why does carbon form four covalent bonds
tetravalence is the state of an with four available for chemical in its outermost, giving the atom a of four. An example is ): the tetravalent atom forms a covalent bond with four. The carbon atom is called tetravalent because it forms 4 covalent bonds. A carbon atom has a total of six electrons occupying the first two shells, i. e. , the K-shell has two electrons and the L-shell has four electrons. This distribution indicates that in the outermost shell there are one completely filled 's' orbital and two half-filled 'p' orbitals, showing carbon to be a. But in actuality, carbon displays tetravalency in the combined state. Therefore, a carbon atom has four valence electrons. It could gain four electrons to form the C. Both these conditions would take carbon far away from achieving stability by the. To overcome this problem carbon undergoes bonding by sharing its valence electrons. This allows it to be covalently bonded to one, two, three or four carbon atoms or atoms of other elements or groups of atoms. Let us see how carbon forms the single, double and triple bonds in the following examples. A carbon atom has four electrons in its outermost valence shell. So, it needs four more electrons to complete its octet. A carbon atom completes its octet only by sharing its valence electrons with other atoms. As a result, a carbon atom forms four covalent bonds by sharing valence electrons with other atoms. This is known as tetravalency of carbon ("tetra" means four). These four valences of carbon are directed towards four corners of a tetrahedron, and inclined to each other atomic an angle of a 109` 28d. The carbon atom is assumed to be atomic the center of the tetrahedron. In common use, the four valences of carbon are shown by four bonds around a carbon atom as shown alongside molecule: Each carbon atom has four electrons in its outermost shell.
Thus, it requires four more electrons to acquire a stable noble gas configuration. Each of the hydrogen atoms has only one electron in its outermost shell and requires one more electron to complete its outermost shell (to acquire He configuration). To achieve this, one carbon atom forms four single covalent bonds with four hydrogen atoms. molecule: Each carbon atom has four electrons in its outermost shell and each atom has six electrons in its outermost shell. Thus, each carbon atom requires four, and each oxygen atom requires two more electrons to acquire noble gas configurations. To achieve this, two oxygen atoms form a double covalent bond with carbon. molecule: Each carbon atom has four electrons in its outermost shell and each hydrogen atom has only one electron in its outermost shell. Two carbon atoms share two electrons each with hydrogen atoms to form single bonds. Each carbon then requires three more electrons to acquire a stable configuration of the nearest noble gas (neon). This is done by mutually sharing three pairs of electrons between the two carbon atoms to form a triple bond. Most people are not in the slightest bit interested in the answer to this question. For the 1% who are though, this vexing little question is at the heart of organic chemistry - that bitch of subject that seems to drive people alternately mad/ecstatic. I always knew C formed 4 bonds, and got through uni organic chem and some other stuff without really knowing/remembering why. So. years later, I decided to sort myself out, with a particular interest in why the 4 single bonds carbon makes are done with 'sp3' orbitals and why a carbon double bond uses 'sp2' etc.
The numbers 3 and 2 didn't seem to have anything to do with anything - but they do, as you can see below. The trivial answer is that a carbon atom that forms 4 covalent bonds is in a lower energy state that one that forms 2. But what happens when this occurs? Carbon's valence shell is configured 2s2 2p2 (remember the p shell can contain up to 6 electrons, in a 3-D 2px, 2py, 2pz shape). Two half-filled p orbitals should mean stable molecules like CH2 - the 2 electrons from the H filling the 2px and 2py subshells. But. these don't normally exist. instead carbon likes to form 4 covalent bonds with other atoms. Why 4 and not 2? Because the 2s2 2p2 shell hybridizes. Why does it hybridise? Because it results in a lower energy state. After hybridization there are NO LONGER any s or p shells, and we're left with 4 hybrid orbitals that are 'a little bit s and a little bit p'. This is really important to understand. They've gone forever! Why are the 4 hybrid orbitals called 'sp3'? Because the new hybrid effectively replaces 1 s and 3 p orbitals, to give 4 single bonding opportunities. 2s2 2p2 becomes : sp3 sp3 sp3 sp3 (the hybrid). What's an sp3 shell look like? sp3 shells look nothing like s or p shells. more like a short stocky baseball bat (whereas an s orbital is a sphere, and a p orbital is like 3 double baseball bats without the handles! ). They're called sp3 because the s and the 3 p shells combine to make 4 sp3 orbitals. Because there are 4 of them, and they like to be as far away from each other as possible, C forms (single bonding) tetrahedral shapes ie. 4 triangle surface planes. I admit that wasn't a great job explaining one of organic chemistry's most fundamental concepts, but hopefully it helped.
The most important thing I realised was that sp3 orbitals are not s and not p. They are completely different, and the old s and p orbitals have gone. How does a double bond work? Think of a carbon double bond in a molecule like ethylene as a carbon with 3 bonds. The hybridization logic is the same as for sp3, but this time we end up with 2 single bonds, and 1 double bond. Using ethylene as an example: 1 x s and 2 p orbitals create 3 x sp2 orbitals. The remaining p orbital on the 2 C atoms forms a pi bond. In conjunction with a sigma bond, the pi+sigma form the double bond between the carbons. The hybrids are called sp2 because they're made up of 1 x s orbital, and 2 of the p orbitals (as opposed to 3 of the p orbitals in sp3 hybridization). A triple bond's no different: 1 x s and 1 x p form an sp orbital. 2 p's then form 2 pi bonds (used for the triple bond in conjunction with the remaining sigma bond). Between the carbon atoms, you've got one sigma bond (from the sp's touching), and 2 pi bonds to make 3! (the sp hybrid has nothing to do with the normal p pi bonding). Easy eh? Hopefully this explanation also clears up that confusing issue of naming the hybrid orbitals. sp3 for 4 bonds, sp2 for double, and sp for triple never made any sense to me, until I realised they're just the formula that represents how many of the old s and p bonds combined. And a quick note on terminology: the orbitals are named 's', 'p' and 'sp3' etc. but the bonds are called 'sigma' or 'pi', depending on their source. * A more advanced description is available at: And a good explanation is also available at:. The Carey book is an excellent resource for organic chemistry.
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