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Atomic structure of carbon

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  Atomic orbitals The atomic orbitals available for the six electrons of carbon are the  s  orbital in the first shell, the  s  orbital in the second shell and the three  p  orbitals in the second shell. The 1 s  and 2 s  orbitals are spherical in shape. The 2 p  orbitals are dumbbell in shape and can be assigned 2 p x ,  2 p y  or 2 p z  depend-ing on the axis along which they are aligned. Energy levels The 1 s  orbital has a lower energy than the 2 s  orbital which has a lower energy than the 2 p  orbitals. The 2 p  orbitals have equal energy (i.e. they are degenerate). Electronic configuration Carbon is in the second row of the periodic table and has six electrons which will fill up lower energy atomic orbitals before entering higher energy orbitals (aufbau principle). Each orbital is allowed a maximum of two elec-trons of opposite spin (Pauli exclusion principle). When orbitals of equal energy are available, electrons will occupy separate orbitals before pairing up (Hund’s rule). T
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Covalent bonding and hybridization

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  Covalent bonding When two hydrogen atoms approach each other, their 1 s  atomic orbitals interact to form a bonding and an antibonding molecular orbital (MO). A stable covalent bond is formed when the bonding MO is filled with a pair of electrons and the antibonding MO is empty. Sigma bonds Sigma ( σ ) bonds are strong bonds with a circular cross-section formed by the head-on overlap of two atomic orbitals. Hybridization The electronic configuration of atomic carbon implies that carbon should form two bonds. However, it is known that carbon forms four bonds. When carbon is part of an organic structure, it can ‘mix’ the 2 s  and 2 p  orbitals of the valence shell in a process known as hybridization. There are three possible types of hybridization –  sp 3 ,  sp 2  and  sp  hybridization.   Covalent bonding A covalent bond binds two atoms together in a molecular structure and is formed when atomic orbitals overlap to produce a  molecular orbital  – so called because the orbital belongs
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sp3 Hybridization

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  Definition In  sp 3  hybridization, the  s  and the  p  orbitals of the second shell are ‘mixed’ to form four hybridized  sp 3  orbitals of equal energy. Electronic configuration Each hybridized orbital contains a single unpaired electron and so four bonds are possible. Geometry Each  sp 3  orbital is shaped like a deformed dumbbell with one lobe much larger than the other. The hybridized orbitals arrange themselves as far apart from each other as possible such that the major lobes point to the cor-ners of a tetrahedron.  sp 3  Hybridization explains the tetrahedral carbon in saturated hydrocarbon structures. Sigma bonds Sigma ( σ ) bonds are strong bonds formed between two  sp 3  hybridized car-bons or between an sp 3  hybridized carbon and a hydrogen atom. A σ  bond formed between two  sp 3  hybridized carbon atoms involves the overlap of half filled  sp 3  hybridized orbitals from each carbon atom. A σ  bond formed between an  sp 3  hybridized carbon and a hydrogen atom involves a
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sp2 Hybridization

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  Definition In  sp 2  hybridization, a 2 s  orbital is ‘mixed’ with two of the 2 p  orbitals to form three hybridized  sp 2  orbitals of equal energy. A single 2 p  orbital is left over which has a slightly higher energy than the hybridized orbitals. Electronic configuration For carbon, each  sp 2  hybridized orbital contains a single unpaired electron. There is also a half-filled 2p orbital. Therefore, four bonds are possible. Geometry Each  sp 2  orbital is shaped like a deformed dumbbell with one lobe much larger than the other. The remaining 2 p  orbital is a symmetrical dumbbell. The major lobes of the three  sp 2  hybridized orbitals point to the corners of a triangle, with the 2 p  orbital perpendicular to the plane. Alkenes Each  sp 2  hybridized carbon forms three  σ  bonds using three  sp 2  hybridized orbitals. The remaining 2 p  orbital overlaps ‘side on’ with a neighboring 2 p  orbital to form a pi ( π ) bond. The  π  bond is weaker than the  σ  bond, but is strong enough
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