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Lone pairs
A lone pair is a (valence) electron pair without bonding or sharing with other atoms. They are found in the outermost electron shell of an atom, so lone pairs are a subset of a molecule's valence electrons. They can be identified by examining the outermost energy level of an atom — lone electron pairs consist of paired electrons as opposed to single electrons, which may appear if the atomic orbital is not full. Electron pairs are therefore considered lone pairs if two electrons are paired but are not used in bonding. Thus, the number of lone electrons plus the number of bonding electrons equal the total number of valence electrons from a compound.

A single lone pair can be found with atoms in the nitrogen group such as nitrogen in ammonia, two lone pairs can be found with atoms in the the chalcogen group such as oxygen in water and the halogens can carry three lone pairs such as in hydrochloric acid.

Angle Changes

The pairs often exhibit a negative polar character with their high charge density and are located closer to the atomic nucleus on average compared to the bonding pair of electrons. The presence of a lone pair decreases the bond angle between the bonding pair of electrons, due to their high electric charge which causes great repulsion between the electrons. They are also used in the formation of a dative bond. For example, the creation of the hydronium (H3O+) ion occurs when acids are dissolved in water and is due to the oxygen atom donating a lone pair to the hydrogen ion.

This can be seen more clearly when looked at in two more common molecules. For example methane (CH4) has an angle between the hydrogens of 109.5o, whereas in water (H2O) the angle between the hydrogens is just 104.5 o. As you can clearly see, if lone pairs are present (in water there are two) then the other hydrogens are pushed further away to a point where there is the least repulsion from the lone pair but also from the other electrons. That's an illustration of the VSEPR theory.

Unusual lone pairs

A stereochemically active lone pair is also expected for divalent lead and tin ions due to their formal electronic configuration of ns2. In the solid state this results in the distorted metal coordination observed in the litharge structure adopted by both PbO and SnO. The formation of these heavy metal ns2 lone pairs which was previously attributed to intra-atomic hybridization of the metal s and p states[1] has recently been shown to have a strong anion dependence[2]. This dependence on the electronic states of the anion can explain why some divalent lead and tin materials such as PbS and SnTe show no stereochemical evidence of the lone pair and adopt the symmetric rocksalt crystal structure[3],[4].

In molecular systems the lone pair can also result in a distortion in the coordination of ligands around the metal ion. The lead lone pair effect can be observed in supramolecular complexes of Lead(II) nitrate and in 2007 a study [5] linked the lone pair to lead poisoning. Lead ions in the human metabolism replace native metallic ions in several key proteins, for example: zinc cations in the ALAD protein, which is also known as Porphobilinogen synthase. This seems to be the molecular basis of "lead poisoning", or "saturnism" ("plumbism"). Computational experiments reveal that although the coordination number does not change upon substitution in calcium-binding proteins, the introduction of lead distorts the way the ligands organize themselves to accommodate such an emerging lone pair: consequently, these proteins are perturbed. This lone-pair effect becomes dramatic for zinc-binding proteins, such as the above-mentionned porphobilinogen synthase, as the natural substrate cannot bind anymore: in those cases the protein is inhibited.

Group 14 lone pairs manifest themselves in triple bonds as well. The familiar alkynes have bond order 3 with 180° bond angles (A) but going down the row germanium to germanium formal triple bonds have an effective bond order 2 with one lone pair (B) and trans-bent geometries. In lead the bond order is even 1 with lone pairs for each lead atom (C). In the organogermanium compound D, the bond order is also 1 with complexation of the acidic isonitrile groups based on interaction with germaniums empty 4p orbital [6]

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Lone pair trends in group 14 triple bonds

See also

References

1. ^ Stereochemistry of Ionic Solids J.D.Dunitz and L.E.Orgel, Advan. Inorg. and Radiochem. 1960, 2, 1-60
2. ^ Electronic origins of structural distortions in post-transition metal oxides: experimental and theoretical evidence for a revision of the lone pair model D.J.Payne, R.G.Egdell, A.Walsh, G.W.Watson, J.Guo, P.-A.Glans, T.Learmonth and K.E.Smith, Phys. Rev. Lett. 2006, 96, 157403 doi:10.1103/PhysRevLett.96.157403
3. ^ The origin of the stereochemically active Pb(II) lone pair: DFT calculations on PbO and PbS A.Walsh and G.W.Watson, J. Sol. Stat. Chem. 2005, 178, 5 doi:10.1016/j.jssc.2005.01.030
4. ^ Influence of the Anion on Lone Pair Formation in Sn(II) Monochalcogenides: A DFT Study A.Walsh and G.W.Watson, J. Phys. Chem. B 2005, 109, 18868 doi:10.1021/jp051822r
5. ^ ''Is an Electronic Shield at the Molecular Origin of Lead Poisoning? A Computational Modeling Experiment'' C.Gourlaouen and O.Parisel Angew. Chem. Int. Ed. 2007, 46, 553 –556 doi:10.1002/anie.200603037
6. ^ Lewis base induced tuning of the Ge–Ge bond order in a digermyne G.H.Spikes and P.P.Power Chem. Commun., 2007, 85 - 87, doi:10.1039/b612202g
In chemistry, valence electrons are the electrons contained in the outermost, or valence, electron shell of an atom. Valence electrons are important in determining how an element reacts chemically with other elements: The fewer valence electrons an atom holds, the less
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atom (Greek ἄτομος or átomos meaning "indivisible") is the smallest particle still characterizing a chemical element.
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An electron shell, also known as a main energy level, is a group of atomic orbitals with the same value of the principal quantum number n. Electron shells are made up of one or more electron subshells, or sublevels
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molecule is defined as a sufficiently stable electrically neutral group of at least two atoms in a definite arrangement held together by strong chemical bonds.[1][2] In organic chemistry and biochemistry, the term molecule
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In chemistry, valence electrons are the electrons contained in the outermost, or valence, electron shell of an atom. Valence electrons are important in determining how an element reacts chemically with other elements: The fewer valence electrons an atom holds, the less
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energy levels are possible. The term energy level is most commonly used in reference to the electron configuration in atoms or molecules. In other words, the energy spectrum can be quantized (see continuous spectrum for the more general case).
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An atomic orbital is a mathematical description of the region in which an electron may be found around a single atom.[1] Specifically, atomic orbitals are the possible quantum states of the individual electrons in the electron cloud around a single atom.
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In chemistry, valence electrons are the electrons contained in the outermost, or valence, electron shell of an atom. Valence electrons are important in determining how an element reacts chemically with other elements: The fewer valence electrons an atom holds, the less
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The Nitrogen group elements (a.k.a. group VA) are also known as IUPAC Group 15 (formerly Group V) of the periodic table.

This group has the defining characteristic that all the component elements have 5 electrons in their outermost shell, that is 2 electrons in the s
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Ammonia is a compound with the formula NH3. It is normally encountered as a gas with a characteristic pungent odor. Ammonia contributes significantly to the nutritional needs of the planet as a precursor to foodstuffs and fertilizers.
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The chalcogens (with the "ch" pronounced with a hard "c" as in "chemistry") are the name for the periodic table group 16 (old-style: VIB or VIA) in the periodic table. It is sometimes known as the oxygen family.
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halogens or halogen elements are a series of nonmetal elements from Group 17 (old-style: VII or VIIA; Group 7 IUPAC Style) of the periodic table, comprising fluorine, F; chlorine, Cl; bromine, Br; iodine, I; and astatine, At.
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hydrochloric acid is the aqueous (water-based) solution of hydrogen chloride gas (HCl). It is a strong acid, the major component of gastric acid and of wide industrial use. Hydrochloric acid must be handled with appropriate safety precautions because it is a highly corrosive liquid.
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Flavour in particle physics
 

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The nucleus of an atom is the very small dense region of an atom, in its center consisting of nucleons (protons and neutrons). The size (diameter) of the nucleus is in the range of 1.
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coordinate covalent bond (also known as dative bond) is a description of covalent bonding between two atoms in which both electrons shared in the bond come from the same atom.
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In chemistry, hydronium is the common name for the cation H3O+ derived from protonation of water. It is the simplest type of an oxonium ion.

Nomenclature

According to IUPAC ion nomenclature, the hydronium ion should be referred to as
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2, −1
(neutral oxide)
Electronegativity 3.44 (Pauling scale)
Ionization energies
(more) 1st: 1313.9 kJmol−1
2nd: 3388.3 kJmol−1
3rd: 5300.5 kJmol−1

Atomic radius 60 pm
Atomic radius (calc.
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1, −1
(amphoteric oxide)
Electronegativity 2.20 (Pauling scale) More

Atomic radius 25 pm
Atomic radius (calc.) 53 pm
Covalent radius 37 pm
Van der Waals radius 120 pm
Miscellaneous

Thermal conductivity (300 K) 180.
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molecule is defined as a sufficiently stable electrically neutral group of at least two atoms in a definite arrangement held together by strong chemical bonds.[1][2] In organic chemistry and biochemistry, the term molecule
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Methane is a chemical compound with the molecular formula CH4. It is the simplest alkane, and the principal component of natural gas. Methane's bond angles are 109.
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Water is a common chemical substance that is essential to all known forms of life.[1] In typical usage, water refers only to its liquid form or state, but the substance also has a solid state, ice, and a gaseous state, water vapor.
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Valence shell electron pair repulsion (VSEPR) theory (1957) is a model in chemistry, which is used for the representation of shapes of individual molecules, based upon their extent of electron-pair electrostatic repulsion, determined using steric numbers[1].
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2
(Amphoteric oxide)
Electronegativity 2.33 (scale Pauling)
Ionization energies
(more) 1st: 715.6 kJmol−1
2nd: 1450.5 kJmol−1
3rd: 3081.
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TIN may refer to:
  • Tax identification number
  • Triangulated irregular network, a data structure used in a geographic information systems

See also

  • Tin
This article is about the metallic chemical element.

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Lead(II) nitrate is a chemical compound, the inorganic salt of nitric acid and lead, with the formula Pb(NO3)2. It is a colourless crystal or white powder and a strong, stable oxidizer. Unlike most other lead(II) salts, it is soluble in water.
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Lead poisoning
Classification & external resources

ICD-10 T 56.0
ICD-9 984.9

Lead poisoning is a medical condition, also known as saturnism, plumbism or painter's colic, caused by increased blood lead levels.
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Porphobilinogen synthase (or ALA dehydratase, or Aminolevulinate dehydratase) synthesizes porphobilinogen through the asymmetric condensation of two molecules of aminolevulinic acid.
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In chemistry, coordination number (c.n.), as defined originally in 1893 by Alfred Werner, is the total number of neighbors of a central atom in a chemical compound.[1][2] In methane the coordination number for the carbon atom is 4.
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Enzyme inhibitors are molecules that bind to enzymes and decrease their activity. Since blocking an enzyme's activity can kill a pathogen or correct a metabolic imbalance, many drugs are enzyme inhibitors. They are also used as herbicides and pesticides.
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