P2bL12 Inside the Atom

Key Words

Electrical force - force of attraction or repulsion due to electric charges.

Plum Pudding Model - an early attempt to explain the structure of the atom.

Strong Nuclear force - a force that stops the nucleus flying apart.

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Physics GCSE
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Grade E

Models of the Atom

When the atom was first discovered, physicists gave a lot of thought as to what the atom consisted of.  J J Thompson proposed the plum pudding model, in which electrons were placed like cherries in a matrix of positive charge.  The neutron had not yet been discovered.

Most people thought that the atom was like this.  Nobody had any reason to believe otherwise until Ernest Rutherford, a New Zealand physicist, proved otherwise in the early 1900s.

 

Rutherford bombarded a thin layer of gold atoms with alpha particles.  He was using the alpha particles like bullets, expecting to see the atoms burst like watermelons.  He described his experiment as firing artillery shells at tissue paper.  To his surprise he found that:

  • Most particles went straight though;

  • While other particles were deflected.

  • A tiny proportion of the alpha particles bounced back in the direction they came from. 

Grade C

This is what Rutherford found:

This alpha scattering showed some amazing facts about the nucleus:

  • The nucleus is very small;

  • The nucleus is very dense;

  • Most of the atom is empty space;

  • The repulsion of the positively charged alpha particle showed that the nucleus is positively charged.

  • The electrons are orbiting the nucleus.

The alpha particles are NOT refracted.

 

This discovery led to the idea of the nuclear atom.  This was developed further by Neils Bohr, a Danish physicist (and goalkeeper of the Danish Olympic football team).  It is the model of the atom shown at the start of this topic.  The neutron was discovered twenty years later by an English physicist, Chadwick.

 

Since the nucleus is so small, the size of an atom is governed by the size of the electron shells.  Therefore big atoms and small atoms are all roughly the same size, about 10-10 m in diameter.

Grade A

If we have all the protons packed into one tiny space, there is an immense force of repulsion, since two positives will repel.  A calculation will show that the repulsive force is about 30 N, a massive force on something so tiny.  Therefore there must be something that stops the nucleus flying apart.  The protons and neutrons are held together by the strong nuclear force:

  • It is immensely powerful, about 100 stronger than the electrical force;
  • It has a very short range (10-15 m).