Helium (element)
氦 centers on the element helium, a light, non-reactive gas. Its meaning is specific to this chemical element.
氦 is a modern character created for the chemical element helium. It combines the gas radical 气 with the phonetic component 亥 (ガイ), which provides the on-reading.
The outer enclosure 气 signals a gas, and the inner 亥 suggests the sound. Picture a helium balloon with the character 亥 floating inside the gas symbol.
For ガイ, imagine a guy inhaling helium from a balloon and speaking in a high voice: guy -> ガイ, and the gas makes him sound funny.