* 34 protons: This defines selenium as the element with atomic number 34.

* 34 electrons: In a neutral atom, the number of protons and electrons is equal.

* 45 neutrons (on average): The number of neutrons can vary, leading to different isotopes of selenium. The most common isotope, Selenium-80, has 46 neutrons.

Important Note: The number of neutrons can vary in different isotopes of an element. While the number of protons defines the element, the number of neutrons determines the specific isotope.