The identity of every chemical element rests on a single, defining value that dictates its physical and chemical behavior. Helium (He) is a colorless, odorless, monatomic gas and the second most abundant element in the observable universe, following only hydrogen. Its unique characteristics, from its extreme lightness to its complete lack of chemical reactivity, all stem from this fundamental numerical property. Understanding this number is necessary for placing the element within the organization of the Periodic Table.
Defining the Atomic Number
The atomic number for Helium is 2. This value, represented by the symbol Z, is the count of positively charged protons found within the atom’s nucleus. Every atom of Helium contains exactly two protons, and it is this unchangeable quantity that separates it from every other element. If an atom possessed only one proton, it would be Hydrogen (Z=1), and if it had three, it would be Lithium (Z=3). The modern Periodic Table of Elements is structured according to increasing atomic number, which means Helium is positioned immediately after Hydrogen.
The Structure of the Helium Atom
The atomic number of two dictates the arrangement of the subatomic particles within the helium atom. The nucleus contains the two protons, which are balanced by two orbiting electrons in a neutral helium atom. The most common isotope, Helium-4, also contains two neutrons, giving it an atomic mass near four. The two electrons completely fill the first electron shell, which is the atom’s lowest energy level. This configuration satisfies the “duplet rule,” making the atom exceptionally stable and classifying Helium as a noble gas, the first in Group 18 of the Periodic Table.
Essential Properties and Uses
The stable atomic structure resulting from the atomic number of two governs all of Helium’s observable properties. Because its outer electron shell is full, the atom is chemically inert and non-reactive, having little tendency to gain, lose, or share electrons. Helium possesses the lowest boiling point of any element, liquefying at -268.9 °C. This property makes liquid Helium indispensable for cryogenics, the science of very low temperatures. Its largest commercial use is cooling superconducting magnets in Magnetic Resonance Imaging (MRI) machines, and it is also used as a lifting gas in weather balloons and airships due to its low density.