-small proportion bounced back

Factors affecting amount of deflection

-speed of alpha particles; faster particles deflect through smaller angles

-nuclear charge; more highly charged nuclei produce greater deflections

-distance between alpha particles and nucleus; electric force diminishes with square of distance

Rutherford's nuclear model

-theory of atom like a plum pudding: positive sphere contained negative electrons

-deductions: atom is mostly empty space; positive charge is concentrated in a small central region of atom; nucleus is 1/10,000 of the diameter of the atom

Nuclear Fission The nucleus of a Uranium-235 atom can be split into two if a low energy neutron is fired at it. The 'slow' neutron is absorbed by the Uranium-235 (parent nucleus) to become an unstable Uranium-236 atom. The unstable Uranium-236 atom then splits apart into two daughter nuclei, three neutrons and gamma radiation. If the three neutrons produced are absorbed by other nuclei, this results in a chain reaction. Rapidly increasing numbers of atoms split apart and release energy. If this is uncontrolled, then this is called a nuclear bomb.

Generating Electricity with Nuclear Fission