Atom

• Atoms are the building blocks of all matter
• They are incredibly small, with a radius of only 1 × 10^-10 m

  • This means that about one hundred million atoms could fit side by side across your thumbnail
• Atoms have a tiny, dense nucleus at their centre, with electrons orbiting around the nucleus

  • The radius of the nucleus is over 10,000 times smaller than the whole atom, but it contains almost all of the mass of the atom
  • They consist of small dense positively charged nuclei, surrounded by negatively charged electrons
• An atom: a small positive nucleus, surrounded by negative electrons

  • the atom is around 100,000 times larger than the nucleus
• Atom structure:

  A) negative electrons

  B) small, positively charged nucleus

  C) 10^-15m

  D) 10^-10m

  
• In 1909 a group of scientists were investigating the Plum Pudding model

  • Physicist, Ernest Rutherford was instructing two of his students, Hans Geiger and Ernest Marsden to carry out the experiment
• Ernest Rutherford Experiment

  • This involved the scattering of alpha (α) particles by a sheet of thin metal supports the nuclear model of the atom
  • A beam of alpha particles (He 2+ ions) were directed at a thin gold foil
  • They expected the alpha particles to travel through the gold foil, and maybe change direction a small amount
• From the experiment, they discovered that :

  • Most of the alpha particles passed straight through the foil
  • Some of the alpha particles changed direction but continued through the foil
  • A few of the alpha particles bounced back off the gold foil
• The bouncing back of the alpha particles could not be explained by the Plum Pudding model, so a new model had to be created

  • This was the first evidence of the structure of the atom
• When α-particles are fired at thin gold foil, most of them go straight through but a very small number bounce straight back

  A) atoms

  
• When α-particles are fired at thin pieces of gold foil:

  • The majority of them go straight through (A) - this happens because the atom is mainly empty space
  • Some are deflected through small angles (B) - this happens because the positive α-particles are repelled by the positive nucleus which contains most of its mass
  • A very small number are deflected straight back (C) - this is because the nucleus is extremely small
• An ion is an electrically charged atom or group of atoms formed by the loss or gain of electrons

  • An atom will lose or gain electrons to become more stable
• A stable atom is normally electrically neutral

  • This means it has the same number of protons (positive charge) and electrons (negative charge)
• Positive ions are therefore formed when atoms lose electrons

  • There will be more protons than electrons
• Negative ions are therefore formed when atoms gain electrons

  • There will be more electrons than protons
• You may hear the term 'net charge'

  • This just means the 'overall' charge of the atom
  • If an atom has 5 protons, 5 neutrons and 6 electrons, it has a net negative charge because it's a negative ion (more electrons than protons)
• Remember which way around the charges are by proton being positive.
• Difference between negative and positive ions

  A) neutral

  B) same number

  C) negatively charged

  D) more

  E) positively chargedd

  F) more