Atomic models

The accepted model of the atom has changed throughout history

1) Some ancient Greeks thought all matter was made from invisible particles

2) At the start of the 19th C, John Dalton described them as solid spheres, and each atom has a different type of sphere

Experimental evidence showed that atoms weren’t solid spheres

In 1897, J J Thomson did experiments that proved they weren’t spheres.

1) His measurements of charge and mass showed that an atom must contain smaller, negatively charged particles. ‘Corpuscles’, we call them electrons.

2) New model made, Plum Pudding model, a positively charged sphere with negative electrons embedded in it.

Rutherford showed that the Plum Pudding model was wrong

1) In 1909, Ernest Rutherford conducted the gold foil test where they fired alpha particles at a thin sheet of gold.

2) The pudding model would mean most of these particles would be very slightly deflected.

3) In fact, most particles went straight through the gold atoms, and a small few were deflected backwards

4) Rutherford came up with a model that could explain this, the nuclear model of the atom.

Rutherford’s model was modified several times

1) Henry Moseley discovered that the charge of the nucleus increased form one element to another, in units of one.

2) This led Rutherford to investigate the nucleus further. He finally discovered that in contained positively charged particles he called protons. The charges of nuclei of different atoms could then be explained - the atoms of different elements have a different number of protons in their nucleus.

3) Still one problem, the nucleus was still heavier than if they only contained proton. Rutherford predicted there were particles that had mass but no charge. James Chadwick eventually discovered Neutrons.

The Bohr model was a further improvement

Scientists realised that electrons in a cloud around a nucleus would spiral down into the nucleus and cause the atom to collapse.

Niels Bohr proposed:

• Electrons can only exist is fixed orbital, or shells, and not anywhere in between

• Each shell has a fixed energy

• When an electron moves between shells electromagnetic radiation is emitted or absorbed

• Because the energy of shells is fixed, the radiation will have a fixed frequency

  This helped explain why some elements are inert, because shells can only hold a fixed number of electrons. Atoms will react in order to gain full shells of electrons. When ius has full shells of electrons, it is stable and does not react.

There’s more than one model of atomic structure in use today

The most accurate involves complicated quantum mechanics, you never know where an electron is or what direction it is going, but you can say how likely it is to be at a point in the atom. Oh, and electrons can act as waves as well as particles.