FACTOID # 15: A mere 0.8% of West Virginians were born in a foreign country.
 
 Home   Encyclopedia   Statistics   States A-Z   Flags   Maps   FAQ   About 
   
 
WHAT'S NEW
 

SEARCH ALL

FACTS & STATISTICS    Advanced view

Search encyclopedia, statistics and forums:

 

 

(* = Graphable)

 

 


Encyclopedia > Electron counting

Electron counting is a formalism used for classifying compounds and for explaining or predicting electronic structure and bonding. Many rules in chemistry rely on electron-counting: A chemical bond is the physical phenomenon (or phenomena) responsible for the attractive interactions between atoms that confers stability to di- and polyatomic chemical compounds. ...

Atoms that do not obey their rule are called "electron-deficient" (when they do not have enough electrons to achieve a Noble gas configuration, or "hypervalent" (when they have an excess of electrons). Since these compounds tend to be more reactive than compounds that do obey their rule, electron counting is an important tool in the identification of reactivity of molecules. The bonding in carbon dioxide The octet rule is a simple chemical rule of thumb that states that atoms tend to combine in such a way that they each have eight electrons in their valence shells, similar to the electronic configuration of a noble gas. ... General Name, Symbol, Number carbon, C, 6 Chemical series nonmetals Group, Period, Block 14, 2, p Appearance black (graphite) colorless (diamond) Atomic mass 12. ... General Name, Symbol, Number nitrogen, N, 7 Chemical series nonmetals Group, Period, Block 15, 2, p Appearance colorless Atomic mass 14. ... General Name, Symbol, Number oxygen, O, 8 Chemical series Nonmetals, chalcogens Group, Period, Block 16, 2, p Appearance colorless (gas) very pale blue (liquid) Atomic mass 15. ... The valence shells of a transition metal can accommodate 18 electrons: 2 in each of the five d orbitals, to give 10 in total; 2 in each of the 3 p orbitals, to give 6 in total; and 2 in the s orbital (see Electron counting). ... Inorganic chemistry is the branch of chemistry concerned with the properties and reactions of inorganic compounds. ... It has been suggested that Organometallic compounds be merged into this article or section. ... In chemistry, the term transition metal (sometimes also called a transition element) has two possible meanings: It commonly refers to any element in the d-block of the periodic table, including zinc, cadmium and mercury. ... The lanthanide series comprises the 15 elements from lanthanum to lutetium on the periodic table, with atomic numbers 57 through 71. ... The actinide series encompasses the 15 chemical elements that lie between actinium and lawrencium on the periodic table with atomic numbers 89 - 103. ... The polyhedral skeletal electron pair theory or Wades rules in chemistry are electron counting rules that apply to electron-poor cages such as boron hydrides which consist of many boron atoms linked together. ... In chemistry, a cluster is an array of bound atoms that is intermediate in character between a molecule and a solid. ... General Name, Symbol, Number boron, B, 5 Chemical series metalloids Group, Period, Block 13, 2, p Appearance black/brown Atomic mass 10. ... A polyhedron is a geometric shape which in mathematics is defined by three related meanings. ...

Contents

Counting rules

Two styles of electron counting are popular and both give the same result. The neutral counting approach is usually considered easier but the "ionic counting" approach rewards the user with a knowledge of oxidation states, which can be valuable. One can check one's calculation by counting employing both approaches.


Neutral counting

  • Locate the central atom on the periodic table and determine the number of its valence electrons. One counts valence electrons for main group elements differently from transition metals.
E.g. in period 2: B, C, N, O, and F have 3, 4, 5, 6, and 7 valence electrons, respectively.
E.g. in period 3: K, Ca, Ti, V, Cr, Fe, Ni have 1, 2, 4, 5, 6, 8, 10 valence electrons respectively.
  • Add one for every halide or other anionic ligand which binds to the central through a sigma bond.
  • Add two for every lone pair bonding to the metal (e.g. each Lewis base binds with a lone pair). Unsaturated hydrocarbons such as alkenes and alkynes are considered Lewis bases. Similarly Lewis and Bronsted acids (protons) contribute nothing.
  • Add one for each homoelement bond.
  • Add one for each negative charge, and subtract one for each positive charge.

A halide is a binary compound, of which one part is a halogen atom and the other part is an element or radical that is less electronegative than the halogen, to make a fluoride, chloride, bromide, iodide, or astatide compound. ...

Ionic counting

  • Calculate the number of electrons of the element, assuming an oxidation state
e.g. for a Fe2+ has 6 electrons
S2- has 8 electrons
  • Add two for every halide or other anionic ligand which binds to the metal through a sigma bond.
  • Add two for every lone pair bonding to the metal (e.g. each phosphine ligand can bind with a lone pair). Similarly Lewis and Bronsted acids (protons) contribute nothing.
  • For unsaturated ligands such as alkenes, count the number of carbon atoms binding to the metal. Each carbon atom provides one electron.

A halide is a binary compound, of which one part is a halogen atom and the other part is an element or radical that is less electronegative than the halogen, to make a fluoride, chloride, bromide, iodide, or astatide compound. ...

Electrons donated by common fragments

Ligand Electrons contributed
(neutral counting)
Electrons contributed
(ionic counting)
X 1 2 (X-; X = F, Cl, Br, I)
H 1 2 (H-)
O 2 4(O2-)
N 3 6 (N3-)
NR3 2 2 (NR3; R = H, alkyl, aryl)
CR2 2 4 (CR22-)
Ethylene 2 2 (C2H4)
cyclopentadienyl 5 6(C5H5-)
benzene 6 6 (C6H6)

A halide is a binary compound, of which one part is a halogen atom and the other part is an element or radical that is less electronegative than the halogen, to make a fluoride, chloride, bromide, iodide, or astatide compound. ... A hydride is a compound of hydrogen with more electropositive elements. ... An oxide is a chemical compound of oxygen with other chemical elements. ... Definition The nitride ion is very very gay and retarded A nitride (compound) is a compound that has nitrogen with more electropositive elements. ... The general structure of an amine Amines are organic compounds and a type of functional group that contain nitrogen as the key atom. ... A transition metal carbene complex in organometallic chemistry is a compound bearing a formal carbon-metal bond. ... Ethylene (or IUPAC name ethene) is the chemical compound with the formula CH2CH2. ... A cyclopentadienyl complex is a metal complex with one or more cyclopentadienyl groups (C5H5-, abbreviated as Cp). ... Benzene, also known as benzol, is an organic chemical compound with the formula C6H6. ...

"Special cases"

The numbers of electrons "donated" by some ligands depends on the geometry of the metal-ligand ensemble. Perhaps the most famous example of this complication is the M-NO entity. When this grouping is linear, the NO ligand is considered to be a three-electon ligand. When the M-NO subunit is strongly bent at N, the NO is treated as a pseudohalide and is thus a one electron (in the neutral counting approach). The situation is not very different from the η-3 vs. η-1 allyl. Another unusual ligand from the electron counting perspective is sulfur dioxide. The nitrosyl group is NO, a nitrogen atom plus an oxygen atom. ...


Examples of electron counting

  • CH4, for the central C
neutral counting: C contributes 4 electrons, each H radical contributes one each: 4+4(1) = 8 valence electrons
ionic counting: C4- contributes 8 electrons, each proton contributes 0 each: 8 + 4(0) = 8 electrons.
Similar for H:
neutral counting: H contributes 1 electron, the C contributes 1 electron (the other 3 electrons of C are for the other 3 hydrogens in the molecule): 1 + 1(1) = 2 valence electrons.
ionic counting: H contributes 0 electrons (H+), C4- contributes 2 electrons (per H), 0 + 1(2) = 2 valence electrons
conclusion: Methane follows the octet-rule for carbon, and the duet rule for hydrogen, and hence is expected to be a stable molecule (as we see from daily life)
  • H2S, for the central S
neutral counting: S contributes 6 electrons, each hydrogen radical contributes one each: 6+2(1) = 8 valence electrons
ionic counting: S2- contributes 8 electrons, each proton contributes 0: 8+2(0) = 8 valence electrons
conclusion: with an octet electron count (on sulfur), we can anticipate that H2S would be pseudotetrahedral if one considers the two lone pairs.
  • SCl2, for the central S
neutral counting: S contributes 6 electrons, each chlorine radical contributes one each: 6+2(1) = 8 valence electrons
ionic counting: S2+ contributes 4 electrons, each chloride anion contributes 2: 4+2(2) = 8 valence electrons
conclusion: see discussion for H2S above. Notice that both SCl2 and H2S follow the octet rule - the behavior of these molecules is however quite different.
  • SF6, for the central S
neutral counting: S contributes 6 electrons, each fluorine radical contributes one each: 6+6(1) = 12 valence electrons
ionic counting: S6+ contributes 0 electrons, each fluoride anion contributes 2: 0+6(2) = 12 valence electrons
conclusion: ionic counting indicates a molecule lacking lone pairs of electrons, therefore its structure will be octahedral, as predicted by VSEPR. One might conclude that this molecule would be highly reactive - but the opposite is true: SF6 is inert, and it is widely used in industry because of this property.
  • TiCl4, for the central Ti
neutral counting: Ti contributes 4 electrons, each chlorine radical contributes one each: 4+4(1) = 8 valence electrons
ionic counting: Ti4+ contributes 0 electrons, each chloride anion contributes two each: 0+4(2) = 8 valence electrons
conclusion: Having only 8e (vs. 18 possible), we can anticipate that TiCl4 will be a good Lewis acid. Indeed, it reacts (in some cases violently) with water, alcohols, ethers, amines.
neutral counting: Fe contributes 8 electrons, each CO contributes 2 each: 8 + 2(5) = 18 valence electrons
ionic counting: Fe(0) contributes 8 electrons, each CO contributes 2 ech: 8 + 2(5) = 18 valence electrons
conclusions: this is a special case, where ionic counting is the same as neutral counting, all fragments being neutral. Since this is an 18-electron complex, it is expected to be isolable compound.
neutral counting: Fe contributes 8 electrons, the 2 cyclopentadienyl-rings contribute 5 each: 8 + 2(5) = 18 electrons
ionic counting: Fe2+ contributes 6 electrons, the two aromatic cyclopentadienyl rings contribute 6 each: 6 + 2(6) = 18 valence electrons on iron.
conclusion: Ferrocene is expected to be an isolable compound.


please note: these examples show the methods of electron counting, they are a formulism, and don't have anything to do with real life chemical transformations. Most of the 'fragments' mentioned above do not exist as such; they cannot be kept in a bottle: e.g. the neutral C, the tetraanionic C, the neutral Ti, and the tetracationic Ti are not free species, they are always bound to something, for neutral C, it is commonly found in graphite, charcoal, diamond (sharing electrons with the neighboring carbons), as for Ti which can be found as its metal (where it shares its electrons with neighboring Ti atoms!), C4- and Ti4+ 'exist' only with appropriate counterions (with which they probably share electrons). So these formulisms are only used to predict stabilities or properties of compounds! Methane is a significant and plentiful fuel which is the principal component of natural gas. ... Flash point -82. ... Sulphur Dichloride Sulpur dichloide is a cherry-red liquid at room temperature that is produced from the chlorination of [S2Cl2] (disulphur dichloride). ... Sulfur hexafluoride (SF6) is a gas whose molecules consist of one sulfur atom and six fluorine atoms. ... Geometry of the water molecule Molecules have fixed equilibrium geometries--bond lengths and angles--that are dictated by the laws of quantum mechanics. ... Titanium tetrachloride (or titanium(IV) chloride) is the chemical compound with the formla TiCl4. ... The valence shells of a transition metal can accommodate 18 electrons: 2 in each of the five d orbitals (10 in total); 2 in each of the 3 p orbitals (6 in total); and 2 in the s orbital (see Electron counting). ... Iron pentacarbonyl, also known as iron carbonyl, is the compound with formula Fe(CO)5. ... Ferrocene Fe(C5H5)2 is the prototypical metallocene, a type of organometallic chemical compound, consisting of two cyclopentadienyl rings bound on opposite sides of a central iron atom and forming an organometallic sandwich compound. ... A cyclopentadienyl complex is a metal complex with one or more cyclopentadienyl groups (C5H5-, abbreviated as Cp). ...


See also


  Results from FactBites:
 
Electron counting (403 words)
In the inorganic chemistry and organometallic chemistry of transition metals, electron counting is a formalism used for characterizing a compound and for understanding its electronic structure and bonding.
The valence shells of a transition metal are filled when they contain 18 electrons: 2 each in the 5 d orbitals, or 10 total; 2 each in the 3 p orbitals, or 6 total; and finally 2 in the single valence shell s orbital.
The electrons contributed by the metal atom or ion are summed with the electrons contributed by each ligand (ie, those valence electrons of each ligand participating in some way in a bonding interaction with the metal, and not otherwise occupied only in intraligand bonding or in lone-pairs not interacting with the metal center).
The Organometallic HyperTextBook: Electron Counting (1139 words)
Each derives from a simple count of the number of electrons that may be accommodated by the available valence orbitals (one s and three p for organic chemists; organometallic chemists get five bonus d-orbitals in which to place their electrons).
There are two distinct methods that are used to count electrons, the neutral or covalent method and the effective atomic number or ionic method.
Therefore, the methyl group is a one electron donor, not a two electron donor as it is under the ionic formalism.
  More results at FactBites »

 
 

COMMENTARY     


Share your thoughts, questions and commentary here
Your name
Your comments

Want to know more?
Search encyclopedia, statistics and forums:

 


Press Releases |  Feeds | Contact
The Wikipedia article included on this page is licensed under the GFDL.
Images may be subject to relevant owners' copyright.
All other elements are (c) copyright NationMaster.com 2003-5. All Rights Reserved.
Usage implies agreement with terms, 1022, m