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Atmospheric Pollution and Pollutants
Published in Wayne T. Davis, Joshua S. Fu, Thad Godish, Air Quality, 2021
Wayne T. Davis, Joshua S. Fu, Thad Godish
All HC species are characterized by covalent chemical bonds between C and H, C and C, or C and other atomic species such as O. The covalently bonded atoms share electron pairs to attain chemical stability. A single covalent bond is formed when an electron pair is shared. In the simplest HC, CH4, each H is bonded to C by a single bond. HC compounds that contain only single bonds are called alkanes or paraffins. Larger molecules contain two or more carbons covalently bonded to each other. The paraffins include compounds that are straight chained (e.g. CH4 and ethane [C2H6]), branched (e.g. isobutane), and cyclic (e.g. cyclopropane). The alkanes or paraffins are relatively unreactive and therefore chemically stable; for example, n-butane has a relatively long atmospheric lifetime (~5 days).
Bonding and Properties of Materials
Published in Ashutosh Kumar Dubey, Amartya Mukhopadhyay, Bikramjit Basu, Interdisciplinary Engineering Sciences, 2020
Ashutosh Kumar Dubey, Amartya Mukhopadhyay, Bikramjit Basu
Sidgwick and Powell (1940) proposed a model to obtain a molecular geometry or shape based on valence electron pairs.5 This theory was modified by Nyholm and Gillespie (1957)6 as valence shell electron pair repulsion (VSEPR) theory, which is based on the repulsion between the electron pairs (bond and/or lone pairs [lp]) of the outermost (valence) shell of a central atom. These electron pairs try to orient in a position having minimum repulsion or maximum distance which provides a definite shape to the molecule. The lone pairs (lp) exist on the central atom, while unpaired electrons shared with the unpaired electrons of another atom for bond formation. However, lone pairs (lp) occupied more space than bond pairs (bp).7 Therefore, lp–lp repulsion > lp–bp repulsion > bp–bp repulsion.6,7
Chemical Bond I: Lewis Scheme
Published in Franco Battaglia, Thomas F. George, Understanding Molecules, 2018
Franco Battaglia, Thomas F. George
Regarding the molecular shape, when a group AXp of atoms appears in a Lewis structure, where A is a central atom with d lone pairs and to which are linked p atoms, it is possible to assign a geometrical shape making use of the so-called VSEPR rules (the cumbersome acronym means valence-shell electron-pair repulsion). To apply them, one should first evaluate the steric number SN, defined as SN ≡ d + p. Then, each SN value is correlated to a geometrical shape according to the principle that a directional character is attributed to each electron pair, and that two pairs repel from each other with a strength which is greatest between two lone pairs and smallest between two bond pairs.
Compositional dependence of physical parameters of Sb-doped InSe nanochalcogenide alloys
Published in Phase Transitions, 2023
Diksha Thakur, Vir Singh Rangra
Chalcogenide glasses are generally called lone pair semiconductors. This is due to the two-nonbonding p-orbits of group VI chalcogens in two-fold coordination. The excellent properties of chalcogenides such as high thermal stability and high sensitivity in small wavelength region are attributed to lone pair p shell electrons. Lone pair electrons are the nonbonding electron pair lying in the valance shell of chalcogenide glasses. A stable glass is characterized by severak lone pairs [13]. Electron lone pair (L) in a chalcogenide glass system is calculated by using the following formula [41]: where V and <r> are the average valance electrons and average coordination number, respectively. With increasing Sb content the number of lone pairs decreases (Figure 6).
Structural transition and thermo-physical study of quaternary (Se80Te20)94-x Ge6Pb x (0 ≤ x ≤ 12) alloys
Published in Phase Transitions, 2022
Priyanka Vashist, Balbir Singh Patial, Suresh Bhardwaj, A.M. Awasthi, S.K. Tripathi, Nagesh Thakur
The exceptional properties of high thermal stability and high sensitivity in small wavelength regions have been attributed to the lone-pair p-shell electrons. The existence of localized states in the energy band gap is the basis of these amazing properties which make chalcogenide glasses suitable for radiation detectors. The non-bonded pair of electrons lying in the valence band of chalcogenide glasses is called lone-pair electrons. The stability of viterous state in chalcogenide system is primed by the presence of lone pairs. The interaction of cations in the chalcogenide glasses with the lone-pair electrons of bridging chalcogen atom influences the glass forming ability of the system. The valence shell electron pair repulsion theory provides an insight into the role of lone-pair electrons. The number of lone-pair electrons of the investigated chalcogenide system has been calculated as L = V − <r> [28].
Concentration-dependent physicochemical behaviors and micellar interactions in polyalkoxylated fatty alcohol-based binary surfactant systems
Published in Journal of Dispersion Science and Technology, 2021
Chaw Jiang Lim, Chan Kiang Lim, Gwendoline Cheng Lian Ee
In the PAFA-APG system, the alkoxylate headgroups and glycoside headgroup are solvated in water via dipole–dipole interaction. The cooperative van der Waals and hydrophobic interactions between hydrocarbon chains of disparate surfactants confer the mixed micelle formation. Intercalation of APG into PAFA at different mixed ratios does not significantly vary the zeta potential values (–5.77 to –7.02 mV) and the PDI values are in the range of 0.058–0.096 indicating monomodal size distribution. Nonetheless, the presence of lone pair electrons around oxygen atoms in a large number of hydroxyl headgroups of APG could promote valence shell electron pair repulsion (VSEPR). An increase in steric effect between headgroups of PAFA and APG could alter the geometry and size of mixed micelles, as the steric repulsion could decrease micelle aggregation number.[55] As a result, the hydrodynamic sizes of mixed micelles of PAFA-APG are lower than their respective individual surfactants PAFA and APG.