The Atom (CIE IGCSE Physics)

• 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

(Note: the atom is around 100,000 times larger than the nucleus!)

EXTENDED

• 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
• 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 (He²⁺ 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
• Instead, 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 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 

• 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

The difference between positive and negative ions