Carbon: atomic number electrons

Thus, 87 gives a satisfactory picture for a ten w-electron naphthalene-like system, but the localization of charges is unsatisfactory because abundant chemical evidence shows that carbon atoms numbered 2 and 5 are more positive than carbon atoms 3 and 4, and according to 87, mesomeric displacement of the negative charge on atom 3a would lead to the opposite conclusion. 

However, when carbon atoms number 14, 18, and more, a molecule becomes a planar aromatic system again. Many years of persistent research had been spent before the compounds C14H14, CigHig and others were obtained. The authors called them [14] and [18]annulenes respectively. [18]annulene is the first (after benzene) cyclic polyene with (4A + 2) K-electrons, whose planar conformation is stable at room temperature. It is less stable than benzene, but is more stable than the corresponding acyclic polyene (nonaene) ... 

Thinking it Through The first step in applying the VSEPR method is to count the number of electron regions around the central atom. In this case, the Lewis structure is given, showing four pairs of electrons around the carbon atom. These electron pairs are all used to form bonds, and there are no lone pairs. Electron pair repulsion is minimized when the four electron pairs form a tetrahedron around the carbon atom, choice (D). Observe the similarity to methane, CH4, which was probably the first tetrahedral molecule you studied. 

The atomic number, the number of protons and hence the positive charge of the nucleus, determines the number of electrons that surround it. An electron has the same magnitude of electric charge as a proton, but opposite in sign. Therefore, for an atom to be electrically neutral the number of electrons outside the nucleus must be the same as the number of protons inside the nucleus. That is, the number of electrons is equal to the atomic number. Thus, hydrogen (atomic number 1) has one electron, carbon (atomic number 6) has six electrons, and so on, up to livermorium with its 116 electrons. Electrons are much lighter than protons and neutrons (by a factor of nearly 2,000), so their presence barely affects the mass of an atom. They have a profound effect on the chemical and physical properties of the element, and almost all chemistry can be traced to their behaviour. 

SOLUTION The element is carbon, atomic number 6. The number of protons or electrons equals the atomic number and is 6. The number of neutrons is determined by subtracting the atomic number from the mass number 14 - 6 = 8. 

The electron configuration of carbon, atomic number 6, is ls 2s 2p. The four outer (valence) electrons are shown by the Lewis symbol... 

Unlike the forces between ions which are electrostatic and without direction, covalent bonds are directed in space. For a simple molecule or covalently bonded ion made up of typical elements the shape is nearly always decided by the number of bonding electron pairs and the number of lone pairs (pairs of electrons not involved in bonding) around the central metal atom, which arrange themselves so as to be as far apart as possible because of electrostatic repulsion between the electron pairs. Table 2.8 shows the essential shape assumed by simple molecules or ions with one central atom X. Carbon is able to form a great many covalently bonded compounds in which there are chains of carbon atoms linked by single covalent bonds. In each case where the carbon atoms are joined to four other atoms the essential orientation around each carbon atom is tetrahedral. 

These absorptions are ascribed to n-n transitions, that is, transitions of an electron from the highest occupied n molecular orbital (HOMO) to the lowest unoccupied n molecular orbital (LUMO). One can decide which orbitals are the HOMO and LUMO by filling electrons into the molecular energy level diagram from the bottom up, two electrons to each molecular orbital. The number of electrons is the number of sp carbon atoms contributing to the n system of a neuhal polyalkene, two for each double bond. In ethylene, there is only one occupied MO and one unoccupied MO. The occupied orbital in ethylene is p below the energy level represented by ot, and the unoccupied orbital is p above it. The separation between the only possibilities for the HOMO and LUMO is 2.00p. 

Carbanions are negatively charged organic species with an even number of electrons and the charge mainly concentrated on a carbon atom. In alkyl, alkenyl, and alkynyl anions all of the... 

There exist a number of d -synthons, which are stabilized by the delocalization of the electron pair into orbitals of hetero atoms, although the nucleophilic centre remains at the carbon atom. From nitroalkanes anions may be formed in aqueous solutions (e.g. CHjNOj pK, = 10.2). Nitromethane and -ethane anions are particularly useful in synthesis. The cyanide anion is also a classical d -synthon (HCN pK = 9.1). 

As a consequence of the alternative distribution of an even number (2n) TT electrons on an odd number (2n - 1) carbon atoms, centers of the methine chain susceptible to nucleophilic attack are effectively the even carbons atoms starting from nitrogen, as it has been proven experimentally (103), particularly with a ketomethyiene giving a neutrocyanine compound (53, 67). 

A neutral carbon atom has four valence electrons Five electrons are assigned to the CH2OH carbon therefore it has an oxidation number of -1 Seven electrons are assigned to the CH3 carbon therefore it has an oxidation number of-3 As expected this method gives an oxidation number of -2 for oxygen and +1 for each hydrogen... 

The free radicals that we usually see in carbon chemistry are much less stable than these Simple alkyl radicals for example require special procedures for their isolation and study We will encounter them here only as reactive intermediates formed m one step of a reaction mechanism and consumed m the next Alkyl radicals are classified as primary secondary or tertiary according to the number of carbon atoms directly attached to the carbon that bears the unpaired electron... 

With an atomic number of 28 nickel has the electron conflguration [Ar]4s 3c (ten valence electrons) The 18 electron rule is satisfied by adding to these ten the eight elec Irons from four carbon monoxide ligands A useful point to remember about the 18 electron rule when we discuss some reactions of transition metal complexes is that if the number is less than 18 the metal is considered coordinatively unsaturated and can accept additional ligands... 

Chiral Center. The chiral center, which is the chiral element most commonly met, is exemplified by an asymmetric carbon with a tetrahedral arrangement of ligands about the carbon. The ligands comprise four different atoms or groups. One ligand may be a lone pair of electrons another, a phantom atom of atomic number zero. This situation is encountered in sulfoxides or with a nitrogen atom. Lactic acid is an example of a molecule with an asymmetric (chiral) carbon. (See Fig. 1.13b.)... 

In thin sections natural graphite is translucent, strongly pleochroic, and uniaxial. It has a negative sign of birefringence and two extinctions per revolution under crossed Nicol prisms. The atomic number of carbon accounts for its low absorption coefficient for x-rays and electrons. 

Electron donation from pyrrole-like nitrogen, or to a lesser extent from analogous sulft or oxygen atoms, helps electrophilic attack at azole carbon atoms, but as the number c heteroatoms in the ring increases, the tendency toward electrophilic attack at both C an N decreases rapidly. 

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