Inside the nucleus

 

	1. The  atom
Picture 1.6 An atom has a nucleus with electrons outside it.	Inside the nucleus The atom is made from a central nucleus with electrons around the outside. The electrons have a negative charge and very little mass. Most of the mass of the atom is concentrated in the nucleus, which has a positive charge. The positive charge in the nucleus is balanced by the negative electrons, making an atom neutral. But what's inside the nucleus? And what are the fundamental particles that make up an atom?
proton neutron electron m 1 1    1    1800 q + 1 0 - 1 Table 1. The relative mass(m) and the relative charge (q) of the three main sub–atomic particles.	What's in the nucleus? There are two types of particle in the nucleus of an atom: the proton and the neutron. The proton has a positive charge and a large mass (1800 times more than an electron – see table 1). The number of protons in a nucleus is the same for all the atoms of a particular element. It corresponds to the atomic number of that element. For example, if a nucleus contains three protons, then it is part of an atom of lithium (atomic number 3). The neutron has a similar mass to the proton but has no charge. The neutrons help keep the nucleus together.
Picture 1.7 Lithium has 3 positve protons in its nucleus. name symbol protons Z hydrogen H 1 1 helium He 2 2 lithium Li 3 3 carbon C 6 6 Table 2. Some atoms and their atomic numbers.	What do protons do? All atoms of a given element have the same number of protons in the nucleus. This number is the atomic number of the element and we give it the symbol Z. For example, lithium atoms contain 3 protons. Therefore, the atomic number of lithium is 3. Any atom that has 3 protons must be a lithium atom. We can show this information about an atom using its symbol and atomic number: (The symbol for lithium is Li - note it starts with a capital letter although its name, lithium, starts with a small letter.) The atomic number is written to the left bottom of the symbol. Neutrons The elements in the periodic table are arranged in increasing atomic mass. Their atomic numbers correspond to their position in the sequence (as well as to the number of protons in their nuclei). However, the atomic mass is not proportional to the atomic number (see page 6). This tells us that there must be something else in the nucleus as well as protons. For small atomic numbers, the relative atomic mass is approximately double the atomic number. For higher atomic numbers, it is more like 2.5 times the atomic number. i.e. they are not proportional. We can see this clearly by looking at hydrogen and helium.

Picture 1.8 The atomic mass is not proportional to the number of protons. Therefore, there must be something else in the nucleus too.	Helium has atomic number 2 and a relative atomic mass of about 4. Hydrogen has atomic mass and relative atomic mass of 1. If protons were the only particles in the nucleus, then a helium nucleus would have two protons and therefore only twice the mass of hydrogen. However, it actually has four times the mass of hydrogen. This suggests that there is something else in the nucleus as well as protons. This new particle – the neutron - would have no charge. So how could it be detected? Finding the neutron The difficulty with finding the neutron is that it has no effect on charged particles (because it is neutral). This means that it will not knock electrons out of atoms making ions. So it will not leave any visible trace in a cloud chamber or register in a Geiger counter because both of these rely on ionisation to register the presence of particles. However, the neutron does feel the strong nuclear force – the short-range force that holds the nucleus together. So if it passes close enough to a proton it will make it move.
Picturing sub-atomic particles In diagrams, all these particles are made to look a bit like small coloured snooker balls. This is because we have to draw them somehow. However, it is important to understand that they are not like tiny coloured snooker balls – solid with well-defined edges. • They do not have any colour because they are smaller than the wavelength of visible light. So light waves pass straight over them without changing direction. • We cannot talk about their being solid because a solid is a collection of atoms. These particles are smaller than atoms. • They do not have well-defined edges but can influence other particles over a long distance.	In 1932, James Chadwick, who had worked with Rutherford, detected neutrons and measured their mass in an invisible game of billiards. He fired the neutrons at a block of paraffin wax, which has a high concentration of hydrogen and is therefore rich in protons. Some of the neutrons collided with protons in the wax and knocked them out. Chadwick could then detect these protons and measure their energy. Using his knowledge of energy and momentum, he was able to work out the mass of the neutrons from the range of energies of the protons that they knocked out. He found that its mass was slightly more than that of a proton. Chadwick, like Rutherford, used an ingenious method to probe into what cannot be seen.
	Question 3 a) Which of the main sub-atomic particles (proton, neutron, electron) are: i. found in the nucleus? ii. fundamental? b) Why was it so difficult to find direct evidence of the neutron?