chemistry Chemistry is the scientific study of the properties and behavior of matter. It is a physical science within the natural sciences that studies the chemical elements that make up matter and chemical compound, compounds made of atoms, molecules a ...

, electron counting is a formalism for assigning a number of valence electrons to individual atoms in a molecule. It is used for classifying compounds and for explaining or predicting their electronic structure and bonding. Many rules in chemistry rely on electron-counting: *

Octet rule The octet rule is a chemical rule of thumb that reflects the theory that main-group elements tend to bond in such a way that each atom has eight electrons in its valence shell, giving it the same electronic configuration as a noble gas. The ru ...

is used with

Lewis structure Lewis structuresalso called Lewis dot formulas, Lewis dot structures, electron dot structures, or Lewis electron dot structures (LEDs)are diagrams that show the chemical bond, bonding between atoms of a molecule, as well as the lone pairs of elec ...

s for

main group element In chemistry and atomic physics, the main group is the group (periodic table), group of chemical element, elements (sometimes called the representative elements) whose lightest members are represented by helium, lithium, beryllium, boron, carbon ...

s, especially the lighter ones such as

carbon Carbon () is a chemical element; it has chemical symbol, symbol C and atomic number 6. It is nonmetallic and tetravalence, tetravalent—meaning that its atoms are able to form up to four covalent bonds due to its valence shell exhibiting 4 ...

nitrogen Nitrogen is a chemical element; it has Symbol (chemistry), symbol N and atomic number 7. Nitrogen is a Nonmetal (chemistry), nonmetal and the lightest member of pnictogen, group 15 of the periodic table, often called the Pnictogen, pnictogens. ...

, and

oxygen Oxygen is a chemical element; it has chemical symbol, symbol O and atomic number 8. It is a member of the chalcogen group (periodic table), group in the periodic table, a highly reactivity (chemistry), reactive nonmetal (chemistry), non ...

, * 18-electron rule in

inorganic chemistry Inorganic chemistry deals with chemical synthesis, synthesis and behavior of inorganic compound, inorganic and organometallic chemistry, organometallic compounds. This field covers chemical compounds that are not carbon-based, which are the subj ...

and

organometallic chemistry Organometallic chemistry is the study of organometallic compounds, chemical compounds containing at least one chemical bond between a carbon atom of an organic molecule and a metal, including alkali, alkaline earth, and transition metals, and so ...

transition metal In chemistry, a transition metal (or transition element) is a chemical element in the d-block of the periodic table (groups 3 to 12), though the elements of group 12 (and less often group 3) are sometimes excluded. The lanthanide and actinid ...

s, *

Hückel's rule In organic chemistry, Hückel's rule predicts that a planar ring molecule will have aromatic properties if it has 4''n'' + 2 π-electrons, where ''n'' is a non-negative integer. The quantum mechanical basis for its formulation was f ...

for the π-electrons of aromatic compounds, *

Polyhedral skeletal electron pair theory In chemistry the polyhedral skeletal electron pair theory (PSEPT) provides electron counting rules useful for predicting the structures of clusters such as borane and carborane clusters. The electron counting rules were originally formulated by ...

for polyhedral

cluster compound Nanoclusters are atomically precise, crystalline materials most often existing on the 0-2 nanometer scale. They are often considered kinetically stable intermediates that form during the synthesis of comparatively larger materials such as semic ...

s, including transition metals and main group elements and mixtures thereof, such as boranes. Atoms are called " electron-deficient" when they have too few electrons as compared to their respective rules, or " hypervalent" when they have too many electrons. Since these compounds tend to be more reactive than compounds that obey their rule, electron counting is an important tool for identifying the reactivity of molecules. While the counting formalism considers each atom separately, these individual atoms (with their hypothetical assigned charge) do not generally exist as free

species A species () is often defined as the largest group of organisms in which any two individuals of the appropriate sexes or mating types can produce fertile offspring, typically by sexual reproduction. It is the basic unit of Taxonomy (biology), ...

Counting rules

Two methods of electron counting are "neutral counting" and "ionic counting". Both approaches give the same result (and can therefore be used to verify one's calculation). *The neutral counting approach assumes the molecule or fragment being studied consists of purely

covalent bond A covalent bond is a chemical bond that involves the sharing of electrons to form electron pairs between atoms. These electron pairs are known as shared pairs or bonding pairs. The stable balance of attractive and repulsive forces between atom ...

s. It was popularized by Malcolm Green along with the L and X

ligand In coordination chemistry, a ligand is an ion or molecule with a functional group that binds to a central metal atom to form a coordination complex. The bonding with the metal generally involves formal donation of one or more of the ligand's el ...

notation. It is usually considered easier especially for low-valent transition metals. *The "ionic counting" approach assumes purely

ionic bonds Ionic bonding is a type of chemical bonding that involves the electrostatic attraction between oppositely charged ions, or between two atoms with sharply different electronegativities, and is the primary interaction occurring in ionic compound ...

between atoms. It is important, though, to be aware that most chemical species exist between the purely covalent and ionic extremes.

Neutral counting

* Neutral counting assumes each bond is equally split between two atoms. * This method begins with locating the central atom on the periodic table and determining the number of its valence electrons. One counts valence electrons for main group elements differently from transition metals, which use

d electron count The d electron count or number of d electrons is a chemistry formalism used to describe the electron configuration of the valence electrons of a transition metal center in a coordination complex. The d electron count is an effective way to unders ...

. :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 4: K, Ca, Sc, Ti, V, Cr, Fe, Ni have 1, 2, 3, 4, 5, 6, 8, 10 valence electrons respectively. * One is added for every

halide In chemistry, a halide (rarely halogenide) is a binary chemical compound, of which one part is a halogen atom and the other part is an element or radical that is less electronegative (or more electropositive) than the halogen, to make a fl ...

or other anionic ligand which binds to the central atom through a sigma bond. * Two is added 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 base A Lewis acid (named for the American physical chemist Gilbert N. Lewis) is a chemical species that contains an empty orbital which is capable of accepting an electron pair from a Lewis base to form a Lewis adduct. A Lewis base, then, is any sp ...

s. Similarly Lewis and Bronsted acids (protons) contribute nothing. * One is added for each homoelement bond. * One is added for each negative charge, and one is subtracted for each positive charge.

Ionic counting

* Ionic counting assumes unequal sharing of electrons in the bond. The more electronegative atom in the bond gains electron lost from the less electronegative atom. * This method begins by calculating the number of electrons of the element, assuming an

oxidation state In chemistry, the oxidation state, or oxidation number, is the hypothetical Electrical charge, charge of an atom if all of its Chemical bond, bonds to other atoms are fully Ionic bond, ionic. It describes the degree of oxidation (loss of electrons ...

. :E.g. for a Fe²⁺ has 6 electrons :S²⁻ has 8 electrons * Two is added for every

or other anionic ligand which binds to the metal through a

sigma bond In chemistry, sigma bonds (σ bonds) or sigma overlap are the strongest type of covalent chemical bond. They are formed by head-on overlapping between atomic orbitals along the internuclear axis. Sigma bonding is most simply defined for diat ...

. * Two is added 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, one electron is added for each carbon atom binding to the metal.

Electrons donated by common fragments

"Special cases"

The numbers of electrons "donated" by some ligands depends on the geometry of the metal-ligand ensemble. An example of this complication is the M– NO entity. When this grouping is linear, the NO ligand is considered to be a three-electron 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 versus the ''η''¹ allyl. Another unusual ligand from the electron counting perspective is

sulfur dioxide Sulfur dioxide (IUPAC-recommended spelling) or sulphur dioxide (traditional Commonwealth English) is the chemical compound with the formula . It is a colorless gas with a pungent smell that is responsible for the odor of burnt matches. It is r ...

Examples

* H₂O For a water molecule (H₂O), using both neutral counting and ionic counting result in a total of 8 electrons. Water_molecule_covalent_counting

The neutral counting method assumes each OH bond is split equally (each atom gets one electron from the bond). Thus both hydrogen atoms have an electron count of one. The oxygen atom has 6 valence electrons. The total electron count is 8, which agrees with the octet rule. Water_molecule_ionic_countin

With the ionic counting method, the more electronegative oxygen will gain electrons donated by the two hydrogen atoms in the two OH bonds to become O^2-. It now has 8 total valence electrons, which obeys the octet rule. * 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 pseudo-tetrahedral 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. 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. * RuCl₂(bpy)₂ RuCl₂(bpy)₂ is an octahedral metal complex with two bidentate

2,2′-Bipyridine 2,2′-Bipyridine (bipy or bpy, pronounced ) is an organic compound with the formula . This colorless solid is an important isomer of the bipyridine family. It is a bidentate chelation, chelating ligand, forming complexes with many transition meta ...

(bpy) ligands and two chloride ligands. In the neutral counting method, the Ruthenium of the complex is treated as Ru(0). It has 8 d electrons to contribute to the electron count. The two bpy ligands are L-type ligand neutral ligands, thus contributing two electrons each. The two chloride ligands hallides and thus 1 electron donors, donating 1 electron each to the electron count. The total electron count of RuCl₂(bpy)₂ is 18. In the ionic counting method, the Ruthenium of the complex is treated as Ru(II). It has 6 d electrons to contribute to the electron count. The two bpy ligands are L-type ligand neutral ligands, thus contributing two electrons each. The two chloride ligands are anionic ligands, thus donating 2 electrons each to the electron count. The total electron count of RuCl₂(bpy)₂ is 18, agreeing with the result of neural counting. * 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. * Fe(CO)₅ :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 each: 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. * Ferrocene, (C₅H₅)₂Fe, for the central Fe: :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.

References

{{DEFAULTSORT:Electron Counting Inorganic chemistry Chemical bonding