The concepts of oxidation state and atomic charge are entangled in modern materials science. We distinguish between these quantities and consider their fundamental limitations and utility for understanding material properties. We discuss the nature of bonding between atoms and the techniques that have been developed for partitioning electron density. While formal oxidation states help us count electrons (in ions, bonds, lone pairs), variously defined atomic charges are usefully employed in the description of physical processes including dielectric response and electronic spectroscopies. Such partial charges are introduced as quantitative measures in simple mechanistic models of a more complex reality, and therefore may not be comparable or transferable. In contrast, oxidation states are defined to be universal, with deviations constituting exciting challenges as evidenced in mixed-valence compounds, electrides and highly correlated systems. This Perspective covers how these concepts have evolved in recent years, our current understanding and their significance.
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C.R.A.C. is grateful for many discussions with A. M. Stoneham on the topic in this Perspective. A.W. thanks R. G. Egdell and A. Regoutz for discussions on X-ray photomemission. A.A.S. is indebted to L. N. Kantorovich for discussions of the electron groups theory and structural elements. The research was supported by the EPSRC (grant nos EP/K016288/1 and EP/N01572X/1), the Leverhulme Trust, and the Royal Society. D.O.S. acknowledges support from the European Research Council (grant no. 758345). This work was carried out with funding from the Faraday Institution (https://faraday.ac.uk; EP/S003053/1), grant no. FIRG003.
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Nature Materials (2019)