Atoms in molecules and structural formulae

As there is no bonding interaction between the (peripheral) H atoms, no stroke is shown connecting their symbols. A similar representation is given for the isoelectronic and isogeometric species NH4. The removal of a hydrogen ion, H , from the ammonium ion yields NH3 whose structural formula now includes a stroke located on the N symbol corresponding to 

Single bonds are also considered in the water molecule, which means two non-bonding electron pairs  

Although the use of strokes to represent bonds between atoms in molecules comes from the nineteenth century, the electron pair concept as necessary for the understanding of chemical bonding was introduced by G.N. Lewis (1875-1946) in 1916 (ref. 90) following Bohr s, then recently proposed, model of the atom. Indeed, the Lewis model still lies at the basis of much of present-day chemical thinking, although it was advanced before both the development of quantum mechanics and the introduction of the concept of electron spin. In a more quantitative way, it found a natural theoretical extension in the valence-bond approximation to the molecular wavefunction, as expressed in terms of the overlap of (pure or hybridized) atomic orbitals to describe the pairing of electrons, coupled with the concept of electron spin. 

The success of such a well-estabhshed representation is obtained in spite of the fact that structural formulae do not even allow a proper visualization of the actual (van der Waals) shapes of the molecules. These are represented by envelopes of constant electronic charge density averaged over thermal motions of the nuclei, as given experimentally by X-ray and electron ] 

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