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Atomic number, mass number and isotopes
What do we call isotopes that don't undergo radioactive decay?
Here is an atom. This one has a nucleus consisting of three protons and three neutrons... And around the nucleus there are three electrons. One of these "threes" tells us that it is a lithium atom. But which of these threes determines that?
What if there was one less electron? Then you would get an ion, but it would still be an ion of the same element, lithium. What if there was one more neutron? Would that make it another element? No.
There are lithium atoms with four neutrons, so that's not the deciding factor, either. But if we remove or add a proton, then it's not the same element any more. It's the number of protons that determines what element an atom belongs to. There's a special name for the proton count in an atom. It's called the atomic number.
Hydrogen has the atomic number one. That means that an atom with one proton is always hydrogen, no matter how many neutrons or electrons it has. The next element is helium with the atomic number two. It always has two protons. So it was the three protons that made this...
A lithium atom. The number of protons - the atomic number - determines which element it is. What about the number of neutrons? Let's look at these two versions of lithium. If you count the total number of particles in the nucleus, this atom has three protons plus four neutrons, or a total of seven - nuclear particles.
This type of lithium atom is called "lithium-seven". The lithium atom with three neutrons has six nuclear particles in total, so that is "lithium-six". Lithium-7 and lithium-6 are two isotopes of lithium. They have the same proton count, but their neutron count differs. The number of neutrons doesn't affect the chemical properties much, but it does give the atom a different mass.
That's why the number of particles in the nucleus is called the mass number. Just like the atomic number, the mass number is always a whole number, since you can't have 3.5 protons or 1.2 neutrons. Are there more isotopes of lithium than these two? Yes. But...
They are not stable. We can create them in a laboratory, but these other isotopes of lithium are short-lived, and can't be found in nature. Some elements have only one stable isotope. Fluorine, for instance. All fluorine atoms found naturally have nine protons and ten neutrons, so they all have a mass number of..
19. Other elements have a wide range of different isotopes. Tin holds the record, with ten different stable isotopes. They all have the same number of protons - 50, but the neutron count can vary between 62 and 74! There's almost no difference in the chemical behaviour of the isotopes.
It's only the mass that differs. Sometimes, you need to know the average mass of all the atoms of an element. We use that to calculate the number of atoms in a sample by weighing it. The average mass is of course, determined by the mix of isotopes in the element. If more of the heavier isotopes are present, this increases their average mass.
The mass number is the number of protons plus the number of neutrons, or the total number of nuclear particles. Atoms with different mass numbers are the isotopes of that element. Most elements are a mix of different isotopes. The atomic number is the number of protons in the nucleus of the atom. That is what determines which element it is.