Atomic Models and Radioactivity NCEA AS 2.5 Text Chapters:

History Greeks: Greeks: 4 types of atoms, earth, air, fire, water Used these atoms to explain why things happened Eg stones fell to the earth because they were made of earth atoms Atomos = “indivisible”

History Early 1800s Early 1800s John Dalton, an observer of weather and discoverer of color blindness among other things, came up with atomic theory John Dalton, an observer of weather and discoverer of color blindness among other things, came up with atomic theory All matter is made up of small indivisble particles known as “atoms” All matter is made up of small indivisble particles known as “atoms” Atoms were solid spheres Atoms were solid spheres Drew the first molecular diagrams Drew the first molecular diagrams

History J.J. Thompson ( ) J.J. Thompson ( ) Studied the “mysterious cathode rays In 1903 he proposed the “Plum pudding model” for the atom the atom is a sphere of positively charged matter with electrons embedded like the currents in a “plum pudding”

Thompson’s Model

History Ernest Rutherford Ernest Rutherford Famous for his gold foil experiment Famous for his gold foil experimentgold foil experimentgold foil experiment Atom is mainly empty space Atom is mainly empty space Small dense positively charged nucleus Small dense positively charged nucleus Electrons orbiting the nucleus Electrons orbiting the nucleus (This is the model you have to be able to explain for this achievement standard) (This is the model you have to be able to explain for this achievement standard)

Since Rutherford…. With the help of quantum theory that was being developed by Planck, Einstein and others, the model continued to evolve… Neils Bohr (1913): electrons occupy fixed energy levels (not fixed positions) Neils Bohr (1913): electrons occupy fixed energy levels (not fixed positions) Louis de Broglie (1924): electrons are waves Louis de Broglie (1924): electrons are waves Erwin Schrodinger (1925): electrons are matter waves whose position is based on a statistical probability (enter quantum mechanics) Erwin Schrodinger (1925): electrons are matter waves whose position is based on a statistical probability (enter quantum mechanics) Chadwick (1935): Discovers the neutron. Chadwick (1935): Discovers the neutron.

Rutherford’s Gold Foil Experiment He fired alpha particles at a very thin piece of gold foil and measured the angles they were scattered at. He fired alpha particles at a very thin piece of gold foil and measured the angles they were scattered at.

The Results Observation 1 Most passed right through the gold foil Explanation 1 Atoms are mostly empty space

The Results Observation 2 Some were deflected Explanation 2 The atom contains a positive charge in its centre or nucleus that deflects alpha particles (which are positively charged)

The Results Observation 3 A rare few bounced directly backwards Explanation 3 The positive charge must be small and densely packed so only a few alpha particles hit it directly head-on and bounce back The positive charge must be small and densely packed so only a few alpha particles hit it directly head-on and bounce back

Rutherford’s Model Positive nucleus surrounded by orbiting negatively charged electrons Positive nucleus surrounded by orbiting negatively charged electrons

Nuclear Reactions 3 types: Radioactive Decay – the spontaneous emission of particles from the nucleus of an atom Nuclear Fission – splitting one large nuclei into two smaller ones Nuclear Fusion – combining two small nuclei into one large one.

Radioactivity 3 types: 3 types: Alpha  Alpha  Beta  Beta  Gamma  Gamma  Named in order of their discovery. Named in order of their discovery. Alpha and beta decay don’t usually occur by themselves, there is usually some gamma that occurs with them. Alpha and beta decay don’t usually occur by themselves, there is usually some gamma that occurs with them.

The Nucleus In small atoms, the number of protons and neutrons are usually the same (roughly) In small atoms, the number of protons and neutrons are usually the same (roughly) In larger atoms, there are usually many more neutrons than protons, in order to keep the nucleus stable. In larger atoms, there are usually many more neutrons than protons, in order to keep the nucleus stable. If a nucleus is unstable, it may spontaneously decay to something more stable by emitting alpha, beta or gamma radiation If a nucleus is unstable, it may spontaneously decay to something more stable by emitting alpha, beta or gamma radiation

Alpha Particles Helium nucleus Helium nucleus Charge of +2 Charge of +2 Mass of 4 (a.m.u) Mass of 4 (a.m.u) Travel slowly ie. 10% of light speed Travel slowly ie. 10% of light speed Don’t travel very far ie. A few cms in air Don’t travel very far ie. A few cms in air Low penetration power – can be stopped by a piece of paper Low penetration power – can be stopped by a piece of paper Very good ionising power – because they’re big and slow. Very good ionising power – because they’re big and slow.

Beta Particles An electron from the nucleus An electron from the nucleus Charge of -1 Charge of -1 Same mass as an electron (effectively 0) Same mass as an electron (effectively 0) Travel relatively fast – up to 95% of light speed Travel relatively fast – up to 95% of light speed Travel about 30 cms in air Travel about 30 cms in air Average penetration power – can be stopped by a few mm of Aluminium Average penetration power – can be stopped by a few mm of Aluminium Average ionising power Average ionising power

Gamma Radiation A wave of electromagnetic radiation (energy) A wave of electromagnetic radiation (energy) No charge No charge No mass No mass Travels at light speed Travels at light speed Travels several metres in air Travels several metres in air High penetration power – Several cms of lead needed to stop it High penetration power – Several cms of lead needed to stop it Low ionising power – because no mass Low ionising power – because no mass

Radiation One way that the different types of radiation can be distinguished is by observing their behaviour in a magnetic field: One way that the different types of radiation can be distinguished is by observing their behaviour in a magnetic field:   

The Nucleus Writing nuclei Writing nuclei X = element symbol A = mass number or nucleon number (the number of p+n) Z = atomic number (the number of protons)

Isotopes Atoms with the same atomic number but different mass numbers Atoms with the same atomic number but different mass numbers Eg: Eg:

Alpha Decay Example: Radium 226 decays to Radon 222 by alpha decay: Example: Radium 226 decays to Radon 222 by alpha decay: Note: Both mass and charge must be conserved Note: Both mass and charge must be conserved (ie 226=222+4, 88=86+2

Beta Decay Cobalt 60 decays by beta decay to Nickel 60 Cobalt 60 decays by beta decay to Nickel 60 Again, mass and charge are conserved Again, mass and charge are conserved NB. the  or  symbols can be used instead of He or e NB. the  or  symbols can be used instead of He or e

Half-life The time it take for the decay rate to have halved, or…. The time it take for the decay rate to have halved, or…. The time taken for half of the original atoms to have decayed The time taken for half of the original atoms to have decayed Usually shown on a graph Usually shown on a graph

Half-life

Detecting Radioactivity Geiger Counter – detects electrical current caused by the ionisation of atoms in a gas Geiger Counter – detects electrical current caused by the ionisation of atoms in a gas 400V DC Supply Counter or speaker - Anode: central wire +Cathode: metal cylinder Geiger-Muller tube filled with low pressure Ar End: thin mica window

Uses of Radioactivity Radiation therapy to treat cancer Radiation therapy to treat cancer Sterilisation Sterilisation Carbon dating Carbon dating Nuclear medicine eg tracers Nuclear medicine eg tracers Smoke detectors Smoke detectors

Nuclear Fission Breaking large unstable nuclei into smaller ones. Breaking large unstable nuclei into smaller ones. Releases a lot of energy Releases a lot of energy Lots of possible combinations of fragments from one initial nucleus Lots of possible combinations of fragments from one initial nucleus Eg: Eg:

Nuclear Fission Only one neutron is needed to start the reaction, but several are produced Only one neutron is needed to start the reaction, but several are produced This starts a “chain reaction” This starts a “chain reaction” n U Ba Kr n n n U Ba Kr n n n U Ba Kr n n n U Ba Kr n n n

Nuclear Fission If the chain reaction is controlled it can be used in a nuclear reactor If the chain reaction is controlled it can be used in a nuclear reactor If it is uncontrolled it explodes as a nuclear or atomic bomb If it is uncontrolled it explodes as a nuclear or atomic bomb

Nuclear Fusion The joining of two small nuclei to form one larger one The joining of two small nuclei to form one larger one Again, a lot of energy is produced Again, a lot of energy is produced This is the process that powers the sun This is the process that powers the sun Eg: Eg:

Nuclear Fusion Fusion requires extreme temperature and pressure to occur, and has not practically and economically been used in power generation (yet….) Fusion requires extreme temperature and pressure to occur, and has not practically and economically been used in power generation (yet….) Hydrogen bombs have been successfully made, but require a fission reaction to provide the necessary temp and pressure. Hydrogen bombs have been successfully made, but require a fission reaction to provide the necessary temp and pressure.