Structure of the atom

You should be able to describe the nuclear model of the atom.

The nuclear model

The structure of the atom

Atoms contain three sub-atomic particles called protons, neutrons and electrons. The protons and neutrons are found in the nucleus at the centre of the atom. The electrons are arranged in energy levels, or shells, around the nucleus.

The table shows the properties of these three sub-atomic particles.

Properties of sub-atomic particles

The number of electrons in an atom is always the same as the number of protons, so atoms are electrically neutral overall. Atoms can lose or gain electrons. When they do, they form charged particles called ions:

• if an atom loses one or more electrons, it becomes a positively charged ion
• if an atom gains one or more electrons, it becomes a negatively charged ion

Check that you remember and understand the things you need to know about the atom for your Chemistry examination questions. Go to this section in the BBC Bitesize AQA Additional Science.

Isotopes

Atomic number and mass number

The number of protons in the nucleus of an atom is called its atomic number:

• the atoms of a particular element all have the same number of protons
• the atoms of different elements have different numbers of protons

The total number of protons and neutrons in an atom is called its mass number.

Chemical symbols

This is the full chemical symbol for chlorine-35.

The proton number is shown below the chemical symbol, and the mass number is shown above. In the example below, the atomic number is 17 and the mass number is 35. This means that each of these atoms has:

• 17 protons
• 17 electrons
• 35 - 17 = 18 neutrons

Isotopes are the atoms of an element with different numbers of neutrons. They have the same proton number, but different mass numbers.

Three isotopes of hydrogen

isotope	symbol
hydrogen-1 1 proton, 0 neutron, 1 electron
hydrogen-2 1 proton, 1 neutron, 1 electron
hydrogen-3 1 proton, 2 neutrons, 1 electron

Radioactive decay

The nuclei of some isotopes are unstable. They can split up or ‘decay’ and release radiation. Such isotopes are called radioactive isotopes or radioisotopes. When a radioactive isotope decays, it forms a different atom with a different number of protons.

Changing elements

When an atom emits alpha or beta radiation, its nucleus changes. It becomes the nucleus of a different element. This is because the number of protons in the nucleus determines which element the atom belongs to. These are the changes that occur to the number of particles in an unstable nucleus when it emits a radioactive particle:

Changes to particles

In each case, a different element is left behind.

Example 1

Uranium-230 nuclei emit alpha radiation and become nuclei of thorium-226:

Remember that an alpha particle is identical to a helium nucleus. Notice that:

• the mass number goes down by 4
• the atomic number goes down by 2

Example 2

Hydrogen-3 nuclei emit beta radiation and become nuclei of helium-3:

Notice that:

• the mass number stays the same
• the atomic number goes up by 1

Now try a Test Bite - foundation

Evidence for atomic structure - higher

You should be able to interpret information about Rutherford's scattering experiment.

Plum pudding model of the atom

An early model - scientific idea - about the structure of the atom was called the plum pudding model. In this model, the atom was imagined to be a sphere of positive charge with negatively charged electrons dotted around inside it like plums in a pudding. Scientific models can be tested to see if they are wrong by doing experiments. An experiment carried out in 1905 showed that the plum pudding model could not be correct.

Rutherford’s scattering experiment

A scientist called Rutherford designed an experiment to test the plum pudding model. It was carried out by his assistants Geiger and Marsden. A beam of alpha particles was aimed at very thin gold foil and their passage through the foil detected. The scientists expected the alpha particles to pass straight through the foil, but something else also happened.

Some of the alpha particles emerged from the foil at different angles, and some even came straight back. The scientists realised that the positively charged alpha particles were being repelled and deflected by a tiny concentration of positive charge in the atom. As a result of this experiment, the plum pudding model was replaced by the nuclear model of the atom.

Check your understanding of the experiment by studying this activity:

Now try a Test Bite - higher