Core electrons Covalent bonds

A high electron density surface (also called a bond electron density surface) shows the core of electron density around each atomic nucleus and regions where neighboring atoms share electrons (covalent bonding regions). 

A chemical constitution is usually represented by a constitutional formula in which the chemical element symbols correspond to the respective atomic cores, the covalent bonds are indicated by lines between the element symbols, and the free valence electrons by dots at the atomic symbols. 

If an ion contains two or more atoms covalently bonded to each other, the total charge on the ion must equal the total core charge less the total number of electrons, shared and unshared ... 

The molecular space is divided into core and valence basins, the latter being classified according to their connectivity to the core basins as monosynaptic basins, associated with electron pairs, or dissynaptic basins, associated with covalent bonds. Moreover, an integration of the electronic population over each basin gives the number of electrons in each of them and the fluctuation between basins is related to the electronic delocalisation [39]. 

---