Valence shell atomic orbitals fluorine

A second example is the minimal-basis-set (MBS) Hartree-Fock wave function for the diatomic molecule hydrogen fluoride, HF (Ransil 1960). The basis orbitals are six Slater-type (i.e., single exponential) functions, one for each inner and valence shell orbital of the two atoms. They are the Is function on the hydrogen atom, and the Is, 2s, 2per, and two 2pn functions on the fluorine atom. The 2sF function is an exponential function to which a term is added that introduces the radial node, and ensures orthogonality with the Is function on fluorine. To indicate the orthogonality, it is labeled 2s F. The orbital is described by... 

Although the boron atom only provides three valence electrons, the four valence shell orbitals are occupied additionally by three a electrons and a pair of n electrons from the fluorine atoms to make up the normal octet of electrons surrounding central atoms of the main group elements. The three canonical forms of the BF3 molecule are shown in Figure 6.5. 

The SbF6 ion has a central antimony atom with a ground state of 5s25p2 which is trivalent. If the 5s electrons were to be unpaired and one promoted to a 5d orbital the antimony would then be five-valent. If it accepted an electron to make it Sb and the electron was accommodated in another 5d orbital the ion would be six-valent and could accept the six electrons from the six ligand fluorine atoms to give a valence shell of six a pairs. This would indicate that the ion should have a regular octahedral shape. 

Similar effects are not shown by the heavier atoms. It is likely that the orbitals occupied by the unshared pairs of these atoms have a larger amount of d and / character, such that there is much less overlap than for the elements of the first row. For chlorine, for example, the 3d orbitals can be hybridized with 3s and 3p with a much smaller promotion energy than is required for the d character of the 2s and 2p orbitals of fluorine, which must go beyond its valence shell (to 3d) for d character. 

For diatomic (and linear polyatomic) molecules, the basis set usually consists of several STOs centered on each atom. Thus the MOs are expressed as LCAOs—linear combinations of atomic orbitals. In a mini- ia/-basis-set calculation, only inner-shell and valence-shell STOs are used. Thus a minimal basis calculation of HF would use as basis functions l.sH, 1 F, 2jf, 2/ ctf, 2p7TF, 2pTTF, where 2poF is a fluorine 2p AO along the internuclear (z) axis (i.e., a 2pz AO), and the 2pir and 2pir AOs are... 

For most atoms there wilt be a maximum of eight electrons in the valence shell (- Lewis octet structure). This is absolutely necessary for atoms of the elements lithium through fluorine since they have only four orbitals (an s and three p orbitals) in the valence shell. It is quite common, as well, for atoms of other elements to utilize only their s and p orbitals. Under these conditions the sum of shared pairs (bonds) and unshared pairs (lone pairs) must equal the number of orbitals—four. This is the maximum, and for elements having fewer than four valence electrons, the octet will usually not be filled. The following compounds illustrate these possibilities ... 

STRATEGY A fluorine atom forms only single bonds, so we anticipate that the Lewis structure consists of a shared pair between the central S atom and each of the four surrounding F atoms. However, each F atom has three lone pairs and supplies one bonding electron, and the S atom already has six elec-5 trons in its valence shell. So, there are two extra electrons. Because sulfur is in Period 3 and has empty 3d-orbitals available, it can expand its octet. 

Six of the seven valence electrons in a fluorine atom are already paired in three filled atomic orbitals and thus are not shared in bonding. The seventh fluorine valence electron, however, is unpaired and can be used in forming a covalent bond to another fluorine. Each atom in the resultant F2 molecule thereby achieves a filled valence-shell octet. The three pairs of nonbonding electrons on each fluorine atom are called lone pairs, or nonbonded pairs, and the shared electrons are called a bonding pair. 

Hence, each fluorine nucleus is involved in one single covalent bond, which contains two electrons, which are accommodated in a single molecular orbital that envelops the two nuclei. These two electrons are shared equally between the two fluorine nuclei. Using the above formula there are seven electrons normally present in the valence shell of the elemental atom, minus the eight that are actually present, plus half of the two that are shared in the single covalent bond, which results in an overall zero charge, i.e. 7-8+1/2(2)=0... 

In the second row, neon has a complete valence shell with both the 2s and 2p orbitals filled (in addition to the filled Is helium core). Sodium has one more eleetron than neon losing that electron gives Na, which has neon s stable filled shell. If a fluorine atom gains one electron to become fluoride, it has the complete octet of neon and a filled shell. 

XeF2 There are 10 valence electrons (8 from valence shell of xenon 5s2 5p6 and two form the two bonded fluorine atoms) to be filled in orbitals. These require five orbitals which can be formed by the hybridisation of one 5s, three 5p and one 5d orbitals (sp3d hybridization). These are directed towards the five comers of a trigonal bipyramid. Two of these contain shared electrons, the other three lone pairs. For greatest stability, the shared pairs are as far apart as possible, so Xe - F bonds are at 180° to each other and the stmcture is linear. 

The orbitals used are hybridized, sp d hybrids, and are directed to the apices of a trigonal bipyramid (Fig. 23.18). The phosphorus and three of the fluorine atoms lie in a plane the remaining two fluorine atoms are placed symmetrically above and below this plane. To promote an electron in nitrogen, the electron would have to be moved out of the valence shell to a shell of higher principal quantum number. The energy required would be too large to be compensated by the formation of two additional bonds. 

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