atomic structure and radioactivity

• electrons can move from 1 energy level to another if they absorb or emit the right amount of energy (as an electromagnetic wave)
• electrons can absorb EM waves to gain energy. They move from a lower energy level to a higher one (electron moves away from nucleus if EM waves are absorbed into the atom)
• electrons can move towards the nucleus by emitting EM waves. The wavelength depends on how many energy levels the electron has dropped. The further the electron has dropped - the shorter the wavelength and the greater the energy of EM wave
• atoms are neutral as number of protons = neutrons, Ions are atoms that have lost one or ore electrons so be come positively charged
• atomic number is the number of protons in a nucleus, the mass number in the total number of protons and neutrons in the nucleus. More experiments are being done to gain a better understanding of the atom.
• The history of the atom started with Dalton's theory that atoms are tiny solid spheres tat are indivisible. Date: 1804
• The second discovery was the discovery of electrons that atoms can be split up into subatomic particles. Thompson discovered this in 1897.
• The next theory was the plum pudding model. This was that an atom was a mass of positive charge with negative electrons embedded within it. This happened straight after the discovery of the electron.
• Next is a discovery by Rutherford, Geiger and Marsden with the alpha scattering experiment. a beam of positive alpha particles is fired at a thin layer of gold foil. The alpha particles emerge at different angles. This was a discovery which proved the plum pudding model wrong as Rutherford, Geiger and Marden proved there is a nucleus in the atom
• Scientists prove there is a nucleus which is a small positive mass in the center of the atom

  
• The next theory is the Bohr in 1913. He suggested that electrons orbit the nucleus at a specific distance from the nucleus, in shells
• The next discovery was one to show the positive charge of the nucleus is made up of smaller positively charged particles called protons.
• The final discovery is the discovery of neutrons in 1932 by Chadwick.

  
• some Isotopes are unstable-meaning the nucleus decays into other elements by emitting radiation to become more stable. This is called radioactive decay.
• The emitted radiation could be one of:

  • Alpha particle
  • Beta particle

  • Gamma ray

  these are all types of ionising radiation
• Ionizing radiation means those types of radiation can knock electrons of their atom meaning they become a positive ion

  how easily they do this is their ionizing power
• Alpha particles have 2 protons and 2 neutrons without any electrons, wit a mass of 4 ,this is the same as a hydrogen atom.
• alpha decay

  Alpha particles have a igh ionizinf power and knock electrons off their atom

  
• Beta ionization: an electron is released when a neutron splits into a proton and an electron

  
• Gamma rays: gamma has no mass or charge, but is a high energy electromagnetic wave travelling through a nucleus

  
• Alpha is the most likely radiation to get stopped or be absorbed.

  Gamma = most penetrating

  Beta = moderately penetrating

  Alpha = least penetrating
• alpha = most ionizing

  beta = moderately ionizing

  gamma = least ionizing
• Fire alarms need to be highly ionizing so we use alpha radiation

  (Radioactive americium releases alpha radiation, which ionises the air inside the detector. Smoke from a fire absorbs alpha radiation, altering the ionisation and triggering the alarm.)
• Safety

  -keep sources in lead lined boxes

  -minimize time of source being out of it's box

  -handle with tongs

  -keep source away from people

  -wear lab coat and gloves to avoid contamination
• alpha particles are absorbed by a sheet of paper of a few cm of air, (most dangerous if inside your body)

  Beta can be absorbed by a few cm aluminum or reduced by a sheet of paper or a sheet of aluminum foil. It's range in air is roughly 5m.

  Gamma rays can be greatly reduced by several meters of concrete or several cm of lead. It's range in air is roughly a few km.
• Nucleus before decay = nucleus after decay + radiation emitted
• Alpha decay decrease the mass and the charge of the nucleus.

  With alpha decay the:

  mass number always decreases by 4

  charge number decreases by 2

  
• Examples:

  1.

  a) mass number (top number) = 222

  b) atomic number = 86

  2.

  a) mass number = 210

  b) atomic number =84

  
• Beta Decay increases the charge of the nucleus by 1, but the mass stays the same
• Examples: Beta

  1.

  a)atomic mass = 234

  b)atomic number =91

  2.

  a)atomic mass = 3

  b)atomic number = 1

  
• Gamma decay does not change the mass or the charge of the nucleus. They remove excess energy from the nucleus.
• this is the symbol for Gamma

  
• this is the symbol for Beta

  
• this is the symbol for alpha

  
• Radio active decay: where an unstable nuclei emits alpha, beta, gamma or a neutron to become more stable

  Radioactive decay is a completely random process

  We can't predict when an individual nucleus will decay or which one will decay next

  we can measure how long it will take for half of the nuclei to decay
• A Geiger - Muller tube + counter will detect radiation emitted, It measures count rate ie. the number of decays per second it receives

  Activity = total number of decays per sec

  Activity is measured in Becquerels, Bq

  e.g 20 bq = 20 decays per second
• We can use the Geiger - Muller tube and counter to look at how the activity will change with time.

  For every radioactive source, the activity will decrease with time.

  how quickly this happens is different for every isotope
• For half life no matter how many nuclei are left in the sample there will be set amount of time for half of the nuclei to decay

  e.g

  was full, then half decrease then halved again

  
• Half life is the time it takes for the number of radioactive nuclei in a sample to decrease to half the original value

  e.g .

  was 8 radioactive nuclei in sample, now there is 4
• For some isotopes the activity will halve in just a few minutes or hours wheras some may take millions of years