P4

Developing the model of the atom

John Dalton- Agreed with Democritus meaning:

• Atom were tiny spheres that couldn’t be broken down any further

100 Years later JJ Thompson discovered electrons could be remove from atoms hence the PLUM PUDDING MODELLL:

• This suggested that atoms were spheres of positive charge with tiny negative spheres stuck in them.

The charges cancelled each other out making the entire atom of neutral charge

Problems

• How does the nucleus not collapse due to electro static attraction

Geiger Marsden experiment (Alpha particle scattering experiment)

knowledge:

• They believed the plum pudding model was correct

• They knew that opposite charges attract and like charges repel

What they expected

• Because the plum pudding model suggested an atom that had no overall charge as the positive sphere and negative electrons cancelled each other out

• Meaning that the positive alpha particles could pass straight through

Results

• Most alpha particles went straight through

• But a small number were deflected

• And around 1 in 10,000 were deflected by more than 90°

Conclusions

• Most of the atom is just empty space

  • Because the majority just passed through

• Most of the mass is focused in a positively charged nucleus

  • Because the alpha particles must have been repelled by a like charge.

• Electrons orbit the nucleus

  • In order to not be drawn into the nucleus by electrostatic attraction

Bohr

• Said that electrons orbit at fixed distances

• The further away the more energy it has

• When electron go down a level they lose energy and emit EM radiation

• When electrons go up a level they absorb EM radiation- gaining energy

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Isotopes and Nuclear radiation

These are atoms With the same Atomic number, different Mass number

Some isotopes are unstable and decay into other material giving out radiation in order to become more stable

• Radioactive decay- An unstable nucleus can become more stable over time by randomly (spontaneously) emitting ionising radiation

• Radioactive substances- Spit out one or more types or radiation

• Ionising power- How easily radiation can knock electrons off atoms making them positive ions

• Penetration power- The ability of radiation to pass through matter

• The random nature of radiation- You can’t predict when a radioactive nucleus will decay or which nucleus will decay next

Alpha

• What is it- A helium nucleus (2 protons, 2 neutron)

• Penetration- Stopped by paper pr a few cm of air

• Ionising- Very strong

Extra- When emitted the element changes as it loses 2 protons and 2 neutrons,

• Mass decreases by 4 charge decreases by 2

Beta

• What is it- a fast moving electron

• Penetration- stopped by aluminium foil

• Ionising- Mid

Extra~ Every beta radiation emitted causes a neutron in the nucleus to turn into a proton, changing the element altogether, as it Gains a proton

~ Charge increases by 1

~A Neutron turns into a fast moving electron that runs away and a proton that stays

~Mass remains unchanged

Gamma Rays

• What is it- A wave of energy

• Penetration- Partially stopped by thick lead/ meters of concrete

• Ionising- weak

Extra~ No change in mass or charge, just a release of energy

Nuclear equations

Shows the process of nuclear decay in the form:

• Atom before decay → Atom after decay + radiation emitted

• The total mass and atomic numbers have to be equal on both sides

Now taking into account the factors from the aforementioned extras you apply this the nuclear equations:

Half life

Each time a radioactive nucleus decays to become stable its radioactivity decrease. Some sources take a few hours to decay whilst others take millions of years to decay, But eventually the radioactivity will drop off.

Half life~ the time it takes for the amount for the number of radioactive nuclei to halve

It is measured in becquerels, Bq 1Bq = 1 decay per second

Background radiation and contamination

• This is the ever present amount of constant radiation in the background at all times.

• Subtract this from radiation results whenever you can

• Radioactivity naturally occurs in:

  • Cosmic rays

  • Fallout from past nuclear explosions or waste or melt downs

• Radiation dose measures the risk of harm to body tissue due to exposure:

• It is measured in sieverts and millisieverts

When an object is exposed to radiation it is IRRADIATED, This is the one to do with sieverts

When an object is contaminated: Radioactive atoms get onto or inside the object

Outside the body: (So commonly irradiation wise)

beta and gamma radiation are the most dangerous as the can penetrate the skin and damage organs

Whereas alpha is stopped by skin and air

Inside the body: (So commonly contamination wise)

• Alpha is most dangerous due to the high ionising power

• Beta as it is absorbed over a wider area or passes out the body all together its less dangerous

• Gamma is least dangerous as it passes out the body completely hand has the lowest ionising power

Fission

This is nuclear the radiation that we are used to:

• It usually starts when a nucleus absorbs a neutron,

• then it decays into to lighter elements and another fast moving neutron

• This fast moving neutron is absorbed by another nucleus

• Which repeats the process causing a chain reaction

• During this reaction lots of energy is released through gamma rays and kinetic energy

• Which can generate heat which heats water to make steam to turn a turbine

• Which makes nuclear energy

• Fission in a reactor is controlled and slowed using rods which absorb neutrons

• In order to stop a nuclear melt down

• Uncontrolled fission chain reactions can lead to a nuclear weapon

Fusion

This is the joining of small nuclei at high speeds e.g. Hydrogen

These nuclei fuse to grate a larger nucleus (e.g. helium)

Some of the mass of the smaller nuclei turns to energy, producing a stupid amount of energy

However the conditions to create a fusion reactor are Hard as hell to make they require insane pressures and temperatures

YES FINALY UUUGHGHTTT P4 FINISHED