The periodic table of the elements is a list of all the different types of atoms we have discovered. It is like the alphabet, which lists all the letters we have. This tutorial will not teach all there is to know about the periodic table, but just how to read each atom’s label. Let’s use hydrogen to start:
1. A letter which tells you what atom you’re dealing with.
2. The number on the top which is called the atomic number and tells you how many protons are in the atom.
3. And the number on the bottom called the atomic weight and tells you how much the atom weighs. When it comes to weight, the proton has a weight of 1 and basically so does the neutron (although neutrons are slightly heavier than protons).
Note that hydrogen only has one proton and no neutron in its nucleus, and its atomic weight is basically 1. So you can see that each proton has a value of essentially 1 when it comes to atomic weight. Helium has 2 protons and 2 neutrons in its nucleus, so it’s atomic number (the number of protons) is 2 and its atomic weight is basically 4 (see below).
So, if you have an atom’s atomic number and atomic weight, you can figure out how many neutrons it has by subtracting the atomic number from the atomic weight. Electrons are so light, they don’t change the weight of the atom very much and won’t affect your calculations. So in the case of Helium, the atomic weight (4.00260) minus the atomic number (2) gives a value of 2.00260. This is translated as meaning there are 2 neutrons in the nucleus.
Just to be clear, the boxes for each atom tells you the same information than the picture models of each atom:
If we go further down the periodic table, we get very large atoms with many protons, electrons and neutrons in them. Hafnium, for example has an atomic number (the number of protons) of 72. But it has an interesting atomic weight of 178.49. If we use our calculation to determine the number of neutrons, we will get the stranger number 106.49. How can we have a half neutron? We don’t. Remember that the atomic weight of an atom is its weight. Not the number of protons added to the number of neutrons. And because the atomic weight is the average weight of the atom. For reasons we will discuss in the next section, atoms of the same type sometimes come in different versions of themselves. That is, with different numbers of neutrons. Scientists use the average number of neutrons to determine atomic weight. So the atomic weight will always be a number with some extra decimal places. It won’t be a whole number.
“But what am I to do with the number 106.49?”
Easy, just round it to the nearest number. And elementary math teaches you that anything under 0.5 is rounded down. Anything that is 0.5 or more is rounded up. Therefore, 106.49 is rounded down to show us that there are 106 neutrons in the atom Hafnium:
Remember that the atoms on the periodic chart are neutral. They carry no charge because they have the same number of electrons as protons. So if you have a neutral atom and you know its atomic number, you will also know the number of electrons it has.
Yet another way you may see the atomic number and atomic weight described is with superscripts and subscripts:
The element represented here is Nickel (Ni). It’s atomic number is 28 which means it contains 28 protons in its nucleus. It’s atomic weight is 59. If we use our formula we can now determine how many neutrons Nickel has: Atomic weight, 59, minus atomic number 28 equals 31. Therefore we now know Nickel has 31 neutrons. Further, because there is no positive or negative sign attached to Nickel, we know there has to be 28 electrons.