Structural differences may occur within these four groups, depending on the molecular constitution. A consideration of molecular symmetry helps to distinguish structurally equivalent from nonequivalent atoms and groups. The ability to distinguish structural differences of this kind is an essential part of mastering organic chemistry.
It will come with practice and experience. Our ability to draw structural formulas for molecules is remarkable. To see how this is done Click Here. Formula Analysis. Although structural formulas are essential to the unique description of organic compounds, it is interesting and instructive to evaluate the information that may be obtained from a molecular formula alone. Three useful rules may be listed: The number of hydrogen atoms that can be bonded to a given number of carbon atoms is limited by the valence of carbon.
The origin of this formula is evident by considering a hydrocarbon made up of a chain of carbon atoms. Here the middle carbons will each have two hydrogens and the two end carbons have three hydrogens each.
Thus, when even-valenced atoms such as carbon and oxygen are bonded together in any number and in any manner, the number of remaining unoccupied bonding sites must be even.
If these sites are occupied by univalent atoms such as H, F, Cl, etc. If the four carbon atoms form a ring, two hydrogens must be lost. Similarly, the introduction of a double bond entails the loss of two hydrogens, and a triple bond the loss of four hydrogens. By rule 2 m must be an even number, so if m The presence of one or more nitrogen atoms or halogen substituents requires a modified analysis.
The above formula may be extended to such compounds by a few simple principles: The presence of oxygen does not alter the relationship. All halogens present in the molecular formula must be replaced by hydrogen. Each nitrogen in the formula must be replaced by a CH moiety. However, the structures of some compounds and ions cannot be represented by a single formula.
For clarity the two ambiguous bonds to oxygen are given different colors in these formulas. If only one formula for sulfur dioxide was correct and accurate, then the double bond to oxygen would be shorter and stronger than the single bond.
This averaging of electron distribution over two or more hypothetical contributing structures canonical forms to produce a hybrid electronic structure is called resonance.
Likewise, the structure of nitric acid is best described as a resonance hybrid of two structures, the double headed arrow being the unique symbol for resonance. The above examples represent one extreme in the application of resonance. Here, two structurally and energetically equivalent electronic structures for a stable compound can be written, but no single structure provides an accurate or even an adequate representation of the true molecule.
In cases such as these, the electron delocalization described by resonance enhances the stability of the molecules, and compounds or ions composed of such molecules often show exceptional stability. The electronic structures of most covalent compounds do not suffer the inadequacy noted above. Nevertheless, the principles of resonance are very useful in rationalizing the chemical behavior of many such compounds. For example, the carbonyl group of formaldehyde the carbon-oxygen double bond reacts readily to give addition products.
The course of these reactions can be explained by a small contribution of a dipolar resonance contributor, as shown in equation 3. Here, the first contributor on the left is clearly the best representation of this molecular unit, since there is no charge separation and both the carbon and oxygen atoms have achieved valence shell neon-like configurations by covalent electron sharing.
If the double bond is broken heterolytically, formal charge pairs result, as shown in the other two structures. The preferred charge distribution will have the positive charge on the less electronegative atom carbon and the negative charge on the more electronegative atom oxygen.
Therefore the middle formula represents a more reasonable and stable structure than the one on the right. The application of resonance to this case requires a weighted averaging of these canonical structures. The double bonded structure is regarded as the major contributor, the middle structure a minor contributor and the right hand structure a non-contributor. Since the middle, charge-separated contributor has an electron deficient carbon atom, this explains the tendency of electron donors nucleophiles to bond at this site.
The basic principles of the resonance method may now be summarized. These are the canonical forms to be considered, and all must have the same number of paired and unpaired electrons. The following factors are important in evaluating the contribution each of these canonical structures makes to the actual molecule. The stability of a resonance hybrid is always greater than the stability of any canonical contributor. Consequently, if one canonical form has a much greater stability than all others, the hybrid will closely resemble it electronically and energetically.
This is the case for the carbonyl group eq. On the other hand, if two or more canonical forms have identical low energy structures, the resonance hybrid will have exceptional stabilization and unique properties.
This is the case for sulfur dioxide eq. To illustrate these principles we shall consider carbon monoxide eq. In each case the most stable canonical form is on the left. For carbon monoxide, the additional bonding is more important than charge separation. Furthermore, the double bonded structure has an electron deficient carbon atom valence shell sextet. A similar destabilizing factor is present in the two azide canonical forms on the top row of the bracket three bonds vs.
The bottom row pair of structures have four bonds, but are destabilized by the high charge density on a single nitrogen atom. All the examples on this page demonstrate an important restriction that must be remembered when using resonance. No atoms change their positions within the common structural framework. Only electrons are moved. A more detailed model of covalent bonding requires a consideration of valence shell atomic orbitals. If the rows of atoms are packed in this third layer so that they do not lie over atoms in either the A or B layer, then the third layer is called C.
Both arrangements give the closest possible packing of spheres leaving only about a fourth of the available space empty. The smallest repeating array of atoms in a crystal is called a unit cell.
A third common packing arrangement in metals, the body-centered cubic BCC unit cell has atoms at each of the eight corners of a cube plus one atom in the center of the cube. Because each of the corner atoms is the corner of another cube, the corner atoms in each unit cell will be shared among eight unit cells.
The BCC unit cell consists of a net total of two atoms, the one in the center and eight eighths from the corners. In the FCC arrangement, again there are eight atoms at corners of the unit cell and one atom centered in each of the faces. The atom in the face is shared with the adjacent cell. FCC unit cells consist of four atoms, eight eighths at the corners and six halves in the faces.
Table 1 shows the stable room temperature crystal structures for several elemental metals. As atoms of melted metal begin to pack together to form a crystal lattice at the freezing point, groups of these atoms form tiny crystals. These tiny crystals increase in size by the progressive addition of atoms.
The resulting solid is not one crystal but actually many smaller crystals, called grains. These grains grow until they impinge upon adjacent growing crystals. The interface formed between them is called a grain boundary. Grains are sometimes large enough to be visible under an ordinary light microscope or even to the unaided eye. The spangles that are seen on newly galvanized metals are grains.
See A Particle Model of Metals Activity Figure 5 shows a typical view of a metal surface with many grains, or crystals. Figure 5: Grains and Grain Boundaries for a Metal. This is not grammatically correct in English. The correct question is:. Asks the same question without the need for "many" and the need for a plural verb. Sign up to join this community. The best answers are voted up and rise to the top. Stack Overflow for Teams — Collaborate and share knowledge with a private group.
Create a free Team What is Teams? Learn more. Could it be a structure of "How many -singular noun- is there? Asked 4 years, 8 months ago. Active 4 years, 8 months ago. Viewed 5k times. We had an argument in our collage if it's possible to have such structure: "How many - singular noun - is there?
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