The element with the smallest atomic size is helium (He). Atomic size refers to the size of the atom, specifically the distance between the nucleus and the outermost electron shell. Helium is located in the top-right corner of the periodic table in Group 18, also known as the noble gases.
Helium has the smallest atomic size for several reasons:
Effective nuclear charge: Helium has a relatively high effective nuclear charge, which is the positive charge experienced by the outermost electrons due to the attraction of the protons in the nucleus. With only two electrons, helium's effective nuclear charge is significant, resulting in a relatively small atomic size.
Electron configuration: Helium has a stable electron configuration of 1s², meaning it has a completely filled first electron shell with two electrons. Since the first shell is the closest to the nucleus, the electrons are held tightly, leading to a smaller atomic size.
Absence of valence electrons: Helium does not have any valence electrons in its outermost shell. Valence electrons are involved in bonding and occupy the outermost energy level of an atom. With no valence electrons, helium's atomic size is minimized.
Overall, helium has the smallest atomic size among the elements because of its high effective nuclear charge, stable electron configuration, and absence of valence electrons.