Editing Neptunium (section)

===Atomic===
A neptunium atom has 93 electrons, arranged in the [[electron configuration|configuration]] <nowiki>[</nowiki>[[Radon|Rn]]<nowiki>]</nowiki>&nbsp;5f<sup>4</sup>&nbsp;6d<sup>1</sup>&nbsp;7s<sup>2</sup>. This differs from the configuration expected by the [[Aufbau principle]] in that one electron is in the 6d [[Electron shell#Subshells|subshell]] instead of being as expected in the 5f subshell. This is because of the similarity of the electron energies of the 5f, 6d, and 7s subshells. In forming compounds and ions, all the valence electrons may be lost, leaving behind an inert core of inner electrons with the electron configuration of the [[noble gas]] radon;<ref>{{cite book|author = Golub, A. M. |title = Общая и неорганическая химия (General and Inorganic Chemistry)|date = 1971|volume = 2|pages=222–7}}</ref> more commonly, only some of the valence electrons will be lost. The electron configuration for the tripositive ion Np<sup>3+</sup> is [Rn]&nbsp;5f<sup>4</sup>, with the outermost 7s and 6d electrons lost first: this is exactly analogous to neptunium's lanthanide homolog promethium, and conforms to the trend set by the other actinides with their [Rn]&nbsp;5f<sup>''n''</sup> electron configurations in the tripositive state. The first [[ionization potential]] of neptunium was measured to be at most {{val|6.19|0.12|u=[[electronvolt|eV]]}} in 1974, based on the assumption that the 7s electrons would ionize before 5f and 6d;<ref name="NIST">{{cite journal |first1=W. C. |last1=Martin |first2=Lucy |last2=Hagan |first3=Joseph |last3=Reader |first4=Jack |last4=Sugan |date=1974 |title=Ground Levels and Ionization Potentials for Lanthanide and Actinide Atoms and Ions |url=https://www.nist.gov/data/PDFfiles/jpcrd54.pdf |journal=J. Phys. Chem. Ref. Data |volume=3 |issue=3 |pages=771–9 |access-date=2013-10-19 |doi=10.1063/1.3253147 |bibcode=1974JPCRD...3..771M |archive-date=2014-02-11 |archive-url=https://web.archive.org/web/20140211144635/https://www.nist.gov/data/PDFfiles/jpcrd54.pdf |url-status=dead }}</ref> more recent measurements have refined this to 6.2657&nbsp;eV.<ref>David R. Lide (ed), ''CRC Handbook of Chemistry and Physics, 84th Edition''. CRC Press. Boca Raton, Florida, 2003; Section 10, Atomic, Molecular, and Optical Physics; Ionization Potentials of Atoms and Atomic Ions.</ref>