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Grade 11 chem Q. Difference between stable isotope and ra

How do you know if an isotope is stable or unstable? Is there a certain limit as how many extra neutrons an atom can have that causes this or is there another reason. Our teacher didn't explain this and neither does the textbook, anyone know the answer to this?

February 15, 2013

1 Comment • Newest first

Oyster

For elements with a small atomic number, the number protons should approx. equal the number of neutrons. As the atomic number increases, the most stable nuclei should contain 1.3~1.7 times amount of neutrons to protons ratios.

Here is a graph: http://en.wikipedia.org/wiki/File:Table_isotopes_en.svg
The chart shows that the stable nuclei should not be greater than 83 protons.

Element 83 (bismuth) is the last "stable" element. Bi-209 is considered "stable" because it has a half-life of 10^19 yrs.

Reply February 15, 2013 - edited