It has an atomic number of 7 (Z=7) because it has 7 protons in its nucleus. We write this in front of the chemical symbol to the bottom-left.
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Some nitrogen atoms have 15 nucleons in the nucleus and therefore have an atomic mass number of 15. Again, the atomic mass number, A, comes before the chemical symbol and is positioned top-left.
How does this tell us the number of neutrons?
A is the number of neutrons plus protons in the nucleus.However, we already know that there are 7 protons.Therefore, there must be 8 neutrons in the nucleus to add up to 15.
Since different isotopes of an element have different numbers of neutrons (but always the same number of protons) they have different mass numbers. The isotopes of nitrogen have mass numbers ranging from 12 to 18. We call them nitrogen-12, nitrogen-13, nitrogen-14 and so on.
Some isotopes of nitrogen are unstable. They give out radiation to become more stable; we say they are radioactive. Nitrogen-14 and nitrogen-15 are both stable isotopes of nitrogen. However, the other 5 isotopes are all unstable. Nitrogen-12 and nitrogen-13 will decay by beta plus emission and nitrogen-16, nitrogen-17 and nitrogen-18 decay by beta minus emission.
We can plot a graph of stable isotopes. The atomic number, Z, is on the x-axis and the number of neutrons (N) is on the y-axis. The graph is a curve see picture 2.2.
If we got a straight line all the way, then this would tell us that the stable isotopes have the same number of neutrons as protons. However, this is not the case. The line curves upwards. Stable isotopes of the heavier elements (top right of the graph) have more neutrons than protons. For example, Gold-197 is stable. It contains 79 protons and 118 neutrons.
The neutrons in a nucleus can be thought of as acting as a kind of glue to hold the nucleus together. The positively charged protons are in a very confined space but would rather not be, due to the fact that they repel each other.
However, protons and neutrons are all attracted to each other as a result of another force - the strong nuclear force (see below). The neutrons don"t contribute any repulsive effects because they are neutral. So having more neutrons around can help to hold the nucleus together. Notice that no amount of neutrons can hold a nucleus together once it has more that 82 protons the line stops at Z = 82! All of the elements with an atomic number greater than 82 have only unstable isotopes.
This is the strong nuclear force. It is an attractive force that only has an effect over a very short range in nucleus (about 1015m - the size of the nucleus). The strong nuclear force binds protons and neutrons together to make the nucleus.
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Neutrons and protons are made from quarks (see page 17).The strong nuclear force is actually a force between quarks and is carried by particles called gluons. Protons and neutrons are made of quarks and they feel the strong nuclear force as well. Electrons do not feel the strong nuclear force. Fundamental particles that don"t feel the strong nuclear force are all in the family of leptons.
i. light nuclei (atomic number less than 15)?ii. heavy nuclei (atomic number about 80)?