Iron (Fe) is not considered a noble element despite having 8 valence electrons because the concept of noble elements is primarily based on the electronic configuration of the outermost energy level or valence shell of an atom.
Noble gases, such as helium (He), neon (Ne), and argon (Ar), have a completely filled valence shell with a stable octet configuration, except for helium, which has only two electrons in its valence shell. This stable configuration makes noble gases highly stable and unreactive, which is why they are called noble or inert gases.
On the other hand, iron (Fe) has an atomic number of 26, and its electron configuration is [Ar] 3d^6 4s^2. While the 3d orbital can accommodate up to 10 electrons, iron only has 6 electrons occupying its 3d orbital, leaving two empty slots. Therefore, iron does not have a completely filled valence shell and cannot be considered a noble element.
Valence electrons are the electrons in the outermost energy level, and for iron, those are the 4s and 3d electrons. Although iron has 8 valence electrons (2 in the 4s orbital and 6 in the 3d orbital), it does not fulfill the stability criterion of a noble gas configuration.
It's worth noting that noble gases have a unique stability due to their filled valence shells, and they exhibit limited chemical reactivity. Other elements with filled valence shells, such as some transition metals like gold (Au) and platinum (Pt), also have relatively low reactivity but are not classified as noble elements. The noble gases are a distinct group due to their complete s and p orbitals in the valence shell.