Atoms of the same element can have different numbers of neutrons!
The different possible versions of each element are called isotopes.
For example, the most common isotope of hydrogen has no neutrons at all; there’s also a hydrogen isotope called deuterium, with one neutron, and another, tritium, with two neutrons.
If you want to refer to a certain isotope, you write it like this: AXZ.
Here X is the chemical symbol for the element, Z is the atomic number, and A is the number of neutrons and protons combined, called the mass number.
For instance, ordinary hydrogen is written 1H1, deuterium is2H1, and tritium is 3H1.
How many isotopes can one element have? Can an atom have just any number of neutrons?
No; there are “preferred” combinations of neutrons and protons, at which the forces holding nuclei together seem to balance best.
Light elements tend to have about as many neutrons as protons; heavy elements apparently need more neutrons than protons in order to stick together.
Atoms with a few too many neutrons, or not quite enough, can sometimes exist for a while, but they’re unstable.
Do atoms just fall apart if they don’t have the right number of neutrons?
Well, yes, in a way. Unstable atoms are radioactive: their nuclei change or decay by spitting out radiation, in the form of particles or electromagnetic waves.
Another way to view three isotopes of the element Hydrogen.
All of these are the same kind of atom. All have the same chemical properties. Yet each has a different total weight!
Adding neutrons does not affect chemistry, but it does affect weight & mass.
9/28 Isotopes of hydrogen: http://kgortney.pbworks.com/w/page/28715420/Class%20Notes
What happens if we build molecules with isotopes? We get two molecules that look the same, and chemically act the same, but have different weights:
Image taken from: http://minerva.union.edu/hollochk/kth/illustrations.htm
So why would this even matter?
We will see that these differences are useful in determining how fossils are made, how nuclear power is produced, and in some extreme cases, how heavy versions of atom are not stable. When they are not stable, they eventually break apart all on their own, releasing radiation, and literally transmuting into new elements! 🙂