Four-bonded atoms

The shape around this carhon atom is tetrahedral. That is, the carbon atom is at the centre of an invisible tetrahedron, with the other four atoms at the vertices of the tetrahedron. This shape results because the electrons in the four bonds repel each other. In the tetrahedral position, the four bonded atoms and the bonding electrons are as far apart from each other as possible. 

Here, rib denotes the interbead distance (i.e., the bond length between two adjacent beads), k is the spring constant that quantifies the rigidity of the bond, and R0 is the maximum extensibility of the spring. The form of torsional potential parameters, describing four bonded atoms, is... 

For a linear sequence of four bonded atoms A-B-C-D, the torsion angle r is defined as the angle between the projections of the bond B-A and C-D when viewed along the direction of B-C. 

The general formula for alkenes implies that at least two carbon atoms in any alkene compound have fewer than four bonded atoms. As a result, chemists refer to alkenes as unsaturated compounds. Unlike saturated compounds, unsaturated hydrocarbons contain carbon atoms that can potentially bond to additional atoms. 

Oo) ]. A third class of interaction dependent on the dihedral angle 4> between four bonded atoms is the torsional potential K [l + cos( -S)] used to account for orbital delocalization and to compensate for other deficiencies in the force field. A harmonic term [V2K ( - is often introduced for dihedral angles 5 that are relatively fixed, such as those in aromatic rings. Coulomb s law [qiqJiATreQe ij)] is the simplest approach to the contribution of electrostatics to the potential V ... 

A positive torsion angle is the clockwise twist about the central of three connected bonds that is needed to make the near (upper) bond (A-R in a series of four-bonded atoms A-B-C-D) eclipse the far (lower) bond (C-D). 

A carbon with four bonded atoms is sp3-hybridized, tetrahedral, and has approximately 109° bond angles. The four atomic orbitals on carbon (an s and three p s) combine, through a process called hybridization, to form new orbitals with different geometric orientations. The four new sp3-orbitals are raindrop shaped and are oriented to the corners of a tetrahedron. All bonds to the carbon are sigma bonds. 

The carbons and oxygen each have four space-occupying groups four bonded atoms for each carbon and two bonded atoms and two non-bonding electron pairs for the oxygen. Tlhese atoms are all sp3-hybridized, tetrahedral, and have approximate bond angles of 109°. 

A torsion angle is determined by how much a bond has to be twisted to cause two substituents on the atoms it connects to be eclipsed (Fig. 28). By definition, a torsion angle is positive if a clockwise twist about the central of three connected bonds that is needed to make the near (upper) bond [A-B in a series of four-bonded atoms A-B-C-D) eclipse the far (lower) bond (C-D) a negative torsion angle is a counterclockwise twist. The direction of view along the bond [that is, which is nearer to you (A-B) or (D-C)] is immaterial reversing the sequence of... 

When a molecule has four bonded atoms and two lone pairs, however, the lone pairs always lie at opposite vertices to avoid the stronger 90° lone pair-lone pair repulsions. This positioning gives the square planar shape (AX4E2), as in xenon tetrafluoride (XeF4) ... 

The simplest crystal structure built of four-bonded atoms is that of diamond. Each carbon atom has four shortest distances to its neighbors. Each atom has four valence electrons, which allows interpretation of all the short contacts as two center-two electron (2c-2e) bonds. The simplest heteroatomic structure built the same way is the... 

In Section 1.7C, we described the formation of a carbon-carbon double bond in terms of the overlap of atomic orbitals. A carbon-carbon double bond consists of one cr bond and one tt bond (Figure 5.1). Each carbon of the double bond uses its three sp hybrid orbitals to form a bonds to three atoms. The unhybridized 2p atomic orbitals, which lie perpendicular to the plane created by the axes of the three sp hybrid orbitals, combine to form two tt molecular orbitals one bonding and the other antibonding. For the unhybridized 2p orbitals to be parallel, thus giving maximum overlap, the two carbon atoms of the double bond and the four bonded atoms must lie in a plane. 

Four Electron Pairs, Four Bonded Atoms... 

In the electron-dot formula of water, H2O, there are also four electron groups, which have minimal repulsion when the electron-group geometry is tetrahedral. However, in H2O, two of the electron groups are lone pairs of electrons. Because the shape of H2O is determined by the two H atoms bonded to the central O atom, the H2O molecule has a bent shape with a bond angle of 109°. Table 10.3 gives the molecular shapes for molecules with two, three, and four bonded atoms. 

TABLE 10.3 Molecular Shapes for a Central Atom with Two, Three, and Four Bonded Atoms... 

If the bonds are polar covalent, draw the electron-dot formula and determine if the dipoles cancel. The electron-dot formula for SiClj has four electron groups and four bonded atoms. The molecule has a tetrahedral shape. The dipoles of the Si—Cl bonds point away from each other and cancel out, which makes SiC a nonpolar molecule. 

In CF4, the central atom C has four bonded atoms and no lone pairs of electrons, which gives it a tetrahedral shape. 

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