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An isotope (iso - "the same," tope = "place") is a variant.

Each element (in the Periodic Table) consists of a nucleus (containing neutrons and protons), with electrons orbiting around it.

The atomic number of an element is the number of protons in the nucleus.

The mass number (*) of an element is the number of (protons + neutrons) in the nucleus.

An isotope is a variant, with the number of neutrons being the variable.

For example, Carbon (with an atomic number of 6) is normally the isotope C-12, with 6 protons + 6 neutrons.

However, there are also C-11 and C-13 isotopes. You subtract one neutron to get C-11, and add one neutron to get C-13.

This is all that an isotope is: a variant on the number of neutrons inside the nucleus of a chemical element.

If the following statement is not instantly clear, please go back and reread this post:

"For a given element, different isotopes have the same atomic number, but a different mass number."

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(*) The mass number of an element is the number of (protons + neutrons) in the nucleus, as stated above. 
 
The key words in the above definition are "mass" and "nucleus," and you'll see why in a moment.
 
The "mass number" is alternatively called the "atomic mass number" and the "nucleon number," for two self-defining reasons:
 
1) It's called the "atomic mass number," as opposed to simply the "mass number," because it represents nearly the entire mass of the entire atom.
2) It's called the "nucleon number" because both protons and neutrons reside inside the nucleus of an atom. 
 
A nucleon is defined as "any subatomic particle (i.e. a proton or a neutron) that resides in the atomic nucleus."
 
Both protons and neutrons are nucleons, but electrons aren't, because electrons revolve around, and reside outside of, the atomic nucleus.
 
The mass number of an element is the number of nucleons in the nucleus.
 
Reading 1) and 2), you might deduce that protons and neutrons have a LOT more mass than electrons, and you'd be correct. How much more mass?
 
If a neutron has a mass of 1, then a proton has a mass of nearly .999, but a revolving electron has a negligible mass of only about .0005 - only 1/2000th as much as a proton or a neutron.
 
This is why nearly 100% of the atomic mass is found inside the nucleus.
 
If you go back and read every sentence in both of these posts, it should all be readily understandable, even trivial: It's just a matter of memorizing a few basic terms and their corresponding definitions, all of which are extremely simple.
 
You now know what an isotope, atomic number, mass number, and nucleon are.
You now know that the nucleus contains virtually the entire mass of an atom.
 
This thread could be titled, "Things I learned in fifth grade, but then forgot."

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I'm always blown away by the thought that everything is 99.999999% empty space. It's as though reality barely exists.

It's funny that you say this, as I'm not convinced that there's any such thing as "empty space"; I think we simply haven't discovered what this "emptiness" is yet.

Planets revolving around a star in a solar system, and electrons revolving around a nucleus in an atom ... the two things are so similar that it can't be mere coincidence.

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Planets revolving around a star in a solar system, and electrons revolving around a nucleus in an atom ... the two things are so similar that it can't be mere coincidence.

Not exactly ... from Atomic Orbitals on chemguide.co.uk:

"When a planet moves around the sun, you can plot a definite path for it which is called an orbit. A simple view of the atom looks similar and you may have pictured the electrons as orbiting around the nucleus. The truth is different, and electrons in fact inhabit regions of space known as orbitals"

"This page tries to explain why the idea that electrons orbit a nucleus like planets around the sun is wrong."

also, this.

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