* Atomic Number: Strontium (Sr) has an atomic number of 38. This means a neutral Strontium atom has 38 protons.
* Neutrality: In a neutral atom, the number of protons (positive charge) equals the number of electrons (negative charge).
* Isotopes: Strontium-90 is an isotope of strontium. The number after the element name refers to the mass number, which is the total number of protons and neutrons. The mass number doesn't affect the number of electrons in a neutral atom.
Therefore, a neutral Strontium-90 atom has 38 electrons.
