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- X + eâˆ’â†’ Xâˆ’
The electron affinity, Eea, is defined as positive when the resulting ion has a lower energy, i.e. it is an exothermic process that releases energy:
- Eea = Einitial âˆ’ Efinal
- Xâˆ’â†’ X + eâˆ’
Electron affinities of the elements
Although Eea varies greatly across the periodic table, some patterns emerge. Generally, nonmetals have more positive Eea than metals. Atoms whose anions are more stable than neutral atoms have a greater Eea. Chlorine most strongly attracts extra electrons; mercury most weakly attracts an extra electron. The electron affinities of the noble gases have not been conclusively measured, so they may or may not have slightly negative values.
Eea generally increases across a period (row) in the periodic table. This is caused by the filling of the valence shell of the atom; a group 7A atom releases more energy than a group 1A atom on gaining an electron because it obtains a filled valence shell and therefore is more stable.
A trend of decreasing Eea going down the groups in the periodic table would be expected. The additional electron will be entering an orbital farther away from the nucleus, and thus would experience a lesser effective nuclear charge. However, a clear counterexample to this trend can be found in group 2A, and this trend only applies to group 1A atoms. Electron affinity follows the trend of electronegativity. Fluorine (F) has a higher electron affinity than oxygen and so on.
The following data are quoted in kJ/mol. Elements marked with an asterisk are expected to have electron affinities close to zero on quantum mechanical grounds. Elements marked with a dotted box are synthetically made elementsâ€”elements not found naturally in the environment.
Molecular electron affinities
The electron affinity of molecules is a complicated function of their electronic structure. For instance the electron affinity for benzene is negative, as is that of naphthalene, while those of anthracene, phenanthrene and pyrene are positive. In silicoexperiments show that the electron affinity ofhexacyanobenzene surpasses that of fullerene.
Electron affinity of Surfaces
The electron affinity measured from a material's surface is a function of the bulk material as well as the surface condition. Often negative electron affinity is desired to obtain efficient cathodes that can supply electrons to the vacuum with little energy loss. The observed electron yield as a function of various parameters such as bias voltage or illumination conditions can be used to describe these structures with band diagrams in which the electron affinity is one parameter. For one illustration of the apparent effect of surface termination on electron emission, see Figure 3 in Marchywka Effect.
The structural formula of a chemical compound is a graphical representation of the molecular structure, showing how the atoms are arranged. The chemical bonding within the molecule is also shown, either explicitly or implicitly. There are several common representations used in publications. These are described below. Also several other formats are used, as in chemical databases, such as SMILES, InChI and CML.
Unlike chemical formulas or chemical names, structural formulas provide a representation of the molecular structure. Chemists nearly always describe a chemical reaction or synthesis using structural formulas rather than chemical names, because the structural formulas allow the chemist to visualize the molecules and the changes that occur.
Many chemical compounds exist in different isomeric forms, which have different structures but the same overall chemical formula. A structural formula indicates the arrangements of atoms in a way that a chemical formula cannot.
Lewis structures (or "Lewis dot structures") are flat graphical formulas that show atom connectivity and lone pair or unpaired electrons, but not three-dimensional structure. This notation is mostly used for small molecules. Each line represents the two electrons of a single bond. Two or three parallel lines between pairs of atoms represent double or triple bonds, respectively. Alternatively, pairs of dots may used to represent bonding pairs. In addition, all non-bonded electrons (paired or unpaired) and any formal charges on atoms are indicated.
In early organic-chemistry publications, where use of graphics was severely limited, a typographic system arose to describe organic structures in a line of text. Although this system tends to be problematic in application to cyclic compounds, it remains a convenient way to represent simple structures:
Parentheses are used to indicate multiple identical groups, indicating attachment to the nearest non-hydrogen atom on the left when appearing within a formula, or to the atom on the right when appearing at the start of a formula:
(CH3)2CHOH or CH(CH3)2OH (2-propanol)
In all cases, all atoms are shown, including hydrogen atoms.
Skeletal formulas are the standard notation for more complex organic molecules. First used by the organic chemist Friedrich August KekulÃ© von Stradonitz the carbon atoms in this type of diagram are implied to be located at the vertices (corners) and termini of line segments rather than being indicated with the atomic symbol C. Hydrogen atoms attached to carbon atoms are not indicated: each carbon atom is understood to be associated with enough hydrogen atoms to give the carbon atom four bonds. The presence of a positive or negative charge at a carbon atom takes the place of one of the implied hydrogen atoms. Hydrogen atoms attached to atoms other than carbon must be written explicitly.
Indication of stereochemistry
Several methods exist to picture the three-dimensional arrangement of atoms in a molecule (stereochemistry).
Stereochemistry in skeletal formulas
Wavy single bonds represent unknown or unspecified stereochemistry or a mixture of isomers. For example the diagram below shows the fructose molecule with a wavy bond to the HOCH2- group at the left. In this case the two possible ring structures are in chemical equilibrium with each other and also with the open-chain structure. The ring continually opens and closes, sometimes closing with one stereochemistry and sometimes with the other.
Newman projection and sawhorse projection
The Newman projection and the sawhorse projection are used to depict specific conformers or to distinguish vicinal stereochemistry. In both cases, two specific carbon atoms and their connecting bond are the center of attention. The only difference is a slightly different perspective: the Newman projection looking straight down the bond of interest, the sawhorse projection looking at the same bond but from a somewhat oblique vantage point. In the Newman projection, a circle is used to represent a plane perpendicular to the bond, distinguishing the substituents on the front carbon from the substituents on the back carbon. In the sawhorse projection, the front carbon is usually on the left and is always slightly lower:
Certain conformations of cyclohexane and other small-ring compounds can be shown using a standard convention. For example, the standard chair conformation of cyclohexane involves a perspective view from slightly above the average plane of the carbon atoms and indicates clearly which groups are axial and which are
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Answers:The electron dot structure of the Al is just Al3+. This is because its three valence electrons are gone. The structure of the O is O(2-) with ** on three sides and oo on the fourth side. The *'s are O electrons and the o's are electrons gained from Al. Put down 2 such Al's for every 3 O's. Calcium oxide: The same as for aluminum oxide, except Ca is 2+ and there is one Ca2+ for every O=. Carbon dioxide: Draw O C O and leave a space between each. Draw ** on the top and bottom of each O and between each O and C. Draw oo between C and each O. *'s are O valence electrons; o's are C valence electrons. Phosphorus pentoxide: Draw O...............O ....P...O...P O...............O Put *o between the center O and each P. Put oo between each P and each of the four other O's. Put ** on the top and bottom of the center O and the remaining three sides of the other four O's. Notice that P provides both electrons to the pairs that are shared between it and the outer four O's. These are coordinate covalent bonds. Iodine pentoxide: The same as for P2O5. Just remember that you have to have an extra oo on one side of each iodine, because they have 7 valence electrons.