Here's the breakdown of protons, neutrons, and electrons in a selenium atom:

* Protons: The number of protons defines an element. Selenium (Se) has 34 protons.

* Neutrons: To find the number of neutrons, you need to know the isotope of selenium. The most common isotope is selenium-80, which has 46 neutrons (80 - 34 = 46). Other isotopes of selenium will have different numbers of neutrons.

* Electrons: In a neutral atom, the number of electrons is equal to the number of protons. So, selenium has 34 electrons.