Noble gases are a group of elements found in Group 18 (VIII A) of the periodic table, also known as Group 8 or Group 0. They are characterized by their extremely low reactivity, which is primarily due to their full complement of valence electrons. Valence electrons are the electrons in the outermost energy level (also known as the valence shell) of an atom, and they play a crucial role in determining an element's chemical behavior.
Valence electrons each noble gas has:
Helium (He) - 2 valence electrons
Neon (Ne) - 8 valence electrons
Argon (Ar) - 8 valence electrons
Krypton (Kr) - 8 valence electrons
Xenon (Xe) - 8 valence electrons
Radon (Rn) - 8 valence electrons
Helium (He) is unique among the noble gases because it has only two electrons in total, and both of these electrons are in its first (and only) energy level, which is also its valence shell. Helium's electron configuration is 1s², indicating that it has two electrons occupying the 1s orbital.
In general, the noble gases are stable because their valence electron shells are completely filled with electrons. This gives them a high degree of chemical stability and makes them very unreactive. Helium achieves this stability with just two valence electrons because the 1s orbital can hold a maximum of two electrons. Once the 1s orbital is filled, helium has a full valence shell, which is equivalent to the stable electron configuration of the noble gases in Group 18.
So, even though helium only has two electrons, it is considered a noble gas because it achieves a full valence shell and stability with this electron configuration. This is in contrast to other elements in the periodic table that typically require eight valence electrons to achieve a full valence shell, like the other noble gases in Group 18.
