Intro 3 Electron Density Terminology 101: Electron Density:The presence of energetically accessible valence electrons around an atom. Electrons are represented by a probability distribution spread out over a region of space defined by the nature of the orbital: s, p, d, f, and/or hybrid orbitals such sp3, sp2, sp, etc. Atoms with quite a few valence electrons such as Pt(0) d10and/or contracted orbitals have a high electron density. Atoms with fewer valence electrons (e.g., Na+) and/or diffuse orbitals (electrons spread out over a larger region of space) can be considered to have low electron densities. Do not confuse electron density with electronegativity or how electron-rich or poor an atom is classified. Electron-rich:Atoms that are willing to readily donate electrons or electron pairs to other atoms are considered electron rich. Ease of ionization is another property associated with electron-rich atoms. The willingness to share or donate electron pairs is related to lower electronegativity, number of valence electrons, good donor groups on the atom in question, negative charges, or some combination of these factors. Using organic terminology I would consider an electron-rich atom to be a good nucleophile (electron pair donating).