When the nucleus of a nitrogen atom absorbs a proton, it transforms into an oxygen atom. Here's why:

* Nitrogen (N) has an atomic number of 7, meaning it has 7 protons in its nucleus.

* Oxygen (O) has an atomic number of 8, meaning it has 8 protons in its nucleus.

When a proton is absorbed, the number of protons in the nucleus increases by 1. This changes the element from nitrogen (7 protons) to oxygen (8 protons).