chem notes

Identification and Production of Materials
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Ethene, Polymers and Ethanol
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Catalytic Cracking
Oil refineries need to balance their outputs of various products (petrol, diesel, fuel oil, etc.) to match the demands of the marketplace.
Catalytic cracking is the process in which high molecular weight (high boiling point) fractions from crude oil are broken into lower molecular weight (lower boiling point) substances in order to increase the output of high-demand products.
The column in which this occurs is called a cat cracker.
Catalysts used for cracking alkanes are inorganic compounds called zeolites  crystalline compounds of Al, Si, and O with some metal ions.
Zeolites are effective as catalysts because they have a very large surface area per unit mass.
Mixtures of alkanes can also be cracked by thermal or steam cracking.
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Properties of Alkanes and Alkenes
Alkenes have similar physical properties to the corresponding alkanes. They are both non-polar molecules with weak dispersion forces being the only intermolecular forces involved.
Ethene, propene, and butenes are gases at room temp. as are the corresp alkanes. The higher alkenes, like the alkanes, are liquids.
However, they are different chemically.
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Reactions of Alkanes
Important alkane reactions:
Burns in air to form carbon dioxide and water
Reacts with chlorine, bromine, and iodine only when the mixtures are exposed to UV light.
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Reactions of Alkenes
Presence of double bond make alkenes very reactive. There are many substances which react with alkenes by opening out the double bond to form two single bonds  these are called addition reactions.
An alkyl group is an alkane molecule with one hydrogen atom missing  it does not exist on its own, but is part of another molecule.
Alkyl groups are named by deleting the -ane from the parent alkane and adding -yl. e.g. methyl
Important alkene reactions:
Reaction of chlorine or bromine in a non-aqueous solvent.
Reaction of chlorine or bromine in aqueous solution.
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Industrially Important Reactions of Ethene
Reactive double bond in ethene means that it can easily be converted into a range of very useful products such as ethanol and the starting materials of several important plastics / polymers.
1) Reaction of ethene with water to form ethanol.
Ethanol belongs to a group of compounds called alkanols  alkanes with one H atom replaced by an OH group.
Naming alkanols  delete the -e of the parent alkane and add -ol.  add a number prefix to denote the position of the alcohol group
Alkanols are a sub-group of a class of compounds called alcohols. All alcohols contain the -OH function group.
Ethanol  widely used industrially as a reactant and as a solvent in the synthesis of products ranging from pharmaceuticals and perfumes to varnishes and plastics.  in home used as an antiseptic and as a solvent in food colourings and flavourings and in medications.
2) Catalysed reaction of ethene with oxygen to form ethylene oxide
Ethylene glycol used in manufacture of polymers and as automotive antifreeze. Made from ethene.
3) Catalysed reaction of ethene to form vinyl chloride
Vinyl chloride is the starting material for making the important plastic PVC, polyvinyl chloride.
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Polymerisation of Ethene
The most important reaction of ethene is conversion to polyethylene n a reaction called polymerisation.
Polymerisation is a chemical reaction in which many identical small molecules combine together to form one large molecule. The small molecules are called monomers while the large product molecule is called a polymer.
CH2= CH2 CH2= CH2 CH2= CH2
CH2 - CH2 - CH2 - CH2 - CH2 - CH2 -
The structure is frequently written:
(CH2 - CH2 ) n
Polyethelyene is called an additional polymer. This means that it forms by molecules adding together without the loss of any atoms.
Each double bond 'opens out' to form single bonds with neighbouring molecules
In older processes, the product had significant chain branching. This means that at some carbon atoms one hydrogen atom has been replaced by an alkyl group. Consequently the alkane chains cannot pack close togteher in an orderly fashion. This is called low density polyethylene.
The newer process forms unbranched polyethylene molecules which are able to packclosely together in an orderly fashion. This product is more crystalline and has a higher density than the branched-chained product.
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Some other Addition Polymers
Several other common polymers are addition polymers made from monomers in which one of the H atoms of ethene has been replaced by a different atom or group such as Cl or CH3- .
-CH2-CH-CH2-CH-CH2-CH-CH2-CH-CH2-CH-
| | | | | X X X X X
( )n
e.g. PVC ( )n

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Relating Properties and Uses to Structure
Properties of polymers are affected by their structure. Important structural features for addition polymers are:
Chain length
 number of monomer units, depends upon the conditions used in the polymerisation process  the longer the polymer chain, the higher the melting point and the harder the substance is.
Extent of chain branching  unbranched chains, orderly arrangement, and high degree of crystallinity leads to high density, a high melting point and a relatively hard and tough material. E.g. high density polyethylene is easily recognised by the way it crackles when crumpled. Low density polyethylene is much softer and 'clingy.'
Chain stiffening  Stiffening a polymer involves putting a bigger side-group into the linear chain to reduce its flexibility. E.g. Cl atom in PVC or benzene ring in polystyrene restricts flop or movement.  Polystyrene used in screwdriver handles, car battery cases, some plastic furniture and the like.
Cross-linking  Process in which two or more linear chains are joined together to form a more extended 2-d network. More cross-linking makes the polymer harder and more springy.
Most addition polymers are insoluble or not easily wetted by water.
Most bonds of polymers are strong C-C and C-H bonds  fairly stable.
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Will Raw Materials Run Out?
Raw materials for making polymers come from crude oil. There is considerable concern that the world is going to use up all its available oil reserves within the next few decades.
It would be prudent for the plastics industry to develop alternative sources of ethene and propene.
Ethanol is the prime candidate for an alternative source of ethene. Ethanol can be produced by fermentation from a variety of agricultural crops and it can be easily converted to ethene.
Cellulose is another possible source. Natural polymers  cellulose, proteins, starch
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Condensation Polymers
Condensation polymers are polymers that form by the elimination of a small molecule (often water) when pairs of monomer molecules join together.
Cellulose  naturally occurring condensation polymer. The monomer from which it forms is glucose. Glucose  molecular formula C6H12O6. Polymerisation occurs by eleimination of water molecules from between pairs of glucose molecules.
H} O- C6H10O4 - {OH H} O- C6H10O4 - {OH
-O- C6H10O4 -O- C6H10O4-O- C6H10O4-O + x(H20)
Another synthetic condensation polymers includes nylon.
Proteins are condensation polymers made from amino acids. Amino acids are compounds with a -COOH group at one end and an -NH2 group at the other.
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Structure and Possible Chemical Uses of Cellulose

Has 5 C atoms and 1 O atom forming puckered ring. OH groups on the 5 C atoms.
When glucose molecules combine to form cellulose:
For bonding to occur alternate glucose units must be inverted
This bonding produces a very linear molecule.
Cellulose is major component of plant material or of biomass. Biomass is material produced by living organisms; mainly it is plant material though the term also includes animal excreta and material made by algae.
Identify cellulose by adding drops of calcofluor. If UV light shines on it, it emits visible light.
Decomposition of cellulose  Naturally, bacteria, fungi and insects decompose cellulose to carbon dioxide and water.  These organisms decompose the cellulose to glucose as a first step, but don't stop, decomposing it further, thus decomposition cannot be commercial sources of glucose or ethanol.
Cellulose is widely used as cotton, particularly for textiles, and as paper and cardboard. However, there is no simple or efficient chemical way of breaking cellulose into glucose.
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Synthesis and Use of Biopolymers
Biopolymers are polymers that are made totally or in large part by living organisms.
Partially synthetic biopolymers based on cellulose have been used commercially for nearly a century.
Rayon, or viscose  reconstituted cellulose as is cellophane.
Cellulose nitrate is a synthetically modified cellulose that was widely used for photographic and movie film early this century. It was also used as an explosive and a plastic called celluloid. Unfortunately it was highly flammable and was replaced.
Cellulose acetate, which is much less flammable, is used today for overhead projector slides.
A major problem for petroleum-based polymers  they are not biodegradable. Carelessly discarded synthetic plastics are causing harm to marine and bird life.
One solution is to blend natural and synthetic polymers while another is to produce biopolymers with similar properties to synthetic polymers but retain biodegrdability.
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A Polypropylene-like Biopolymer
There are a range of microorganisms that under suitable conditions can make polymers that have similar properties to polypropylene with the exception they are biodegradable. It is called PHA.
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Ethanol as a Source of Ethene
Ethanol is a source of ethene for the plastics industry. It has the structure:

Ethene is made from ethanol by dehydration. This is a reaction which involves the removal of water.
CH3-CH2-O-H  CH2=CH2 + H2O
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Ethanol as a Solvent
Ethanol widely used as a common solvent in:
Cosmetics
Food colourings and flavourings
Medicinal preparations
Some cleaning agents
Ethanol is a good solvent because it is a very polar molecules: the C-O and O-H bonds are quite polar, because O is much more electronegative that C or H.
Ethanol is therefore a good solvent for polar substances. Ethanol can form hydrogen bonds with many substances and this increases its ability to dissolve such substances. Because of hydrogen bonding, ethanol and water are completely miscible.
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Synthesis of Ethanol from Plant Material
Until 50 years ago, the major source of ethanol was fermentation of starches and sugars from plant material. Ethene was cheaply available in huge amounts after cat cracking of crude oil meant that ethene was a byproduct. Thus industrial ethanol is produces from the ethene.
With falling crude oil reserves, fermentation may become a very important industrial process.
Fermentation is a process in which glucose is broken down to ethanol and carbon dioxide by the action of enzymes present in yeast.
For fermentation:
Suitable grain or fruit is mashed up with water
Yeast is added
Air is excluded
Mixture is kept at blood temp, 37°C
Enzymes ( biological catalysts) in the mixture first convert any starch or sucrose in the mixture into glucose and fructose, then other enzymes convert glucose or fructose into ethanol and carbon dioxide.
C6H12O6(aq)  2CH3-CH2-OH(aq) + 2CO2(g) yeast
Yeast can produce ethanol contents up to about 15%. Alcohol concentrations above this level kill the yeast and stop further fermentation. Higher concentrations require distillation.
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Ethanol as a Fuel
Ethanol is a liquid which readily burns:
C2H5OH(l) +3O2(g)  2CO2(g) + 3H2O(g)
It has been readily proposed as a possible alternative liquid fuel for automobiles.
Advantages:
Renewable resource, reduces non-renewable oil. It is made from carbon dioxide, water, and sunlight. When burnt it returns to water and carbon dioxide.
Could reduce greenhouse gas emissions
Petrol containing 10-20% ethanol can be used in petrol engines without modification.
Disadvantages:
Large areas of agricultural land to grow suitable crops, large energy and resource input, problems such as erosion, deforestation, fertilizer run-off
Disposal of large amounts of smelly waste after ethanol removal presents major environmental problems.
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Heat of Combustion of Ethanol
The molar heat of combustion of a substance is the heat liberated when one mole of the substance undergoes complete combustion with oxygen at a constant pressure of exactly one atmosphere with the final products being carbon dioxide gas and liquid water.
Molar heat of combustion = minus enthalpy change H.
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Electrochemistry
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Displacement Reactions
A displacement reaction  a reaction in which a metal converts the ion of another metal to the neutral atom.
Example:
Brown copper wire is dipped into colourless soln of silver nitrate, black silver forms on copper and solution becomes blue from copper ions.
Cu(s) + 2Ag+(aq)  2Ag(s) + Cu2+(aq)
Copper loses two electrons to form Cu2+ and hence is oxidised. Silver ions gain electrons to form Ag and so are reduced.
Oxidation half reaction: Cu  Cu2+ + 2e-
Reduction half reaction: Ag+ + e-  Ag
Oxidation is loss of electrons O I L
Reduction is gain of electrons R I G
Oxidation-reduction reactions are also called redox reactions and electron transfer reactions.

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Displacement reaction and the Activity Series
Of two metals, the more reactive metal is the one which will displace the other metal from a soln of its ions.
Activity Series:
K Na Li Ba Ca Mg Al Zn Fe Sn Pb Cu Ag Pt Au
The metal further to the left of the Activity series will displace the other metal from a solution of its ions.
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Valence and Oxidation States
For positive monatomic ions the oxidation state is the charge on the ions.
A change in oxidation state of such monatomic species corresponds to a loss or gain of electrons.
E.g. Fe2+  Fe3+ + e- oxidation
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Electricity from Redox Reactions
Redox reactions involve transfer of electrons from one reactant to another. An electric current is a flow is electrons through a wire. We can make redox reactions generate electricity by arranging for the oxidation and reduction half reactions to occur at different locations, and by providing a wire for the electrons to flow through. This occurs in all the batteries we use.
2 beakers, one with copper nitrate solution and one with silver nitrate solution. A strip of Cu metal in Cu soln and Ag wire in Ag soln.
The 2 solns are connected by a U-tube filled with a solution of KNO3 held in place by plugs of cotton wool. This U-tube which makes electrical contact between the 2 solns is called a salt bridge.

Several changes occur:
Metallic silver deposits on the silver wire
Some of the copper strip dissolves
Concentration of Ag ions falls
Concentration of Cu ions increases
Thus, electricity has been produced by the occurrence of chemical reaction.
Cu  Cu2+ + 2e- Ag+ + e-  Ag
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Some technical terms
Galvanic cell or Voltaic cell  a device which makes a chemical reaction occur in such a way that it generates electricity. Car batteries and dry cell batteries are galvanic cells.
Electrodes  conductors of a cell which get connected to the external circuit.
The solutions in a galvanic cell are called electrolyte solutions.
An electrolyte is a substance which in solution or molten conducts electricity.
The chemical reactions occurring at the electrodes are called electrode processes or electrode reactions.
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Purpose of the Salt Bridge
Electrical neutrality of each solution must be preserved by the increase or decrease in the concentration of the spectators ions, i.e. nitrate ions. This implies that there has been a migration of nitrate ions away from the silver nitrate soln through the salt bridge and into the copper nitrate soln.
The purpose of the salt bridge is to allow this migration of ions to occur.
It is important that the saltbridge soln does not form a ppt with the ions in the soln.
When a galvanic cell produces electricity:
1. One electrode reaction liberates electrons which flow out of the metal of the electrode and into the external circuit.
2. These electrons flow through the metallic conductor of the external circuit to the other electrode.
3. The reaction at the other electrode consumes these electrons
4. Ions migrate through the solns and connecting salt bridge to maintain electrical neutrality.

A galvanic cell is an 'electron pump'; it pumps electrons out of the negative terminal into the external circuit and 'sucks' them back into the positive terminal. It can do this because a redox reaction is occurring in the cell.

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Cell Diagrams
A shorthand way of representing cells:
Cu | Cu2+ || Ag+ | Ag
Single line denotes a change in phase and double line denotes a salt bridge
Such cell diagrams are read as: a piece of copper metal dips into a soln containing Cu ions. This solution is connected by a salt bridge to a soln containing Ag ions into which dips a piece of silver metal.
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Anode and Cathode
The anode is the electrode in which oxidation occurs.
The cathode is the electrode in which reduction occurs.
In a galvanic cell, the anode is the negative terminal while the cathode is the positive terminal. AN OX, RED CAT Anode oxidation, Cathode reduction
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Some commercial Galvanic cells
The ordinary dry cell ( Leclanche cell ):
Widely used in torches, calculators, radios, clocks.
The two metals used are zinc and manganese  zinc is anode, negative, oxidation
Advantages  Robust, easy to store and use, most common, cheapest, causes minimal environmental problems on disposal.
Disadvantages  does not contain very large current, can develop leaks when it goes flat
Alkaline cell:
Appears similar to Leclanche cell and uses the same metals, i.e. Zn and Mn.
Used for photographic flash cameras, tape recorders, and toys.
Uses different electrolyte paste, KOH instead of NH4Cl.
Reactions occur readily under alkaline conditions.
Advantages  able to deliver higher currents longer than ordinary cells. Appear to be more expensive than Leclanche cells but similar cost effectiveness. Relatively small, robust and very practical to use.
Disadvantages  leakages problems are more severe because of their alkalinity.
Silver oxide cell (button cell):
Widely used in miniature appliances such as watches, hearing aids, and cameras.
Small cells can provide considerable amounts of electricity at a very constant voltage over a along period of time.
2 metals are Ag and Zn
Overall reaction: Zn + Ag2O  ZnO + 2Ag
Flat Battery
Galvanic cells go flat when one of the reactants, usually the more expensive one, is all used up. No further chemical reaction is possible, hence no more electrical current can be produced.
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Electrolysis
Electrolysis  the process in which an electric current is used to bring about a chemical reaction which does not occur spontaneously.
Copper reacts with chlorine to form copper chloride: Cu(s) + Cl2(g)  CuCl2(s)
If we pass a current through CuCl2 soln using inert electrodes, Cu deposits at the -ve electrode and Cl gas at the +ve. CuCl2 is decomposed by electrolysis.
The voltage source is an electron pump. It pushes electrons out of its negative terminal into the electrode attached to it. The positive copper ions are attached to this negative electrode and they take electrons from it and deposit on the surface as neutral copper atoms: Cu2+ + 2e  Cu
The voltage source also sucks electrons out of the positive terminal. Negative Cl ions are attracted to it and give up electrons to it, forming neutral Cl atoms, which combine to form Cl2 gas.
2Cl-  Cl2(g) + 2e
CuCl2(aq)  Cu(s) + Cl2(g)
A cell in which electrolysis occurs is called an electrolytic cell, in contrast to a galvanic cell which produces electricity. A name covering both types of cell is electrochemical cell.

In electrolysis the anode is the positive electrode and the cathode is the negative electrode. This is the opposite of galvanic cells.
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Purifying Copper by Electrolysis
The electrode reactions which occur during electrolysis depend upon:
The nature of the ions present
The concentration of the ions
The nature of the electrodes used
Electrolytic refining of copper  impure blister copper is placed at the +ve anode and pure copper at the -ve cathode, in a soln of CuSO4. Copper builds up around the pure copper electrode and impurities fall to the bottom. These are called anode mud.
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Electroplating
Electroplating is the electroplating process of depositing a thin film of one metal on the surface of an object made of another (generally cheaper) metal.
Common e.g.'s of electroplating include silver-plated cutlery and other household objects, silver and gold-plated jewellery, nickel-plated hardware items and chrome-plated taps, bathroom fittings and motor car parts.
To carry out electroplating the object to be plated is made the -ve cathode and the metal to be plated onto it is the anode with a soln containing the metal ion as the electrolyte. During electrolysis metal atoms of the anode give up electrons to form cations which migrate to the cathode where they accept electrons and deposit as metal. This process is virtually the same as that used to purify copper.
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Rechargeable Batteries (cells)
Some galvanic cells can be recharged by passing an electric current through them in the opp direction to that in which they delivered current.
The two common rechargeable commericail cells are the nickel-cadmium cell and the lead accumulator / lead-acid battery (car battery).
Some cells are rechargeable because if we attempt to recharge them, we bring about a different reaction rather than the reverse direction.
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Standard Electrode Potentials
The electromotive force or EMF of a galvanic cell is the potential difference (voltage) across the electrodes of the cell when a negligibly small current is being drawn. It is the maximum voltage that the cell can deliver.
The standard electrode potential, E°, of an electrode is the potential of that electrode in its standard state relative to the standard hydrogen electrode.
Standard electrode potentials are assigned not only to the electrodes but also to the reduction half reactions associated with the electrodes.
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Calculating EMFs for Redox Reactions and Cells
{Standard EMF for complete reaction} =
{Standard EMF for reduction half reaction} +
{Standard EMF for oxidation half reaction}
EMF°(total) = EMF°(red) + EMF°(oxid)
Doubling the half reaction does not alter E°.
To calculate the EMF of a redox reaction, we use the above equation together with the fact that the EMF of an oxidation half reaction is minus the electrode potential of the corresponding reduction half reaction.
To calculate the EMF of a galvanic cell 
When considering a galvanic cell with electrodes A and B, EMF can be calculated as:
EMF°(cell) =
{standard electrode potential of electrode A} --
{standard electrode potential of electrode B}
If the EMF of a redox reaction calculated is positive, then the reaction occurs as written, if the calculated EMF turns out to be negative, then the reaction does not occur as written, but rather occurs in the reverse direction.
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Nuclear Chemistry
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Radioactivity
Elements such as uranium, radium, polonium, and thorium were found to emit some form of radiation. These elements were called radioactive. This phenomenon was called radioactivity.
For some elements all isotopes are radioactive, while for others only one or some isotopes are radioactive. Hence we talk about radioactive isotopes or radioisotopes rather than radioactive elements. Because the radioactive emission comes from the nucleus of the isotope, so scientists talk about unstable nuclei and stable nuclei.
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Isotopes and Symbols for Them
Isotopes  atoms of the one element that differ by having different numbers of neutrons in their nuclei.
Isotopes have the same atomic number but different mass numbers.
Special symbol:

A = mass number Z = atomic number
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Stable and Unstable Isotopes
If we plot the number of neutrons against the number of protons in the nucleus for isotope that are stable we find they lie in a narrow band  called the zone or band of stability.
An isotope is unstable if its: atomic number is greater than 83, or if its ratio of neutron to protons places it outside the zone of stability.
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Types of Radiation Emitted
There are 3 different types of radiation which were originally called alpha (), beta (), and gamma () rays because their true identities were unknown.
Alpha particles  heavy, +ively charged particles with low penetrating power, a sheet of paper could stop them. They are helium nuclei  2 protons and 2 neutrons stuck together.
Beta particles  were much lighter negatively charged particles with greater penetrating power. Can pass through paper and 0.5mm aluminium but not pass 0.5mm lead. They are simply elelctrons.
Gamma rays  like X-rays, genuine radiation rather then particles. Carry no charge and extremely penetrating. Can only be stopped by several cms of lead or many cms of concrete. Gamma rays are a type of EM radiation with shorter wavelengths and carry large amounts of energy.
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Alpha-Emitting Radioisotopes
The common radioisotopes of uranium and radium are alpha emitters.
U  He + 234/90 Th
We see that the substance resulting is now thorium.
The atomic and mass numbers must balance, as they represent numbers of protons and neutrons.
Radioisotopes which emit alpha particles often emit gamma rays as well.
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Beta-Emitting Radioisotopes
Beta particles, electrons, also come from the nucleus. This happens because a neutron decomposes into a proton and an electron. n  p + e
n  p + 0/-1e
Cabalt-60, a beta emitter
60/27 Co  0/-1 e + 60/28 Ni
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Decay of Natural Uranium
Two naturally occurring isotopes of Uranium are 238-U and 235-U. Both are radioactive alpha emitters. 238-U decayed to 234-Th, which decays further. Eventually it converts to the stable lead isotope 206-Pb.
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Nuclear Fission
Nuclear fission involves bombarding certain nuclei with neutrons: this splits the target nucleus into two roughly equal fragments.
235/92 U  141/56 Ba + 92/36 Kr + 3(1/0n) + energy
Nuclear fission is the basis of both atomic bombs and nuclear power stations. The amount of energy released per g of uranium is enormous.
Nuclear reactors produce several new elements and can be used to make new isotopes of naturally occurring elements.
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Transuranic Elements from Nuclear Reactions
When very unstable natural elements such as 238-U get hit by neutrons they form unnatural elements such as plutonium or nemptunium.
Transuranic elements or transuranium elements are these elements formed by bombarding heavy nuclei with high speed positive particles such as helium or carbon nuclei.
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Half-life
An important property of radioisotopes is their stability, or in other words how long they survive before completely decaying to other isotopes. This is measured by what is called their half-life.
The half-life of a radioisotope is the time required for half the atoms in a given sample to undergo radioactive decay; for any particular radioisotope, the half-life is independent of the initial amount of the isotope present.
Radioisotope decay displays exponential decay.
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Commercial Isotopes from Nuclear Reactors
Cobalt-60 is a gamma ray emitter, used for cancer treatment. Half-life 5 years. Gamma rays can penetrate deeply into body tissue and destroy biological tissues including cancer cells. Also used in industry to destroy bacteria in food and to sterilise medical supplies.
Technicium-99m  half-life 6 hours. Injected into the patient to help in diagnosis of medical conditions such as heart problems and blood clots.
Sodium-24  half-life 15 hours, used to detect leaks in water pipes.
Americium-241  alpha emitter used in domestic smoke detectors.