New experimental evidence may lead to a scientific model being changed or replaced.
John Dalton thought that all matter was made of tiny particles called atoms, which he imagined as tiny spheres that could not be divided.
J.J. Thomson discovered the electron. He suggested a plum pudding model of the atom. In this model the atom is a ball of positive charge with negative electrons randomly distributed within it.
Hans Geiger and Ernest Marsden tested the plum pudding model. In the experiment they fired positively charged alpha particles at thin gold foil. Most alpha particles went straight through the foil, but a few were scattered in different directions or were reflected back. Ernest Rutherford explained these results as:
- the mass of an atom is concentrated at its centre, the nucleus (which is very small)
- the nucleus is positively charged
Niels Bohr adapted Ernest Rutherford's model. Bohr did calculations that led him to suggest that electrons orbit the nucleus in fixed energy levels - called shells. The shells are at certain distances from the
Later experiments led to the idea that the positive charge of any nucleus could be subdivided into a whole number of smaller particles, each particle having the same amount of positive charge. The name proton
was given to these particles.
James Chadwick found evidence for the existence of particles in the nucleus with mass but no charge. These particles are called neutrons.
Atoms are made from three types of subatomic particles: protons, neutrons and electrons.
On the Periodic Table you will find every element that we know about, which in turn is a list of every atom that we have discovered.
Next to each element's symbol you will find several numbers:
- The atomic number - this is the number of protons
- The mass number - this is the total number of protons and neutrons
Every atom of the same element has the same number of protons as electrons. This is because every atom is neutral.
An atom has a radius of about 0.1 nm (10–10 m), with about 99% of an atom being empty space! The radius of a nucleus is less than 1/10000 (that's really small!) of that of the atom. Almost all of the mass of an atom is in the nucleus.
The proton number of an element defines the element. Every atom of one element must have the same number of protons.
All atoms have the same number of protons and electrons. This is because atoms are neutral, and this is achieved by having the same number of positive charges (protons) and negative charges (electrons).
Not all atoms have the same number of neutrons. All atoms of carbon have 6 protons, making its atomic number 6. Most of those atoms will have 6 neutrons, making the mass number 12 (6+6). However, some atoms
of carbon have a different number of neutrons, making the mass number different too.
Isotopes are different forms of the same element, they have:
- the same atomic number (same number of protons and electrons)
- a different mass number (different number of neutrons)
Not all atoms have a whole number mass. Chlorine, as an example, has a recorded mass of 35.5. This is because we have to take the masses of all an element's isotopes into account.