Structures of Molecules Having Single Bonds

One of the most important factors when describing molecules that have only single bonds is the repulsion that exists between electrons. Repulsion is related to the number of electron pairs both shared and unshared around the central atom. When only two pairs of electrons surround the central atom (as in BeH2), the structure is almost always linear because that gives the configuration of lowest energy. 

When there are four pairs of electrons around the central atom (as in CH4), the structure is tetrahedral. 

From your prior study of chemistry, the hybrid orbital types sp and sp3 used to describe these cases are probably familiar. It is not unusual to hear someone say that CH4 is tetrahedral because the carbon atom is sp3 hybridized. However, CH4 is tetrahedral because that structure represents the configuration of lowest energy, and our way of describing a set of orbitals that matches that geometry is by combining 95 

Based on the requirement that repulsion should be minimized, idealized structures can be obtained based on the number of electrons surrounding the central atom. However, unshared pairs (sometimes called lone pairs) of electrons behave somewhat differently than do shared pairs. A shared pair of electrons is essentially localized in the region of space between the two atoms sharing the pair. An unshared pair of electrons is bound only to the atom on which they reside, and as a result, they are able to move more freely than a shared pair, so more space is required for an unshared pair. This has an effect on the structure of the molecule. 

Number of pairs on central atom and hybrid type 

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When a substance is packed into a liquid from a gas or into a solid from a liquid, the molecules also have a reduced ability to assume the various conformations. This loss of freedom is reflected in A12S, conformational. Different conformations arise from the ability of structures to rotate around single bonds. For example, consider l-bromo-2-chloro-ethane. Viewing the two carbons and the chlorine substituent as co-existing in a plane, we recognize that the bromine atom can occur in the same plane opposite the chlorine atom, or above the plane or behind the plane ... 

A double bond is a covalent bond in which four electrons are shared rather than the two that are typical of a single covalent bond. As a practical matter, if you do not know that a double bond exists in a molecule, it soon becomes apparent that writing a Lewis structure having single bonds does not work. It is then that you should consider a double bond. 

Although the molecular orbital description of bonding has some mathematical advantages, simple valence bond representations of structures are adequate for many purposes. The structures of molecules that have only single bonds (and in some cases unshared pairs of electrons on the... 

Chemical intuition is often better reflected in force constants and distances than in values of bond energies. This is demonstrated by the isoelectronic molecules SiF2, OFF, and SO2. In accordance with the force constants and the distances, single bonds have to be written in the Lewis structure of SiF2 and double bonds in that of SO2 (Binnewies and Schnoeckel, 1990) ... 

This completes the Lewis structures of these two double bonded molecules CI2CO and H2CCH2. In CI2CO, all four atoms have complete octets the two Cl atoms involve one shared and three lone pairs, the O atom two sh ed and two lone pairs, and the C atom four shared pairs of electrons. On both the oxygen and carbon atoms, two of the shared electron pairs are involved in a double bond. In H2CCH2, each of the Four H atoms has a single shared two-electron pair, and the two C atoms involve four shared electron pairs, two of which are involved in a double bond. 

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