Why is helium considered a noble gas?
The noble gases are found in group 18 of the periodic table. These elements were considered to be inert gases until the 1960's, because their oxidation number of 0 prevents the noble gases from forming compounds readily. All noble gases have the maximum number of electrons possible in their outer shell. Here Helium has 2 electrons in their outer shells while other noble gases have 8 in their outer shells. That is why they are stable and considered to be a noble gas.
Two comments:
1. How in the world did this get named as a question in organic chemistry. Helium is about as un-organic as you can get.
2. With regard to Yang's answer. That's a great answer EXCEPT that "all noble gases have the maximum number of electrons possible in their outer shell." That isn't quite true since the third shell can hold 18, 32, etc from 2N^2. (It's that word "possible" that bugs me.") Yes, all noble gases have 8 electrons in their outside shell but that shell may hold many more electrons except for He and Ne. And yes, I know I'm being picky.
Helium is considered a noble gas because of its chemical properties and electronic configuration. To understand why, let's break it down:
1. Electronic configuration: Helium has an atomic number of 2, which means it has two electrons. Its electronic configuration is 1s², which indicates that it has a completely filled first energy level. This stable electron configuration is similar to that of the noble gases in the periodic table.
2. Full valence shell: The noble gases, including helium, have a full valence shell, meaning their outermost energy level is complete with the maximum number of electrons it can hold. In the case of helium, its first energy level is filled with two electrons, satisfying the octet rule (an atom tends to gain, lose, or share electrons to achieve a stable configuration with eight electrons in its outermost shell).
3. Non-reactivity: One of the defining characteristics of noble gases is their non-reactivity. Helium is chemically inert, meaning it does not readily form compounds with other elements. This is because its full valence shell provides stability, and there is no need for helium to gain, lose, or share electrons to achieve a stable configuration.
To summarize, helium is considered a noble gas because it has a stable electronic configuration with a completely filled first energy level, a full valence shell, and exhibits non-reactivity. These properties are shared with the other noble gases, which are helium (He), neon (Ne), argon (Ar), krypton (Kr), xenon (Xe), and radon (Rn).