Atoms and the particle model

The particle model

In Year 7 you used the particle model to explain solids, liquids and gases. It is so well supported by evidence that scientists call it the kinetic theory of matter ("kinetic" means moving). Its main ideas are:

• All matter is made of tiny particles called atoms. They are far too small to see, but they have mass.
• Atoms of the same element are identical to each other and different from atoms of other elements.
• Atoms can join (bond) to form larger particles called molecules.
• Particles are always moving. The hotter a substance, the faster its particles move.
• Atoms aren't created or destroyed in chemical reactions; they are just rearranged.

Think of it as a model: imagining atoms as tiny balls lets us explain and predict how substances behave. Like all models, it has limits. Atoms are not really hard little balls, as scientists discovered a little over a century ago.

Inside the atom

For thousands of years people thought atoms were the smallest things there could be. (The word comes from the Greek atomos, "uncuttable".) Then experiments showed that atoms are made of even smaller subatomic particles:

Almost all of an atom's mass is in its tiny, dense nucleus. The electrons are held around it because opposite charges attract.

• The atomic number is the number of protons. It decides which element an atom is: every atom with 6 protons is carbon, and every atom with 79 protons is gold.
• The mass number is the number of protons plus neutrons.
• In a neutral atom, the number of electrons equals the number of protons, so the charges cancel out.

Atoms of the same element can have different numbers of neutrons. These versions are called isotopes. Most carbon is carbon-12 (6 protons, 6 neutrons), but a tiny amount is carbon-14 (6 protons, 8 neutrons), which is radioactive and is used to date ancient objects. If an atom gains or loses electrons, it becomes a charged particle called an ion.

How we found out

Nobody has ever seen inside an atom directly, so how do we know all this? Through clever experiments, each one improving the model:

• 1897: J. J. Thomson discovered the electron using a cathode ray tube (an early version of the tube in old TVs). Atoms could be split after all.
• 1909: Hans Geiger and Ernest Marsden, working with Ernest Rutherford, fired tiny radioactive particles at gold foil only a few hundred atoms thick. Most went straight through, but about 1 in 8000 bounced back. Rutherford said it was "as if you fired a 15-inch shell at a piece of tissue paper and it came back and hit you". He concluded that atoms are mostly empty space, with a tiny, heavy, positive nucleus.
• 1917–1932: Rutherford identified the proton, and James Chadwick discovered the neutron.
• Today: The Large Hadron Collider, a 27 km ring buried under the border of France and Switzerland, smashes particles together at nearly the speed of light. In 2012 it found the Higgs boson, a particle that helps explain why other particles have mass. Protons and neutrons themselves turn out to be made of even smaller particles called quarks.