Does helium have an ionic radius? I searched and looked in an old Chem book, and come up empty. Hydrogen is not listed either. Is there a specific reason for this?
Thank you.
Helium does not possess an ionic radius, and neither does hydrogen. The reason for this is because both helium and hydrogen do not readily form ions in their common chemical reactions.
Ionic radius is a measure of the size of an ion, which is formed when an atom gains or loses electrons. In most cases, when atoms gain or lose electrons, they do so to achieve a stable electron configuration, usually by attaining a full outer electron shell.
Helium and hydrogen are both in the first period (or row) of the periodic table, which means they only have one electron shell. Helium has a full outer shell with two electrons, while hydrogen has a single electron in its outer shell. To achieve the stable electronic configuration of the nearest noble gases, helium doesn't need to gain or lose any electrons, and hydrogen prefers to either gain one electron to form H- (hydride ion) or lose its electron to form H+ (proton or hydrogen ion). However, these ionization processes are rare and typically require extreme conditions, making them uncommon in normal chemical reactions.
Therefore, because helium and hydrogen do not commonly form ions, there is no defined ionic radius for them.