The atom - Architecture and nucleus

Atoms are the preferred buiding blocks as a start. They are not the smallest building block in nature and we will be looking at the atom's components, but it is a fine and manageable item that most people can relate to.

The short description of an atom is that it is spherical and consists of a positively charged core and negatively charged electrons moving around the core. What gives us the difference between atoms is what is in the core and how the electrons move around the core. In atoms we call the core the nucleus. Let us take a closer look at how this works.

If we start with the most simple part of the atom, we have the protons. All atoms have protons; they are the unit in the atom's core/nucleus defining the atom. A proton is a spherical unit with a positive charge, and the number of protons in the nucleus defines the elements. One proton is hydrogen, six protons makes it carbon, 26 protons and we have irons, and so forth.

In the nucleus, along with the protons, we have neutrons. Neutrons are spherical units with no charge, and they are the insulating material in the nucleus. If the nucleus contains more than one proton, the positive charges of the protons will repel each other and split the nucleus, if there is nothing to isolate/shield the charges. Contrary to protons, there is no fixed number of neutrons allowed in the nucleus. An element can exist in several versions having different numbers of neutrons in the nucleus. These versions are called isotopes. Changing the number of neutrons in the nucleus, does not change the atom's properties, except for weight and possibly the stability of the nucleus. This we will get back to. Protons and neutrons are collectively called nucleons.

How do we put this in writing?

Let us start with the simplest nucleus we have at our disposal: One proton. This is the element hydrogen in its simplest form, and we write it like this:

1

1

H. The upper 1 indicates that there is one nucleon in the nucleus, and the lower 1 indicates that there is one proton in the nucleus, i.e. we have no neutrons in this nucleus. This is the only element that does not need to have neutrons in the nucleus, as there are no other positive charges in the nucleus. In all other elements there will always be neutrons present.

Neutrons not being required is not the same as not being present. Hydrogen has an isotope called deuterium. Deuterium is written as

2

1

H, which means that it is a hyftogen atom with two nucleons. One of them must be a neutron, otherwise it would not be hydrogen, so this isotope has one neutron. Similarly the isotope tritium,

3

1

H, is a hydrogen atom with two neutrons.

Often the elements are written as ¹H, as the lower number is redundant information. You cannot have a hydrogen atom with any other number of protons than one; that would make it another element. In some parts of physics it makes sense to have the number present as a help, but in chemistry it is actually better to leave it out.

When protons and neutrons are packed in the nucleus, not all combinations are working equally well, and we have some size limits for the total size of the nucleus. For hydrogen 0 and 1 neutrons are fine. At 2 or more neutrons in the hydrogen's nucleus, the nucleus becomes unstabel and fall apart. The splitting is called fission, and the process is called decay. These are commonly known as radioactive isotopes. At 83 protons, which is the element bismuth, there is a natural limit for the number of protons allowed in a nucleus while still remaining stable. All element from polonium (element no. 84) up have no stable isotopes.

The number of protons and neutrons alone in the nucleus are not the only factor when it comes to stability. The ratio between protons and neutrons is important as well. Technetium and promethium cannot, though they have 43 and 61 protons, respectively, form stable isotopes at all, and e.g. bromine is stable as ⁷⁹Br, unstable as ⁸⁰Br, and stable once again as ⁸¹Br.

The way we have written the isotopes so far, it has been the isotopes in their ground state, i.e. at the lowest enegy level. Some isotopes can exist in an excited state which is so stable you can make measurements. If you have excited isomers of an isotope, you add and m to the isotope number. There is thus both a ⁷⁹Tc and a ^79mTc. If you have multiple excited states for the isotope, they are called m1, m2, etc. as the energy level increases. ¹⁷⁸Hf can thus be found in the ground state ¹⁷⁸Hf and the two excited states ^178m1Hf and ^178m2Hf.


Why is this important?
Several reasons:

1. Radioactivity, which is a byproduct of the unstable isotopes decaying, is hazardous, if it is not handled the proper way.
2. Unstable isotopes decaying gives off energy. This is used in nuclear power plants, where the uranium isotope ²³⁸U decays.
3. There is a big difference in how unstable the isotopes are, what kind of rediation they produce, and how much energy is released when decaying. The isotope ²³⁵U, which is more common than ²³⁸U, is unstable, but not enough to be useful for nuclear energy, which is why ²³⁸U is used.
4. The unstable isotopes have the same chemical properties as the stable isotopes, until decaying. Unstable isotopes with a long lifespan and low radioactivity, e.g. the phosphorus isotope ³²P, can therefore be used to track chemical or biological reactions. If, e.g. you feed a mouse with ³²P marked medicin (i.e. some of the stable phosphorous atoms have been exchanged with the radioactive isotope), you can measure where the radioactivity ended up in the animal. This way you can see if the medicine reaches the right organ or just pass through the mouse and comes out again.