Do you remember how protons and neutrons can be squashed together to make atomic nuclei?

Nuclei have a positive charge, just like protons. However, a nucleus with two protons and two neutrons doesn't have the same positive charge as a single proton: the charge of the protons add up, so you have a charge of +2. Now, imagine that an electron comes wandering by. Of course, it gets attracted by the positive charge of the nucleus and stays there in a kind of partnership. An electron has an electric charge of -1, so the two of them have an overall charge of +1. So, if a second electron comes along, it too gets attracted. Now you have an electrically neutral atom, a helium atom.

You can do the same thing with any nucleus. To get an electrically neutral atom, you always have to have the same number of electrons dancing around the nucleus as there are protons inside the nucleus.

Why do we discuss this rule? Because there are a few laws that forbid the elctrons to go around the nucleus in exactly the same place. They have to dance around the nucleus in shells which are kind of like the layers of an onion.In particle physics you will soon find that all the particles want to have the least possible amount of energy. An atom has the lowest energy level if the outermost electron shell is complete. Atoms can't decide how many electrons they want to have in the outermost shell, that depends on the nucleus. But they can start a partnership with other atoms and share their outer electrons. A whole science is based on these partnerships, chemistry, but we'll leave this chapter to the chemists.

There is a classification system of atoms based on the number of protons in the atom. In the periodic table, as it is called, every possible number of protons gets a name; an atom with one proton is called hydrogen, one with two is helium, etc. We call the different names "elements". You can look them all up in a periodic table. Some elements were named after famous scientists like the 99th element Einsteinium. Right next to it is Fermium, element number 100, named after Enrico Fermi. The Curies got the element Curium with 96 protons. Bohr senior and junior were honored with element number 107, and Lise Meitner with 109. Rutherford's element has 104 protons.

There are two fundamentally different ways to notate an atom. On the left you see the way the chemists do it. For them it doesn't make a difference exactly how many neutrons are in an atom since they always work with millions of atoms at one time, so they just notate the average number of neutrons in a given element.

On the right is the way particle physists notate an atom. There you can find out exactly how many neutrons a specific atom has.

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