Let’s say an atom is missing a neutron or has an extra neutron. That type of atom is called an isotope.
An atom is still the same element if it is missing an electron. The same goes for isotopes. They are still the same element. They are just a little heavier or lighter from other atoms of the same element.
For example, there are a lot of carbon (C) atoms in the Universe. Most are carbon-12. They have 6 neutrons, plus 6 protons.
But a few Carbon atoms may have 7 or 8 neutrons – yet the same number of protons (6). So they have more mass (they “weigh” more.)
You will probably hear about carbon-14. Carbon-14 has 8 neutrons (2 extra). C-14 is an isotope of the element carbon.
Messing with the Mass
If you have looked at a periodic table, you may have noticed that the atomic mass of an element is rarely an even number. That happens because of the isotopes. If you are an atom with an extra electron, it’s no big deal. Electrons don’t have much of a mass when compared to a neutron or proton.
Atomic masses are calculated by figuring out the amounts of each type of atom and isotope there are in the Universe. For carbon, there are a lot of C-12, a couple of C-13, and a few C-14 atoms.
When you average out all of the masses, you get a number that is a little bit higher than 12 (the weight of a C-12 atom).
The average atomic mass for the element is actually 12.011. Since you never really know which carbon atom you are using in calculations, you should use the average mass of an atom.
Bromine (Br), at atomic number 35, has a greater variety of isotopes. The atomic mass of bromine (Br) is 79.90.
There are two main isotopes at 79 and 81, which average out to the 79.90amu value.
The 79 has 44 neutrons and the 81 has 46 neutrons.
While it won’t change the average atomic mass, scientists have made bromine isotopes with masses from 68 to 97.
It’s all about the number of neutrons.
Returning to Normal
If we look at the C-14 atom one more time, we find that C-14 does not last forever. There is a time when it loses its extra neutrons and becomes C-12. The loss of those neutrons is called radioactive decay.
Decay happens regularly, like a clock. For carbon, half of the atoms decay happens in a few thousand years (5,730 years). Some elements take longer, and others have a decay that happens over a period of minutes.
Archeologists use their knowledge of radioactive decay when they need to know the date of an object they dug up. C-14 locked in an object from several thousand years ago will decay at a certain rate. With their knowledge of chemistry, archeologists can measure how many thousands of years old an object is. This process is called carbon dating.