A question about isotopes...

[Barbatos]

My geology instructor was discussing some basic chemistry in today's lecture and claimed that atoms by definition contain an equal number of neutrons and protons and that when the number of neutrons differ it's called an isotope. Now, unless I'm gravely mistaken, the number of neutrons has nothing to do with whether or not something is an atom, otherwise most hydrogen couldn't be classified as being an atom, etcetera. I'm also aware that an isotope is simply an atom of varying numbers of neutrons, regardless of whether or not that number is even with the number of protons in the molecule. This all did make me wonder though, is there a term for an isotope that has an equal number of protons and neutrons?

[Mr Skeptic]

Well, you are right. An atom is anything with one or more protons in the nucleus, and the element is based on the number of protons. The number of neutrons has no bearing on whether something is an atom or not, nor on what element it is. Different isotopes do have slightly, slightly different chemical properties, not that you would notice on anything other than hydrogen isotopes.

 

The ratio of neutrons to protons required for stable isotopes increases as the mass of the atom increases. Only rather light atoms would have a 1:1 ratio, if they are supposed to be stable.

[Barbatos]

That makes sense, thanks so much.

[Horza2002]

Mr Skeptic, you can also use C12/C14, N14/N15 and O16/18 for studing isotope effects (mainly in mechanism determination). Obviously the effect is drasitcally smaller than the hydroge deuterium one but it can be measured......as I've had the joys of doing recently!

[StrontiDog]

My geology instructor was discussing some basic chemistry in today's lecture and claimed that atoms by definition contain an equal number of neutrons and protons and that when the number of neutrons differ it's called an isotope.. . .

 

I think your instructor meant to say 'Element' instead of atom. One very common element in the earth's crust is Thorium. With 90 protons, it's atomic number is 90. The Element should therefore have an atomic weight somewhere around 180, but if you look it up, you'll find it a tad over 232.

 

This is because 'elemental' Thorium does not exist, and has probably never been produced. ²³²Th is listed as 100% abundant, there are only 29 isotopes of Thorium that have ever been identified, and all 28 others make up less than one percent of the Thorium on the planet.

 

Very basically, for the heavier elements. You need extra neutrons in there to keep the protons from repelling each other due to their same charge. For ²³²Th, this means there are 52 extra neutrons in the nucleus to keep it stable. And with a radioactive half-life of around 14 billion years, that ain't bad.

 

The 'lightest' Thorium isotope ever identified , is ²¹⁰Th, with a half-life of 9 milliseconds.

 

If you're studying geology, you're going to see a lot of Thorium, just about everywhere you look. It's all ²³²Th, and it's more radioactive than you might think.

 

A chunk of pure Thorium has a respectable dose rate, and will emit alpha particles, as well. It may be natural, but it’s got some punch to it.

 

Handle with care.

 

Bill Wolfe

[swansont]

Very basically, for the heavier elements. You need extra neutrons in there to keep the protons from repelling each other due to their same charge. For ²³²Th, this means there are 52 extra neutrons in the nucleus to keep it stable. And with a radioactive half-life of around 14 billion years, that ain't bad.

 

To be pedantic here, the protons still repel each other. The extra neutrons are required for the nuclear attraction to exceed the electrostatic repulsion. And Th-232 is unstable, though far less unstable than other isotopes.