Dot and cross structure

The element to the left of nitrogen is carbon. This has four electrons in its valence shell in the uncharged atomic state. Thus, in order to achieve the octet it must combine with four other atoms, and so form four single covalent bonds. Draw the dot and cross structure of the tetrahydride of carbon, i.e. methane. 

There is one more carbon species that is encountered less frequently. Propose an equation for the production of the species that results from the loss of the XT anion from the CX3 species. Draw its dot and cross structure. 

Draw a dot and cross structure for the diatomic molecule of nitrogen. 

In the molecule diimide, HN=NH, there is a double nitrogen/nitrogen bond. Draw the dot and cross structure for this molecule. 

Carbon may also form multiple bonds with many other elements. It is this ability that is one of the reasons for the richness of the chemistry of carbon. One of the commonest heteroatomic multiple bond systems in which carbon partakes involves oxygen. Draw the dot and cross structure of this carbon/ oxygen double bond system. 

Draw the dot and cross structure of the imine group, 1 0= and again suggest the direction of... 

The value of the dot and cross structure here is to help to ensure that all the electrons are accounted for before attempting to deduce the structure. The electron pair repulsion theory gives the result that the CC12 molecule has a trigonal planar shape. There are two C-Cl single bonds and also (formally)... 

There is another, rather unusual, functional group that is related to the cyanide group. It contains a carbon atom that is only doubly bonded to a nitrogen atom, but which is not involved in any further bonding. This is the isocyano group, RNC. Draw the dot and cross structure of this grouping. 

The simplest, commonly encountered, charged hydrocarbon species is the methyl cation, CH3+. Draw a dot and cross structure of this ion, and suggest a shape for it. 

In a carbon centre that is involved in four covalent single bonds, how many electrons are there around the carbon atom Demonstrate your answer by focusing on the carbon atom in a molecule of methane, CH4, and drawing a dot and cross structure for it. 

Write the equation that represents the further addition of another hydrogen cation to the hydroxonium ion. Draw a dot and cross structure for the resultant ion. 

If you are having problems, try to write a balanced equation for the formation of this species from known species, and then try to draw the dot and cross structure. 

Now that the three-dimensional structure of the molecule is important, the dot and cross notation, which was so useful when counting electrons around each atom, is of less utility. First, it is rather cumbersome and secondly, it does not... 

A diatomic molecule is a molecule that consists of two identical atoms joined together by covalent bonds. For example, hydrogen gas exists as diatomic molecules, H2. The structure of a hydrogen molecule, H2, can be shown by using a Lewis structure (electron dot diagram) (Figure 4.24). A pair of electrons can be represented by dots, crosses, a combination of dots and crosses or by a line. Many chemists prefer to use a combination of dots and crosses so that it is clear which atom contributed the electrons. 

Lewis structures may also be drawn with dots and crosses including Venn diagrams (Figure 4.30). This approach is used for complicated molecules, as it allows an easy check to be made of exactly which bonds the electrons are in. 

Schrodinger developed the ideas of quantum (wave) mechanics in 1925. It was then applied to determine atomic and molecular structure. The idea of covalent bonding between two atoms based on the sharing of electron pairs was proposed by Lewis in 1916. The Lewis model (of dots and crosses to represent electrons) is still relevant and useful, but the quantum mechanical model (Chapters 2 and 12), incorporating wave-particle duality, Pauli s exclusion principle and Heisenberg s uncertainty principle, gives a deeper understanding of chemical bonds. 

DNA (deoxyribonucleic acid) a polymer with a double helical structure containing two sugar-phosphate chains with nitrogenous bases attached to them. The sequence of bases forms a code, which is used to form more DNA by replication or to encode mRNA (transcription), dot-and-cross diagram a diagram showing the... 

Dot-and-cross diagrams, structure and bonding and examples of shapes of molecules using the ideas of electron-pair repulsion. 

The formation of three-stranded nucleic acid complexes was first demonstrated over five decades ago [56] but the possible biological role of an extended triplex was expanded by the discovery of the H-DNA structure in natural DNA samples [57-59]. H-DNA is an intermolecular triplex that is generally of the pyrimidine-purine x pyrimidine type ( dot -Watson-Crick pairing and cross Hoogsteen base paring) and can be formed at mirror repeat sequences in supercoiled plasmids [59]. 

Figure 2. Left equilibrium geometries of the two lowest energy isomeric states of Au clusters obtained using LDA or GGA scalar relativistic pseudo-potentials. The ground state is Au for GGA and Auj for LDA (except for n=6, which LDA structure is also Aue). Right difference in the binding energy per atom of the planar and 3D structures given in the left panel for neutral gold clusters with 6

Almost simultaneous with the publication of Kossel s paper there appeared a rival electronic theory. The American chemist Lewis introduced the idea of the covalent electron-pair bond. Like Kossel, he was impressed by the apparent stability of the noble gas configuration. He was also impressed by the fact that, apart from many compounds of the transition elements, most compounds when rendered as molecules have even numbers of electrons, suggesting that electrons are usually found in pairs. Lewis devised the familiar representations of molecules and polyatomic ions (Lewis structures, or Lewis diagrams) in which electrons are shown as dots (or as noughts and crosses) to show how atoms can attain noble gas configurations by the sharing of electrons in pairs, as opposed to complete transfer as in Kossel s theory. It was soon apparent from the earliest X-ray studies that Kossel s theory was more appropriate... 

Figure 4 The structure of Prussian blue and related compounds. If none of the cube centre sites are occupied, the structure is that of ferric ferricyanide (both black and white Fe positions occupied by Fe111) if every second cube centre site (marked with a dotted circle) is occupied by K+, the structure is that of soluble Prussian blue (black = Fe11, white = Fe111) if all the centre sites are occupied by K+ (crosses as well as dotted circles) the structure is that of dipotassium ferrous ferrocyanide...

Figure 1. Sublett basin (solid line, dashed where uncertain) in relation to major Permian structural and paleogeographic elements. Areal extent of Meade Peak indicated by left diagonal rules Retort, by right diagonal rules limit of middle Permian epicontinental shelf deposits indicated by dotted line. Cross-section A-A shown in Figure 3, B-B in Figure 2.

In this way, the valence shell configuration of the central atom, combined with the Lewis representation of the inert gas shell, gives a very useful way of visualising the distribution of the valence shell electrons in this chemical book-keeping exercise. In these Lewis structures all the electrons are equivalent and the dot or cross notation simply indicates the source of the electrons from the central atom or the terminal atoms. 

Figure 7 Principle of a Monte Carlo-based approach to mathematically model polymer degradation and drug diffusion in PLGA-based microparticles. Scheme of the iimer structure of the system (one-quarter of a spherical cross section) (A) at time t = 0 (before exposure to the release medium) and (B) during dmg release. Gray, dotted, and white pixels represent nondegraded polymer, drag and pores, respectively. Source From Ref. 48.

In Figure 5.27 a curve calculated from Equation 5.216 is compared with the predictions of other studies. The dotted line is calculated by means of the Henderson theory. The theoretical curve calculated by Kjellander and Sarmatf for ( ) = 0.357 and h>2 by using the anisotropic Percus-Yevick approximation is shown by the dashed line the crosses represent grand canonical Monte Carlo simulation results due to Karlstrom. We proceed now with separate descriptions of solvation, depletion, and colloid structural forces. 

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