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The Atom

An atom: Is the smallest part of an element that can exist independently

An atom contains 3 main subatomic particles

Particle Location Charge Massproton p+ nucleus + 1 1 amu(atomic mass unit)

neutron no nucleus o 1 amuelectron e- orbits the nucleus - 1 almost zero mass

Thompson (1897)discovered the charge on the electron ( Stoney named the electron )Rutherford discovered the proton (1919)Chadwick discovered the neutron (1932)

Mass number = Number of neutrons + the number of protons

Atomic number = Number of protons

Note : An atom is neutral therefore the number of protons (+)mustequal the number of electrons (-)

Isotopes : Have the same number of protons ( atomic number )butdifferent numbers of neutrons ( mass number )Isotopes have the same chemical properties ( determined by the atomic number )But different physical properties ( usually isotopes are radioactive )12 14

Example : Carbon 12 C and Carbon 14 C6 6

Each atom has 6 protons ( and 6 electrons ) but C 12 has 6 neutrons while C 14 has 8 neutrons

Rutherford`s Gold foil experiment 1909 ( determined the structure of the atom ) Method : He bombarded a thin piece of gold foil with ( alpha ) particles. ( particles carry a + charge)

Result : 1. Most of the particles passed straight through the gold.=> An atom is mostly empty space.

2. A small number were turned back in the direction they came from. =>The + charge is concentrated in a very small space in the center of the atom ( the nucleus )

From this experiment we get our present understanding of the atom as consisting of a small centralpositively charged nucleus (10-15m) surrounded by a vast space containing a small number of negative charges ( electrons). Radius of the atom is 10 -10m

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Emission Spectrum: Is the spectrum given out by a substance when its atoms are excited

There are 2 types of emission spectra.

1.Continous spectrum: Produced by a solid or a liquidthathas been heated e.g. a filament bulb

2.Line spectrum: Atoms of a gaseous element, electrons excited by electricity.Line spectrum is characteristic of the element.Sodium produces yellow lines on a black background.

Spectroscopic analysis used to identify elements where only very small samples are available.

From the study of emission spectra Bohr proposed that electrons travelled in orbits (or energy levels ) around the nucleus.

When an electron is given energy (heat or electrical) it jumps to a higher orbit ( energy level ).But it must fall back to its original orbit, when this happens the energy it absorbed is released in the form of heat (infra red ), ultra violet or visible light ( all forms of electromagnetic radiation )

Energy absorbed frequqncy of the radiation emitted

h = Plank`s constant

Radioactivity was discovered by Becquerel (1896)

This demonstration shows -particles carry a + chargeΒ-particles carry a – chargerays carry no charge

E2 - E1 = h f

E = h f

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Artificial radioactive Isotopes : Are created in a nuclear reactor

Problem : Calculate the number of -particles and β-particles emitted in the decay of U238 to Ra 226

9288

Solution : = He4 β = e0 Therefore a β emission has no effect on the mass number.2 -1

Step 1 : Find the number of emissions. By how many multiples of 4 the mass number decreases. 238 - 226 = 12 = ( 4 x 3) => 3 -emissions If there had only been 3-emissions the result would have been U238à 3He4 + Rn226

92 2 86

Step 2 : Find the number of β-emissions.Rn226->2e0 + Ra 226( always subtract 86 – ( -2 ) = 86 + 2 = 88 ) 86 -1 88

Ans : 3-emissions and 2 β-emissions

The Giger Muller Tube ( used to detect radiation )

Radiation enters through the mica window,causingionisation of the gas.

+ ions accelerate towards the cathode ( - )Electrons (-)accelerate towards the anode ( + )

The accelerating ions produce more ions by collision.

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The electrons reach the anode and a pulse of current Is producer that is detected by the counter.Law of Radioactive decay : Activity = Nunit = Becquerel(Bq)= 1 disintegration per second

Rate of decay Activity = (decay constant) x (number of atoms ( nuclei ) present) Number of disintegrations NNumber of or β emissions

Half life : T ½Is the time taken for half the number of atoms of a radioactive substance to decay

T ½ = natural log 2 T ½ = .693 RATE OF see graphs page 353

decay constant EMISSION

( Note : convert all time to seconds )TIME

The mole : A mole of any substance contains Avogrado`s number of atoms ( 6.02 x 10 23 )

Note : The atomic mass of any element expressed in grams contains Avogrado`s number of atoms

Example : How many atoms ( nuclei ) are there in 8 Kg of Sr90

38

Sr90 => mass number = 90 =>90 grams contains 6.02 x 1023atoms381 g contains 6.02x 10 23 90 1 Kg contains 6.02x 10 23 x 1,000 90 8 Kg contains 6.02x 10 23 x 1,000 x 8 905.35 x 10 25 atoms in 8 Kg of Sr90

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6.02 x 1023 atoms of Sr90 have a mass of 90 g = 90 x 10-3 Kg 1 atom of Sr90 has a mass of 90 x 10 -3 Kg 6.02 x 10 23

4.72 x 10 18 atoms have a mass of 90 x 10 -3 x 4.72 x 1018 = 7.05 x 10 -7 Kg 6.02 x 10 23

Note :-particles cause the most ionisation

-particles have the least penetration

Nuclear Fission : Is the splitting up of a large nucleus

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into two or more small nuclei with the emission of neutrons and a large amount of energy

Critical mass : Is the minimum mass of fissile material that can sustain a chain reaction Chain reaction : When each fission reaction produces at least one further fission reaction An atomic bomb is an uncontrolled chain reaction

A Nuclear reactor is a controlled chain reaction

Fuel rods :Natural Uranium ( U238 ) enriched with (U 235 ) shielding

Moderator : Slows down the neutronsthis increases the chances of the neutrons causing fission with U235 rather than being captured by U238 ( Graphite or Heavy water D2O )Control Rods :Absorb neutrons , this slows down the rate of the reaction. When fully in they shut down the reactor. Shielding: Stops radiation escaping Coolant : Takes the heat from the core to the heat exchanger.

The heat produces steam to turn the turbine to make electricity by electromagnetic induction.

Fusion : Is the joining of light nuclei to form a heavy nucleus with the release of large amounts of energyExamples : The Sun and a Hydrogen bomb is an uncontrolled fission reactor

Note: We cannot sustain a Fusion reaction because Fusion requires extremely high temperatures.Advantages : 1. Fuel (Hydrogen) readily available in the oceans 2H + 3 H ->4 He + 1n 2. Very little radiation produced1 1 2 0

Equivalence of mass-energy E = m c2

Change in energy = ( change in mass ) x ( speed of light )2

E = m c2Note : The energy must be in Joules The mass must be in Kg