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The model of the atom has been developed over the years as advances have been made in scientific methods and technology. The work of many scientists over the years has contributed to the model of the atom we use today.
Dalton’s model
In 1830, John Dalton – a scientist who studied gases in the 19th century – stated that all atomic structure was made up of atoms which he believed to be tiny, hard spheres. Dalton claimed that some substances were made up of identical atoms. These substances we now know as elements. Dalton believed that atoms of an element have the same mass and that each different element had its own type of atom with different masses.
The plum pudding model
In 1897, J.J. Thompson discovered that atomic structure was actually not a hard sphere but in fact contained smaller negatively charged particles which had very little mass. Thompson had discovered the first of the sub-atomic particles: the electron. Thompson thought that an atom looked like a ball of positive charge with electrons dotted through it, like plums in a pudding. This is why the model became known as the plum pudding model.
The Rutherford model
In the early 20th century, the scientist Ernest Rutherford discovered the presence of the nucleus of the atom by looking at the findings from experiments conducted by two scientists working in his laboratory. These two scientists were Hans Geiger and Ernest Marsden, and they had been conducting experiments looking at the behaviour of alpha particles.
Alpha particles will be explained in more detail later in the chapter but in brief they can be described as helium nuclei and are uncharged particles. They fired alpha particles at a very thin layer of gold foil, one atom in depth.
If Thompson’s model of atomic structure was correct, the alpha particles should either pass straight through the gold foil or be only slightly deflected.
The set-up for Geiger and Marsden’s experiment is shown in the diagram below:
The zinc sulphide screen emits flashes of light known as scintillations when it is hit by alpha particles. When they fired the alpha particles at the screen, Geiger and Marsden recorded any flashes which occurred.
They found from their observations that the majority of alpha particles passed through the gold foil as expected. However, there were also a significant number of particles which were deflected more than expected and also some particles which were reflected back away from the foil.
Ernest Rutherford analysed these experimental observations and used them to make some deductions about the structure of the atom. As most of the alpha particles passed straight through the gold foil, he deduced that the majority of each gold atom must be empty space. The particles which were reflected must have hit something large. Rutherford used this observation to deduce that most of the atoms’ mass must be concentrated at the centre of the atoms and that there must be some positive charges in the nuclei which repelled the positive charges of the alpha particles.
The model of the atom Rutherford devised from these deductions included a positively charged nucleus surrounded by a cloud of negative electrons which constantly moved around the nucleus. A scientist called Niels Bohr developed this model and came up with a theory that the electrons orbited the nucleus in specific energy shells or energy levels. Experimental data supported this theory.
In 1932 a scientist called James Chadwick developed these ideas further and used experimental evidence to prove the existence of other sub-atomic particles known as neutrons and discovered that these neutrons were cited in the nucleus along with positively charged sub-atomic particles known as protons. Chadwick’s model is used as the basis of the atomic model used today.
Current atomic modelling
The model used today to describe the structure of the atom shows that atoms contain electrons, protons and neutrons. The electrons are found orbiting the nucleus in energy levels. The protons and neutrons are found in the nucleus. The nucleus is found at the centre of the atom and it has a radius which is 10,000 times smaller than the radius of the atom.
Protons and neutrons can be collectively known as nucleons as they are both found in the nucleus. Protons are positively charged, electrons are negatively charged and neutrons have no charge and are therefore neutral. Protons and neutrons both have a relative mass of one, whereas electrons have practically zero mass.
The relative masses and charges of the sub-atomic particles are summarised in the table below:
| Sub-atomic particle | Relative mass | Relative charge |
| Electron | 0 | -1 |
| Proton | 1 | +1 |
| Neutron | 1 | 0 |
Each element has its own chemical symbol. The chemical symbol of all elements is shown on the periodic table of the elements. The chemical symbol shows you the abbreviated symbol for the element, as well as the atomic number and mass number for that element. The chemical symbol for oxygen is:
Mass number (nucleon number)
The mass number of an element describes the total number of protons and neutrons found in the nucleus of the atom. This may also be known as the nucleon number as it describes the total number of nucleons in the nucleus. The symbol used for the mass number of an element is Ar.
Atomic number
The atomic number of an element describes the number of protons in the nucleus of the atom. This is also known as the proton number. The elements in the periodic table are arranged in order of their proton number.
The mass number and atomic number of the element can be used to determine the number of protons, neutrons and electrons an atom has. For example, the atomic number for oxygen is 8. This tells us that oxygen has 8 protons. Atoms have no overall charge. This means that the number of positively charged particles must be equal to the number of negatively charged particles. Therefore, the number of protons is always equal to the number of electrons in an atom. For example, as oxygen has 8 protons, it must also have 8 electrons.
To work out the number of neutrons in the nucleus of an atom, you subtract the proton number from the mass number for that element. The number of protons and neutrons in the nucleus is 16. If 8 of these nucleons are protons, 8 of them must be neutrons.
Isotopes
The number of protons and electrons in an atom are always equal and all atoms of the same element have the same number of protons and electrons. For some elements there can be a variation in the number of neutrons in the nucleus of the atom.
Atoms of the same elements which has the same atomic number but a different number of neutrons are known as isotopes. If the number of neutrons changes, the atomic mass of the element also changes. For example, carbon can exist as carbon-12 and carbon-14. Carbon-12 has a mass number of 12 and 6 protons in its nucleus. The number of neutrons in carbon-12 is also 6.
Carbon-14 has a mass number of 14. The number of protons is equal to the number of protons in carbon-12. Therefore, carbon-14 also contains 6 protons. However, the number of neutrons in the nucleus of carbon-14 is 8.
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