Both $"^18O$ and $"^16O$ are found in nature. However, $"^16O$ Is the most common. Why?

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Answer 1 · 2016-10-29T18:05:18

$""_8^16"O"$ is the most common isotope because it is one of the main products formed in stars.

Both $""_8^16"O"$ and $""_8^18"O"$ are stable isotopes, and both were originally formed in stars.

Formation of $""_8^16"O"$

When a star runs out of hydrogen to fuse in its core, it begins to collapse until the central temperature rises to $10^8color(white)(l) "K"$ .

At this temperature, the α particles can fuse rapidly, and $""_8^16"O"$ is formed in a "triple alpha" process.

$""_2^4"He" + ""_2^4"He" → ""_4^8"Be"$

$""_4^8"Be" + ""_2^4"He" → ""_6^12"C"$

$""_6^12"C" + ""_2^4"He" → ""_8^16"O"$

Formation of $""_8^18"O"$

In contrast, $""_8^18"O"$ is formed mostly in massive stars.

The processes involved are

$""_6^12"C" → ""_7^13"N" → ""_6^13"C" → ""_7^14"N" → ""_8^15"O" → ""_7^15"N"→ ""_8^16"O" → ""_9^17"F" → ""_8^17"O"→ ""_9^18"F" → ""_8^18"O"$

Thus, $""_8^18"O"$ is much less abundant than $""_8^16"O"$ .

Both "^18O"^18O and "^16O"^16O are found in nature. However, "^16O"^16O Is the most common. Why?