p4 - atomic structure

195 - developing the model of the atom

• 1804 - Dalton - atoms are tiny sphere that can’t be broken up, each element is made of a different type of atom

• JJ Thompson - discovers electrons - proves Dalton wrong as electrons can be transfered - plum pudding model

• Rutherford - alpha particle scattering experiment, fired alpha particles at gold foil expecting them to go through, some bounced back, proving atoms had a nucleus with a positive charge as it repelled the positive alpha particles - showed that most of the atom is made up of empty space as most passed through

• lead to the Nuclear model by Niels Bohr - he said that electrons orbit the nucleus in energy levels

• james chadwick discovered the neutron which explained the imbalance in atomic and mass numbers

• electrons can move within the atom or leave it

• if electrons gain energy from absorbing EM wave radiation, they move to a higher electron shell/energy level, further away from the nucleus

• if they release EM radiation, they move to a lower energy level closer to the nucleus

196 - isotopes and nuclear radiation

• isotopes of an element have the same proton number, but a different amount of neutrons - like carbon 12 and 13

• all elements have different isotopes, but only 1 or 2 of them are stable

• radioactive decay is when unstable isotopes decay into other elements as they try to balance their proton and neutron numbers or get rid of excess energy - they try to become stable and give out radiation when doing so

• radioactive substances give out ionizing radiation from their nucleus - alpha, beta and gamma radiation

• they are all produced by radioactive decay

• they also release neutrons when they decay to try and re-balance their nucleus

• ionizing radiation means radiation with enough power to knock electrons off of other atoms, making positive ions

• ionizing power is how easily the radiation can do this

• alpha - are helium nuclei

• they are 2 protons and 2 neutrons, meaning their atomic numbers are 2 and 4

• they are strongly ionzing due to their size

• they have a short range in air

• they don’t have a strong penetrating power so are absorbed by a thin sheet of paper

• beta - high speed electrons released by the nucleus

• moderatly ionizing

• they have moderate penetrating power, they are absorbed by a sheet of aluminium

• for every beta particle emmited, a neutron in the nucleus has turned into a proton - this is due to beta decay

• gamma - EM waves with a short wavelength given out by the nucleus

• they are weakly ionizing as they pass through material and don’t usually collide with atoms, when they do they cause damage

• they have a large range in air

• strong penetrating power - they pass through tissue and materials easily

• can be absorbed by thick lead or concrete

197 - nuclear equations

• nuclear equations show radioactive decay through element symbols

• they follow the rule : atom before decay - atom after decay + radiation emitted

• the atomic and mass numbers must be the same on both sides, they must add up to equal the numbers on the left side

• alpha decay - since alpha is the radiation being given off, the atom that is decaying has lost 2 protons and 2 neutrons, so in total the element’s atomic number will reduce by 2 and the mass number will reduce by 4 (protons + neutrons)

• top number = mass number (p=e) bottom number = atomic number (p+n)

• beta decay - since beta is the radiation being given off, a neutron in the nucleus turns into a proton to balance out the number and releases an electron

• the nucleus gains one proton, so the atomic number increases by 1

• the mass doesn’t change since it has gained a proton and lost a neutron and they both have the same mass of 1

• gamma rays are a way of getting rid of excess energy from a nucleus, so the mass and atomic numbers don’t change

198 - half life

• half life is the time it takes for half the radioactive nuclei in a sample to decay

• radiation is always given out from the nuclei of radioactive substances - it’s totally random - you can’t predict when a nucleus in a sample will decay

• radiation can be measured by a geiger-muller tube and counter, which records the count rate - the number of radiation reaching it per second

• half life can be used to make predictions about radioactive sources and tell us the rate at which a source decays

• the rate at which a source decays is called its activity - this is measured becquerels / Bq - 1 Bq is 1 decay per second

• each time a radioactive source’s nucleus decays to become stable, the activity will decrease, older sources have a lower activity because they are closer to being stable

• for some sources it takes hours for most of the unstable nuclei to decay and for some it will last for millions of years

• activity will never reach zero, because there is always a chance that there is more unstable nuclei, the number will just get closer and closer to zero - this is why we use half life, to see the time taken for activity to half of its initial value

• you can measure half life on a graph - plot activity against time and half the initial value given at the top of the y axis, follow this down to the x-axis to find the time value, this will be the half life, as this is the time it has taken for the half of the source’s radioactive nuclei to decay

199 - irradiation and contamination

• ionizing radiation is dangerous as it kills living cells or damages them causing things like cancer, they do this by entering living cells and ionzing atoms within them - knocking off electrons and leaving behind a proton

• irradiation is when an object is exposed to radioactivity but doesn’t become radioactive itself

• contamination is when radioactive particles get onto an object and make it radioactive

• reducing irradiation - keeping the source in a lead lined box or keeping it in a different room and handling it with remote controlled arms

• reducing contamination - gloves and tongs should be used when handling sources to avoid particles getting onto your skin or under you nails and decaying inside or on you, you can wear a protective suit to ensure you dont breathe them in

• outside the body - beta and gamma are the most dangerous as they both have high penetrating power and get to organs - alpha cannot penetrate the skin and is stopped by a small air gap

• inside the body - alpha is the most dangerous, as it’s very ionizing but has a low penetrating power so can’t pass out of your body easily - beta sources are less damaging as the radiation is absorbed over a wide area and some passes out of the body - gamma is the least dangerous as they pass out of the body and have the least penetrating and ionizing power