Editing Helium (section)

====In quantum mechanics====
In the perspective of [[quantum mechanics]], helium is the second simplest [[atom]] to model, following the [[hydrogen atom]]. Helium is composed of two electrons in [[atomic orbital]]s surrounding a nucleus containing two protons and (usually) two neutrons. As in Newtonian mechanics, no system that consists of more than two particles can be solved with an exact analytical mathematical approach (see [[3-body problem]]) and helium is no exception. Thus, numerical mathematical methods are required, even to solve the system of one nucleus and two electrons. Such [[computational chemistry]] methods have been used to create a quantum mechanical picture of helium electron binding which is accurate to within < 2% of the correct value, in a few computational steps.<ref>{{Cite news|url=http://www.sjsu.edu/faculty/watkins/helium.htm|last=Watkins|first=Thayer|publisher=San Jose State University|title=The Old Quantum Physics of Niels Bohr and the Spectrum of Helium: A Modified Version of the Bohr Model|access-date=2009-06-24|archive-url=https://web.archive.org/web/20090526074018/http://www.sjsu.edu/faculty/watkins/helium.htm|archive-date=2009-05-26|url-status=live}}</ref> Such models show that each electron in helium partly screens the nucleus from the other, so that the [[effective nuclear charge]] ''Z''<sub>eff</sub> which each electron sees is about 1.69 units, not the 2 charges of a classic "bare" helium nucleus.