Covalent bonding

Covalent bonding is an intermolecular form of chemical bonding characterized by the sharing of one or more pairs of electrons between two components, producing a mutual attraction that holds the resultant molecule together. Atoms tend to share electrons in such a way that their outer electron shells are filled - this is referred to as electron configuration. Such bonds are always stronger than the intermolecular hydrogen bond and similar in strength to or stronger than the ionic bond.

In contrast to the ionic and metallic bond, the covalent bond is directional, i.e. the bond angles have a great impact on the strength of the bond. Because of the directional character of the bond, covalently bound materials are more difficult to deform than metals. The cause of the directionality is the form of the s, p, d, and f orbitals. In organic chemistry, the directionality of the bonding is often described by hybrid orbitals.

Covalent bonding most frequently occurs between atoms with similar electronegativities. For this reason, non-metals tend to engage in covalent bonding more readily since metals have access to metallic bonding, where the easily-removed electrons are more free to roam about. For non-metals, liberating an electron is more difficult, so sharing is the only option when confronted with another species of similar electronegativity.

However, covalent bonding involving metals is particularly important, especially in industrial catalysis and process chemistry. Many polymerization techniques require catalysis involving metal-organic covalent bonds. In their more useful applications, metals often engage in more exotic covalent bonding, such as ...

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Bond order is a term that describes the number of pairs of electrons shared between atoms forming a covalent bond.

1) The most common type of covalent bond is the single bond, sharing only one pair of electrons between two atoms. It usually consists of one sigma bond.

All bonds with more than one shared pair are called multiple covalent bonds.

2) Sharing two pairs is called a double bond. An example is in ethylene (between the carbon atoms). It usually consists of one sigma bond and one pi bond.

3) Sharing three pairs is called a triple bond. An example is in hydrogen cyanide (between C and N). It usually consists of one sigma bond and two pi bonds.

4) Quadruple bonds, though rare, exist. Both carbon and silicon can theoretically form these; however, the formed molecules are explosively unstable. Stable quadruple bonds are observed as transition metal-metal bonds, usually between two transition metal atoms in organometallic compounds. Molybdenum and Ruthenium are the elements most commonly observed with this bonding configuration. An example of a quadruple bond is also found in Di-tungsten tetra(hpp).

5) Quintuple bonds are found to exist in certain chromium dimers.

6) Sextuple bonds, of order 6, have also been observed in transition metals in the gaseous phase at very low temperatures and are extremely rare.

Other more exotic bonds, such as three center bonds are known and defy the conventions of bond order. It is also important to note that bond order is an integer value only in the elementary sense and is often fractional in more advanced contexts.

Resonance

Some structures can have more than one valid Lewis Dot Structure (for example, ozone, O3). In an LDS diagram of O3, the center atom will have a single bond with one atom and a double bond with the other. The LDS diagram cannot tell us which atom has the double bond; the first and second adjoining atoms have equal chances of having the double bond. These two possible structures are called resonance structures. In reality, the structure of ozone is a resonance hybrid between its two possible resonance structures. Instead of having one double bond and one single bond, there are actually two 1.5 bonds with approximately three electrons in each at all times.

A special resonance case is exhibited in aromatic rings of atoms (for example, benzene). Aromatic rings are composed of atoms arranged in a circle (held together by covalent bonds) that may alternate between single and double bonds according to their LDS. In actuality, the electrons tend to be disambiguously and evenly spaced within the ring. Electron sharing in aromatic structures is often represented with a ring inside the circle of atoms.

Peer Reviews

Here's what a star student thought of this essay

crystalclearmagic

10th of Feb 2012

Quality of writing

Generally, the spelling, grammar and punctuation are good. The technical terms were used appropriately in the work, with the exception of the misunderstanding of electron configuration. This misunderstanding would obviously lower the mark as it is one of the fundamentals to the topic of bonding in Chemistry. It would also perhaps have been appropriate to define some of the technical terms (such as what is meant by a sigma bond) just for clarity. Overall, the quality of the piece of work is good and doesnÃ¢â‚¬â„¢t present any major problems that would significantly lower the mark achieved.

Level of analysis

The student, with an obvious understanding of covalent bonds, has analysed the bonding well. They have broken down the bonding into different sections and clearly laid out their work. This in turn improved the fluency of the work. The examples used to illustrate the different covalent bonds were spot on, despite them being the typical examples that most students use. However, this does not affect the mark as all the student is required to do is to demonstrate a clear understanding of the question set and support this understanding by illustrating it with an example. Therefore, the usage of the examples made it clear that they understood the difference and could give an example to support it. No necessary conclusion was required as presumably the question was only about describing covalent bonds.

Response to question

It is hard to judge the question set for such a piece of work as it will vary from one exam board to another. Overall, it seems the question is based on covalent bonding. This, the student addresses and answers very well. However, there are a few questionable areas. The student shows a clear understanding of what a covalent bond is, but generalises too much in the definition. Perhaps distinguishing between single covalent bonding and dative covalent bonding is necessary here. This would show the studentÃ¢â‚¬â„¢s understanding of the topic further by being able to differentiate between different types of covalent bonding right from the start of the work. In turn, this would contribute to the student gaining a higher grade as they have gone Ã¢â‚¬Ëœone step furtherÃ¢â‚¬â„¢ in answering the question. One major error in this is what the student defined as electron configuration. It is not how atoms tend to share electrons, but simply the configuration of electrons in the energy levels in the atom. Their response is very clearly laid out but the diagram of CH4 perhaps lacks a title and a description of why it had been used in the work (i.e. its relevance to the question).

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