An atom of beryllium, in its neutral state, contains 4 protons, 5 neutrons (for the most common isotope, Beryllium-9), and 4 electrons. The number of protons defines the element as beryllium. The number of neutrons can vary, resulting in different isotopes of beryllium. The number of electrons is equal to the number of protons in a neutral atom, ensuring an overall electrical charge of zero.
Understanding the composition of an element, specifically the quantity of its constituent subatomic particles, is fundamental to comprehending its chemical behavior and properties. Knowing the number of protons, neutrons, and electrons enables prediction of how beryllium will interact with other elements to form compounds, and is essential for applications ranging from material science to nuclear physics.