Atomic structure representative elements

Draw correct Lewis structures for atoms of representative elements. (Section 4.1)... 

Draw correct Lewis structures for atoms of representative elements. 

A representation of atomic structure. The various spheres are not drawn to scale. The lump of iron on the left would contain almost a million million million million (10 ) atoms, one of which is represented by the sphere in the top center of the page. In turn, each atom is composed of a number of electrons, protons, and neutrons. For example, an atom of the element iron contains 26 electrons, 26 protons, and 30 neutrons. The physical size of the atom is determined mainly by the number of electrons, but almost all of its mass is determined by the number of protons and neutrons in its dense core or nucleus (lower part of figure). The electrons are spread out around the nucleus, and their number determines atomic size but the protons and neutrons compose a very dense, small core, and their number determines atomic mass. 

Each equivalent atom (the same element, the same number of bonds and lone pairs) has the same formal charge. A check on the calculated formal charges is that their sum is equal to the overall charge of the molecule or ion. For an electrically neutral molecule, the sum of the formal charges is zero. Compare the formal charges of each possible structure. The structure with the lowest formal charges represents the least disturbance of the electronic structures of the atoms and is the most plausible (lowest energy) structure. 

Only a few relevant points about the atomic structures are summarized in the following. Table 4.1 collects basic data about the fundamental physical constants of the atomic constituents. Neutrons (Jn) and protons (ip), tightly bound in the nucleus, have nearly equal masses. The number of protons, that is the atomic number (Z), defines the electric charge of the nucleus. The number of neutrons (N), together with that of protons (A = N + Z) represents the atomic mass number of the species (of the nuclide). An element consists of all the atoms having the same value of Z, that is, the same position in the Periodic Table (Moseley 1913). The different isotopes of an element have the same value of Z but differ in the number of neutrons in their nuclei and therefore in their atomic masses. In a neutral atom the electronic envelope contains Z electrons. The charge of an electron (e ) is equal in size but of opposite sign to that of a proton (the mass ratio, mfmp) is about 1/1836.1527). 

Fig. 6 STM images and partial structural models of the Ag-oxygen overlayers, (a) A 200 A patch of Ag(l 11) covered by the p(4 x 4), c(3 x 5 3)rect, and p(4 x 5 3)rect overlayers, measured at -0.51 nA and —131.5 mV. (b) The Agj g30 model for the p(4 x 4) proposed by Carlisle et Solid (open) large gray balls represent the overlayer (substrate) Ag atoms, and small balls the oxygen atoms, (c, e) STM images of 35 A patches of the p(4 x 4) and c(3 x 5,y3)rect overlayers, respectively, (c) is at —0.42 nA and -21.7 mV, and (e) is at -0.40 nA and —34.2 mV. The inset in (e) displays the six-atom structural element of the c(3 x 5 3)rect phase measured with a tip state that allowed the resolution of the central parts of the proposed Ags triangles (-0.42 nA, —21.7 mV), (d, f) The proposed Ag6-based models for the p(4 x 4) and c(3 x 5 3)rect phases, respectively. Figure adapted with permission from Schnadt et al.. Physical Review Letters, 2006, 96, 146101. American Physical Society.

Another design technique being actively explored involves searching for patterns. Whereas the method described in the previous section attempts to imderstand the behavior of atoms and molecules, this method takes its cue from combinations of these particles. By means of X-ray crystallography and other methods, scientists have already determined the structures of thousands of crystals, composed from a wide variety of elements and compoimds. The set of these structures represents an enormous amoimt of data. Some researchers have begun to use computers to sift through this data, looking for clues as to what elements and compoimds produce which structures. With these clues, prediction of the properties of new materials may be possible. 

In antiquity, problems relating to chemistry were approached by two philosophies held to be mutually exclusive. The compositional view is best illustrated by the four elements as developed by Aristotle. The opposing view of structure, represented by the atomic theory of Democritus, lost out in antiquity because its materialism left no room for the spiritual. The so-... 

The representation of structure factors as vectors in the complex plane (Qr complex vectors) is useful in several ways. Because the diffractive contributions of atoms or volume elements to a single reflection are additive, each contribution can be represented as a complex vector, and the resulting structure factor is the vector sum of all contributions. For example, in Fig. 6.4, F represents a structure factor of a three-atom structure, in which f), f2, and f3 are the atomic structure factors. 

It is time-consuming to draw electron arrangements using Bohr-Rutherford diagrams. It is much simpler to use Lewis structures to represent elements and the valence electrons of their atoms. To draw a Lewis structure, you replace the nucleus and inner energy levels of an atom with its atomic symbol. Then you place dots around the atomic symbol to represent the valence electrons. The order in which you place the first four dots is up to you. You may find it simplest to start at the top and proceed clockwise right, then bottom, then left. 

Significant theoretical and experimental efforts to get a planar arrangement of atoms belonging to representative elements, which are included in simple and elaborate molecular entities, appeared in the last decade. Along this line, an interesting structural pattern 16a-r has been presented by the group of Li, where planar central silicon and carbon atoms with tetra-, penta-, hexa-, and heptacoordination numbers were found for the former and tetra- and penta-coordination numbers for the latter <2004JA16227>. 

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