4 - Atomic Structure and Radioactivity

• Radius: 1x10-10 nucleus radius: less than 1/10 000 of the nucleus 

  • Nucleus contains protons (+) and neutrons (neutral) so overall charge is positive 
  • Nucleus is surrounded by electrons (-) in energy levels (the further away the energy level the higher energy they have than those which are close to the nucleus)  
• Electrons can change energy levels 

  • If the atom absorbs electromagnetic radiation, an electron can move from a lower energy levels to  a higher energy level-  the atom can now emit electromagnetic radiation and the electron returns back to the lower energy level 
• • Isotopes: atoms of an element with different numbers of neutrons- remember: all atoms of an element have the same number of protons 
• Ions are atoms which have an overall charge- they have gained or lost electrons; positive ions have lost electrons and negative ions have gained electron
• Every atom has no overall charge (neutral). This is because they contain equal numbers of positive protons and negative electrons. These opposite charges cancel each other out making the atom neutral.
• • 1897- discovered tiny negative particles in the atom, called electrons. This showed atoms are not spheres and cannt be divided- they have an internal structure 
  • Plum pudding model: suggests that the atom is a ball of positive charge with negative electrons embedded in it 

  
• Alpha Scattering Experiment

  1. First they take a piece of gold foil, they used gold bc you can Hammer gold out into very thin foil, just a few atoms thick
  2. Then scientists fired tiny particles at the gold- called the alpha particles - they have a positive charged
  3. The first thing they noticed is that most of the alpha particles fired straight through the gold foil without changing direction
  4. Sometimes though they were deflected- this means they changed direction as it passed through the gold foil and sometimes the particles jumped straight back off the foil 
• Summary of alpha scattering experiment:

  • Most of the alpha particles went through the gold atmos; therefore  atoms are mainly empty space; this means the plum pudding method is wrong
  • Some particles deflected; Therefore the centre of an atom must have  a positive charge. Alpha particles that come close are repelled and  change direction 
  • Some alpha particles bounced back; This must mean the centre of an atom must contain a great deal of mass- we now call this the nucleus 

  
• Neils Bohr: Suggested electrons orbit the nucleus at a specific distance, his work agreed with other scientists and the orbits are now called energy levels or shells 

  Overall positive charge is due to positive particles called protons, James Chadwick: discovered that the nucleus also contains neutrons- they have no charge they’re neutral 
• Radioactive decay: Some isotopes have unstable nucleus- to become stable, the nucleus gives out radiation.  

  • Its a random process, cannot predict when an atom will emit
  • The activity is the rate at which a source of unstable nuclei decay, measured in becquerel (Bq) 1Bq = 1 decay per second  - measured with a geiger muller tube. 
  • The count rate is the number of decays recorded each second by a detector 
• Alpha Particle (same as a helium nucleus) 

  • 2 protons, 2 neutrons 
• Beta particle  (an electron which is ejected from the nucleus at a very high speed) 

  • Formed inside the nucleus when a neutron changes into a proton and an electrons
• Gamma Rays (type of electromagnetic radiation from the nucleus)

  Neutron (sometimes given out)
• Alpha 

  Range in air: Alpha particles are large- can travel around 5cm in air before they collide with air particles and stop 

  
  

  Penetrating power: Stopped by a single sheet of paper

  
  

  Very strongly ionising 

  • Produce lots of ions when colliding with material
• Beta

  Range in Air: Can travel further- reach 15cm in air before stopping 

  
  

  Penetrating power: Stopped by a few millimetres of aluminium 

  
  

  Quite strongly ionising
• Gamma 

  Range in air: Travel in several metres before stopping

  
  

  Penetrating power: stopped by several cms of lead

  
  

  Weakly ionising
• Nuclear Equation- Alpha decay: 

  Two protons and two neutrons- same nucleus as a helium atom 

  • Atomic number decreases by 2 (bottom number) & mass number decreases by 4 (top number) 

  
• Nuclear Equation- Beta Decay:

  A neutron changes to a proton and an electron, the electron is ejected from the nucleus and that's called a beta particle

  • Atomic number increases by one (bottom number) & mass number doesn’t change

  
• Half life: is the time is take for the number of nuclei of the isotope in a sample to halve

  • The time it takes for the account rate (or activity) from a sample containing the isotope to fall to half its initial level  

  
• Radioactive isotopes decay and emit radiation from their nuclei, radiation can be ionising: it can form charged atoms called ions- ionising radiation can increase the risk of cancer in humans 

  • Irradiation: expositing an object to nuclear radiation (alpha, beta, gamma or neutrons) can be used to sterilise objects 
• Serialising objects with nuclear radiation:

  -The gamma radiation kills any bacteria present- object did not become radioactive because it came in contact with the radiation not the radioactive isotope

  Precautions:

  • Shielding to prevent ionising radiation from entering. (gloves for alpha & lead apron for beta and gamma)  could be lead-glass screen/walls 
  • Monitoring: measure how much radiation has been received- if there's too much then stop them from working with radioactive isotopes 

  
• Radioactive contamination; when unwanted radioactive isotopes end up on other materials, this is called contamination

  • This is hazardous as the radioactive atom decays and emits ionising radiation  
• Alpha radiation: strongly ionising by easily stopped by dead cells in the skin surface

  • If inhaled or swallowed can be dangerous - they crash into living cells and damage DNA 
• Beta radiation: quite ionising and can penetrate skin into the body 

  Gamma radiation: weak ionising- can penetrate body by likely to pass straight through  
• Over the years, scientists explore the effects of radiation on humans and its important that these studies are publish and then shared with other scientists- this allows the findings to be checked (peer reviewed)