Models of the Atom

The word "atom" comes from the Greek word "atmos" which means "uncuttable".  - But are they?

• What makes up an Atom? [2]
  • Nucleus
    • What makes up the nucleus? [2]
      • protons and neutrons.
    • Where is most of the mass of an atom found? [1]
      • the nucleus
    • Does the nucleus give an atom it's overall size? [1]
      • No, the nucleus takes up virtually no space - it's tiny
    • What is the atomic number or proton number? [1]
      • The number of protons in the nucleus
    • What is the mass number, or nucleon number? [1]
      • The total number of protons and neutrons in the nucleus
  • Electrons
    • What charge are electrons? [1]
      • negatively charged
    • Are electrons large particles? [1]
      • No, they are really, really small
    • Where in the atom do you find the electrons? [1]
      • They whizz around the outside of the atom
    • Do the electrons give an atom it's overall size? [1]
      • Their paths take up a lot of space, giving the atom its overall size (though it's mostly empty space!)
• What do you call an atom with the same number of protons (same atomic number) but a different number of neutrons (different mass number)? [1]
  • Isotope
    • Carbon-14 is an unstable isotope of Carbon-12. What is the difference between Carbon-12 and it's isotope Carbon-14? [1]
      • Carbon-14 has two more neutrons than 'normal' Carbon-12
    • Usually such elements only have one or two stable isotopes (like Carbon-12). The other isotopes tend to be radioactive - the nucleus is unstable.  What does this mean? [2]
      • The nucleus decays (breaks down)
      • and emits radiation as it does so
• Why can't we know precisely how long an atomic nuclei takes to Radioactively Decay (break down)? [1]
  • It's a Random Process
    • Can you make a decay happen by physical conditions like temperature or chemical reactions? [1]
      • No. Each nucleus just decays quite spontaneously in its own good time. It's completely unaffected by external factors.
    • When the nucleus does decay it spits out one or more of the three types of radiation. What are they? [3]
      • Alpha
      • Beta
      • Gamma
    • Can this process change an atom into a completely new element (e.g. Mercury to Gold) [1]
      • Yes - when the number of protons in the nucleus changes, the element changes.
• Describe some of the old models of the atom? [2]
  • Tiny Spheres
    • In 1804, John Dalton agreed with Democritus that 'matter' was made up of tiny spheres ("atoms") that couldn't be broken up. How did this model attempt to explain the many different elements we have? [1]
      • The theory suggested each element was made up of a different type of "atom"
  • Plum Pudding Model
    • Nearly 100 years later, J J Thomson discovered that electrons could be removed from atoms.  So Dalton's theory wasn't quite right (atoms could be broken up). How does the 'plum pudding' model attempt to encorporate this discovery? [2]
      • Atoms are spheres of positive charge
      • with tiny negative electrons stuck in them (like plums in a plum pudding)
• Which scientist and experiment disproved the old models and inspired the 'nuclear model' of the atom? [1]
  • Ernest Rutherford's Geiger-Marsden experiment
    • If the plum-pudding model of the atom was correct, what would you expect to see from the alpha particles? [1]
      • pass straight through the gold foil
    • The shocker was that some of the alpha particles bounced back! What did this reveal about the inner structure of the atoms they hit? [3]
      • Most of the mass is concentrated at the centre of the atom (nucleus)
      • The nucleus is small (and most of the atom is empty space) since very few alpha particles are deflected
      • The nucleus is positive in order to repel the positively charged alpha particles
• Classwork
  • 1) List the three particles that make up an atom
  • 2) Sketch an atom, clearly labelling these particles on.
  • 3) What is the atomic number of a nucleus?
  • 4) What is the mass number of a nucleus?
  • 5) What are isotopes of an element?
  • 6) Oxygen atoms contain 8 protons. What is the difference between Oxygen-16 and Oxygen-18?
  • 7) What does 'unstable' mean?
  • 8) Radioactive decay is a totally random process. Explain what this means
  • 9) Describe Geiger and Marsden's gold foil experiment
  • 10) Describe how the results of this experiment helped Rutherford devise a new model of the atom

• Homework
  • 1) Fill in the blanks using the words in brackets. Each word should be used only once. ( radiation / element / protons / neutrons / nuclei / radioactivity )
    • Isotopes are atoms which have the same number of ........................ but different numbers of .......................... . 
    • Some isotopes are ........................... . 
    • Their ................................. are unstable so they break down and spit out ............................
    • When this happens the nucleus often changes into a new .............................
  • 2) Carbon-14 is radioactive but carbon-12 is not.
    • Explain why in terms of the difference between their nuclei.
  • 3) Indicate whether these sentences are true of false
    • The nucleus of an atom takes up almost no space compared to the whole atom
    • Most of an atom's mass is in the electrons
    • Radioactive decay speeds up at higher temperatures