We have covered the significance of protons in this post. Let's move on to neutrons, another type of subatomic particles that you'll find in nucleus. In one word, neutron would determine "isotope".
Isotopes
Recall that we say neutrons have a mass of 1 amu, atoms with the same number of protons but different number of neutrons will have different masses too, although they are atoms of the same element, these are called isotopes.
Almost all elements have different isotopes naturally. For example, hydrogen (\( \ce{H} \)) has three isotopes, hydrogen-1 (\( \ce{^1_1H} \)) has 1 proton and no neutron, hydrogen-2 (\( \ce{^2_1H} \)) has 1 proton and 1 neutron, and hydrogen-3 (\( \ce{^3_1H} \)) has 1 proton and 2 neutrons. Since all of them have the same number of protons, they have the same identity hydrogen, but they do have different masses, ie. 1 amu, 2 amu, and 3 amu respectively.
| Element | Isotopes | ||
|---|---|---|---|
| Hydrogen | hydrogen-1 | hydrogen-2 | hydrogen-3 |
| Carbon | carbon-12 | carbon-13 | carbon-14 |
Typical isotopes of hydrogen and carbon
Isotopes have similar chemical properties, meaning they would behave almost the same during a chemical process. But they would have different physical properties, eg. they have different density.
Naturally, all isotopes of a particular element will mix together in a certain proportion, which does not change with the source of the atoms. For example, no matter where you get the carbon (\( \ce{C} \)) atoms from, you will always find 98.9% being carbon-12 (\( \ce{^12_6C} \)) atoms which have 6 protons and 6 neutrons, and 1.1% being carbon-13 (\( \ce{^13_6C} \)) atoms which have 7 neutrons. We'll talk about how to get these numbers and how to use a symbol to represent an isotope in the next post.
Since the proportion of a certain isotope in atoms of a particular element is not changed, we do not really worry about any particular isotope when we study a chemical process. Instead, we consider atoms of different isotopes as a whole. In other words, we study elements rather than isotopes. The presence of isotopes would simply change how an element behave, but its behavior will be identical in different chemical processes.
Carbon-14 dating
However, isotopes do find their special applications in real life. One typical example is carbon-14 dating.
Carbon-14 (\( \ce{^14_6C} \)) is yet another isotope of carbon which contains 6 protons and 8 neutrons in its nucleus. It is formed in upper atmosphere where radiations from the space would turn nitrogen atoms in atmosphere to carbon-14 atoms. On the other hand, carbon-14 atoms are not very stable, they would gradually turn back into nitrogen atoms. In this way, the content of carbon-14 atoms in atmosphere is roughly maintained the same.
When a tree is alive, it will constantly exchange carbon atoms with the atmosphere, therefore, its trunk will have a content of carbon-14 atoms similar to that in the atmosphere.
However, when the tree is cut down and used to produce a piece of paper, the exchange of carbon stops. Carbon-14 atoms will no longer enter this piece of paper. But we also said that carbon-14 atoms are not stable, and they would keep turning into nitrogen atoms. As a result, the content of carbon-14 atoms in the paper will slowly drop. If we are able to know how fast carbon-14 atoms would turn into nitrogen atoms, as well as to detect the content of remaining carbon-14 atoms in the paper, we may determine the age of the paper by simply calculating how long it takes to have that difference of carbon-14 contents. Sounds pretty simply, doesn't it?
The question is, do we know how fast carbon-14 atoms turn into nitrogen atoms? Yes we do. It is interesting to know that carbon-14 always half its content every 5730 years. For instance, if there are 1000 carbon-14 atoms, after 5730 years, there will be only 500 left, while the other 500 would have been trumed into nitrogen atoms. The rest 500 carbon-14 atoms would then become 250 carbon-14 atoms after another 5730 years, and then 125, 63, 32, and it continues.. The time taken for the number of atoms to become half is called half life.
As a result, if we find that there are 5000 carbon-14 atoms in a gram of tree trunk, but we only see 1250 carbon-14 atoms in 1 gram of a particular piece of paper, then we could say that this piece of paper has been existing for two half-life, ie. 11460 years. This method has been widely adopted to determine the age of artifacts such as paintings.