Question

What forms when carbon-14 decays?

Answer 1

Carbon-14 decays to nitrogen-14.

Explanation:

See https://en.wikipedia.org/wiki/Carbon-14, which states that carbon-14 decays thusly:

#""_6^{14}"C" → ""_7^{14}"N" + e + \overline{ν}_e#

This is a typical electron beta decay. The electron, `e`, carries off a negative charge leaving the nucleus with an additional positively charged proton and one fewer neutron. The additional proton, increasing the atomic number from `6` to `7`, is what converts the carbon to nitrogen. The antineutrino `\overline{ν}_e` is so weakly interacting that it is hardly observed, but it is needed to keep the reaction consistent with physical conservation laws. The half-life of the decay is about 5700 years.

The above reaction can be reversed, regenerating carbon-14, by the action of cosmic rays on atmospheric nitrogen. In this way carbon-14 maintains a continued existence even after billions of years on Earth, long after it "should have" all decayed.

The concentration of carbon-14 in organic matter can be used to date that matter, up to roughly 50,000 years depending on the method and sample preparation. See the Wikipedia sute referenced above, and also https://www.radiocarbon.com/about-carbon-dating.htm.