let's start by defining an atom.
.. "an atom is a tiny particle of matter consisting of a nucleus which contains
.. .at least 1 proton surrounded by a field of electrons. That nucleus MAY
.. .contain 1 more neutrons,"
looks something like this
the blue and red spheres represent protons and neutrons.
now.. imagine this. Imagine you raided your grandmothers pantry and collected a couple of hundred perfectly clean mason jars. and you lined them all up on your lab bench. Then you took a sharpie and wrote on one jar.
.. "atoms with 1 proton"
and on a different jar
.. "atoms with 2 protons"
and on a different jar
.. "atoms with 3 protons"
until all the jars were labeled
now image you have a super powerful microscope. So powerful you could see individual atoms. So powerful that you could actually see inside each atom and see the nucleus. And so powerful that you could actually count protons.
now imagine you have a super sharp pair of tweezers. So sharp that you could pick up a single atom.
now imagine you have a huge pile of atoms. All different shapes and sizes. And you have an infinite amount of time on your hands....
And you start sorting.
.. into the jar labeled "atoms with 1 proton".. ..go all the atoms with 1 proton
.. into the jar labeled "atoms with 2 protons".. go all the atoms with exactly 2 protons
.. into the jar labeled "atoms with 92 protons".. go all the atoms with exactly 92 protons
when you're done sorting, you think to yourself.. "self. atoms with x protons is just too long and too impersonal. I'm going to give these jars better names"
.. and you write on the jar of atoms with exactly 1 proton... "hydrogen"
.. and you write on the jar of atoms with exaclty 2 protons.. "helium"
.. and you write on the jar of atoms with exactly 92 protons.. "uranium"
until all the jars are renamed.
now you start inspecting each jar and you notice something unusual. Although all the atoms have the same number of protons in any jar, they have different numbers of neutrons. The all react chemically about the same. But they have different masses and different stabilities. So you decide you need a naming system to identify atoms in the same jar that have different numbers of neutrons... and you come up with the term.. "ISOTOPES". "Iso" meaning "same".. and "topes" meaning "place". They come from the "same place".. i.e.. they are in the same JAR.
AND, you do one more thing. You identify which of those atoms are stable. And you measure the mass of those atoms on a balance. And you average those masses of those stable atoms (weighted by the percent abundance of each of those stable isotopes) and you write that weighted average mass on the outside of the jar. And you call that weighted average mass... "relative atomic mass"
ATOMS are those individual particles
ELEMENTS are the mason jars
ISOTOPES are atoms with the same number of protons.. with different numbers of neutrons
the PERIODIC TABLE shows the mason jars, and the number of protons in each atom in that jar (the atomic number) and the weighted average mass of the stable atoms of any given element (unless it's shown in parenthesis which means there are no stable isotopes of that element and the atomic mass of the most stable element is shown in the parenthesis).