Isoelectronicity
Encyclopedia
Two or more molecular entities
(atom
s, molecule
s, or ion
s) are described as being isoelectronic with each other if they have the same number of electrons or a similar electron configuration
and the same structure (number and connectivity of atoms), regardless of the nature of the elements involved.
The term valence isoelectronic is used when these molecular entities have the same number of valence electron
s or a similar electron configuration, but may have a different number of atoms or a different bonding.
The statement "These compounds or molecules are isoelectronic" is not just an implementation of the above definition. It has significance by the fact that calculations on molecules and electron density, and therefore capability of reaction, have been performed on many common substances. Identifying a new, rare or odd compound as being isoelectronic with an already known one offers clues to possible properties and reactions.
atom and the O+
radical
ion are isoelectronic because each has five electrons in the outer electronic shell. Similarly, the cations K+
, Ca2+
, and Sc3+
, the anions Cl−
, S2−
, and P3−
are all isoelectronic with the Ar
atom. In such monatomic cases, there is a clear trend in the sizes of such species, with atomic radius
decreasing as charge increases.
CO
, N2 and NO+
are isoelectronic because each has two nuclei and 10 valence electrons: 4+6, 5+5, and 5+5, respectively. It is perhaps more accurate to think of the carbon monoxide molecule as C-O+, considering its coordination, which gives it a 5+5 distribution of electrons like the others.
Isoelectronicity leads to the concept of hydrogen-like atoms
, ions with one electron which are thus isoelectronic with hydrogen
.
The uncharged H2C=C=O molecule
and the zwitterion
ic H2C=N+=N- molecule are isoelectronic.
CH3COCH3 and CH3N2CH3 are not isoelectronic. They do have the same number of nuclei and the same number of valence
electrons, but the atoms' connectivity is different: the first one has both methyl (CH3) groups attached to carbonyl
's (CO's) carbon atom, forming a trigonal planar
structure: H3C-C(=O)-CH3; the second molecule's structure is linear: H3C-N=N-CH3 and its methyl groups are not connected to the same nitrogen atom.
The amino acids cysteine
and serine
are also considered (at least valence) isoelectronic.
Molecular entity
According to the IUPAC Gold Book a molecular entity is "any constitutionally or isotopically distinct atom, molecule, ion, ion pair, radical, radical ion, complex, conformer, etc., identifiable as a separately distinguishable entity"....
(atom
Atom
The atom is a basic unit of matter that consists of a dense central nucleus surrounded by a cloud of negatively charged electrons. The atomic nucleus contains a mix of positively charged protons and electrically neutral neutrons...
s, molecule
Molecule
A molecule is an electrically neutral group of at least two atoms held together by covalent chemical bonds. Molecules are distinguished from ions by their electrical charge...
s, or ion
Ion
An ion is an atom or molecule in which the total number of electrons is not equal to the total number of protons, giving it a net positive or negative electrical charge. The name was given by physicist Michael Faraday for the substances that allow a current to pass between electrodes in a...
s) are described as being isoelectronic with each other if they have the same number of electrons or a similar electron configuration
Electron configuration
In atomic physics and quantum chemistry, electron configuration is the arrangement of electrons of an atom, a molecule, or other physical structure...
and the same structure (number and connectivity of atoms), regardless of the nature of the elements involved.
The term valence isoelectronic is used when these molecular entities have the same number of valence electron
Valence electron
In chemistry, valence electrons are the electrons of an atom that can participate in the formation of chemical bonds with other atoms. Valence electrons are the "own" electrons, present in the free neutral atom, that combine with valence electrons of other atoms to form chemical bonds. In a single...
s or a similar electron configuration, but may have a different number of atoms or a different bonding.
The statement "These compounds or molecules are isoelectronic" is not just an implementation of the above definition. It has significance by the fact that calculations on molecules and electron density, and therefore capability of reaction, have been performed on many common substances. Identifying a new, rare or odd compound as being isoelectronic with an already known one offers clues to possible properties and reactions.
Examples
The NNitrogen
Nitrogen is a chemical element that has the symbol N, atomic number of 7 and atomic mass 14.00674 u. Elemental nitrogen is a colorless, odorless, tasteless, and mostly inert diatomic gas at standard conditions, constituting 78.08% by volume of Earth's atmosphere...
atom and the O+
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
radical
Radical (chemistry)
Radicals are atoms, molecules, or ions with unpaired electrons on an open shell configuration. Free radicals may have positive, negative, or zero charge...
ion are isoelectronic because each has five electrons in the outer electronic shell. Similarly, the cations K+
Potassium
Potassium is the chemical element with the symbol K and atomic number 19. Elemental potassium is a soft silvery-white alkali metal that oxidizes rapidly in air and is very reactive with water, generating sufficient heat to ignite the hydrogen emitted in the reaction.Potassium and sodium are...
, Ca2+
Calcium
Calcium is the chemical element with the symbol Ca and atomic number 20. It has an atomic mass of 40.078 amu. Calcium is a soft gray alkaline earth metal, and is the fifth-most-abundant element by mass in the Earth's crust...
, and Sc3+
Scandium
Scandium is a chemical element with symbol Sc and atomic number 21. A silvery-white metallic transition metal, it has historically been sometimes classified as a rare earth element, together with yttrium and the lanthanoids...
, the anions Cl−
Chlorine
Chlorine is the chemical element with atomic number 17 and symbol Cl. It is the second lightest halogen, found in the periodic table in group 17. The element forms diatomic molecules under standard conditions, called dichlorine...
, S2−
Sulfur
Sulfur or sulphur is the chemical element with atomic number 16. In the periodic table it is represented by the symbol S. It is an abundant, multivalent non-metal. Under normal conditions, sulfur atoms form cyclic octatomic molecules with chemical formula S8. Elemental sulfur is a bright yellow...
, and P3−
Phosphorus
Phosphorus is the chemical element that has the symbol P and atomic number 15. A multivalent nonmetal of the nitrogen group, phosphorus as a mineral is almost always present in its maximally oxidized state, as inorganic phosphate rocks...
are all isoelectronic with the Ar
Argon
Argon is a chemical element represented by the symbol Ar. Argon has atomic number 18 and is the third element in group 18 of the periodic table . Argon is the third most common gas in the Earth's atmosphere, at 0.93%, making it more common than carbon dioxide...
atom. In such monatomic cases, there is a clear trend in the sizes of such species, with atomic radius
Atomic radius
The atomic radius of a chemical element is a measure of the size of its atoms, usually the mean or typical distance from the nucleus to the boundary of the surrounding cloud of electrons...
decreasing as charge increases.
CO
Carbon monoxide
Carbon monoxide , also called carbonous oxide, is a colorless, odorless, and tasteless gas that is slightly lighter than air. It is highly toxic to humans and animals in higher quantities, although it is also produced in normal animal metabolism in low quantities, and is thought to have some normal...
, N2 and NO+
Nitrosonium
The nitrosonium ion is NO+, in which the nitrogen atom is bonded to an oxygen atom with a bond order of 3, and the overall diatomic species bears a positive charge. This ion is usually obtained as the following salts: NOClO4, NOSO4H , and NOBF4. The ClO and BF salts are slightly soluble in CH3CN...
are isoelectronic because each has two nuclei and 10 valence electrons: 4+6, 5+5, and 5+5, respectively. It is perhaps more accurate to think of the carbon monoxide molecule as C-O+, considering its coordination, which gives it a 5+5 distribution of electrons like the others.
Isoelectronicity leads to the concept of hydrogen-like atoms
Hydrogen-like atom
A hydrogen-like ion is any atomic nucleus with one electron and thus is isoelectronic with hydrogen. Except for the hydrogen atom itself , these ions carry the positive charge e, where Z is the atomic number of the atom. Examples of hydrogen-like ions are He+, Li2+, Be3+ and B4+...
, ions with one electron which are thus isoelectronic with hydrogen
Hydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...
.
The uncharged H2C=C=O molecule
Molecule
A molecule is an electrically neutral group of at least two atoms held together by covalent chemical bonds. Molecules are distinguished from ions by their electrical charge...
and the zwitterion
Zwitterion
In chemistry, a zwitterion is a neutral molecule with a positive and a negative electrical charge at different locations within that molecule. Zwitterions are sometimes also called inner salts.-Examples:...
ic H2C=N+=N- molecule are isoelectronic.
CH3COCH3 and CH3N2CH3 are not isoelectronic. They do have the same number of nuclei and the same number of valence
Valence (chemistry)
In chemistry, valence, also known as valency or valence number, is a measure of the number of bonds formed by an atom of a given element. "Valence" can be defined as the number of valence bonds...
electrons, but the atoms' connectivity is different: the first one has both methyl (CH3) groups attached to carbonyl
Carbonyl
In organic chemistry, a carbonyl group is a functional group composed of a carbon atom double-bonded to an oxygen atom: C=O. It is common to several classes of organic compounds, as part of many larger functional groups....
's (CO's) carbon atom, forming a trigonal planar
Trigonal planar
In chemistry, trigonal planar is a molecular geometry model with one atom at the center and three atoms at the corners of a triangle, called peripheral atoms, all in one plane. In an ideal trigonal planar species, all three ligands are identical and all bond angles are 120°. Such species belong to...
structure: H3C-C(=O)-CH3; the second molecule's structure is linear: H3C-N=N-CH3 and its methyl groups are not connected to the same nitrogen atom.
The amino acids cysteine
Cysteine
Cysteine is an α-amino acid with the chemical formula HO2CCHCH2SH. It is a non-essential amino acid, which means that it is biosynthesized in humans. Its codons are UGU and UGC. The side chain on cysteine is thiol, which is polar and thus cysteine is usually classified as a hydrophilic amino acid...
and serine
Serine
Serine is an amino acid with the formula HO2CCHCH2OH. It is one of the proteinogenic amino acids. By virtue of the hydroxyl group, serine is classified as a polar amino acid.-Occurrence and biosynthesis:...
are also considered (at least valence) isoelectronic.