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The Selenite ion Lewis structure is Se–O single bond, Se–O double bond and Se–O single bond; the central Se has 4 electron domains (1 lone pair), giving a tetrahedral electron-domain arrangement and a trigonal pyramidal molecular geometry with a ≈107° bond angle. Se is best described as sp3-hybridized, and Selenite ion is polar because the trigonal pyramidal shape (and/or unequal terminal atoms) prevents cancellation of the bond dipoles.
Figure 1: Lewis Structure of Selenite ion (O3Se-2)
| Chemical Formula | O3Se-2 |
| CAS Number | 14124-68-6 |
| Net Charge | -2 |
| Total Valence Electrons | 26 |
| Lewis Structure | Se–O single bond, Se–O double bond and Se–O single bond |
| Electron-Domain Geometry | tetrahedral |
| Molecular Geometry | trigonal pyramidal |
| Hybridization Model | sp3 (central Se) |
| Bond Angle | ≈107° |
| Polarity | Polar molecule |
In aqueous and process chemistry, O3Se-2 exists as the anion Selenite ion ion. It is encountered as a counterion and a controlled source of Se in salts, buffers, electrochemistry and analytical protocols, and as a reactant in syntheses that require a defined equivalent of Se-containing material.
To draw the Selenite ion Lewis structure we first count the total valence electrons. O3Se-2 contains 3×6 (O) + 6 (Se) − 2 (net anion); the net charge of -2 gives a total of 26 valence electrons to place.
The central atom is Se. It is the least electronegative atom capable of forming multiple bonds and is surrounded by the more electronegative terminal atom(s); hydrogen, when present, always occupies a terminal position. Electronegativity considerations place the electron density toward the terminal atom(s).
Total valence electrons = 3×6 (O) + 6 (Se) − 2 (net anion) = 26 electrons.
Place Se at the center and connect each surrounding atom with a single bond, complete octets on the terminal atoms, and place any remaining electrons on the central atom. The resulting electron distribution is: Se–O single bond, Se–O double bond and Se–O single bond; one lone pair on Se.
Counting bonding and nonbonding electrons around Se therefore satisfies the octet/expanded-octet requirement for the drawn structure.
This species is best represented by resonance structures; the π bonding is delocalized over the equivalent terminal atom(s), so all Se–terminals bonds are equivalent in the resonance hybrid.
VSEPR theory states that the 4 electron domains around Se arrange to minimize mutual repulsion, giving an tetrahedral electron-domain geometry.
Because 1 of these domains are lone pair(s) — and lone pairs repel more strongly than bonding pairs — the observed molecular geometry is trigonal pyramidal; the Se bond angle is set to ≈107° rather than the ideal value for the electron-domain geometry.
A useful VSEPR approximation is: lone pair–lone pair repulsion > lone pair–bonding pair repulsion > bonding pair–bonding pair repulsion. With 1 lone pair(s) on Se, the lone-pair domains occupy positions that minimize overall repulsion while pushing the bonding domains (and therefore the bonded atoms) closer together.
In the conventional localized-bond model, Se uses sp3 hybrid orbitals to form the 4 equivalent electron-domain directions (3 bonding + 1 lone pair).
Formal charge on central Se = V − N − ½B = 6 − 2 − ½×8 = 0. The preferred Lewis structure carries zero formal charge on the central atom (and on the terminals), so it is the dominant contributor.
Selenite ion is polar. The trigonal pyramidal shape prevents cancellation of the bond dipoles. The bond dipoles do not cancel, leaving a net molecular dipole, which strongly influences solvation behavior and intermolecular interactions.
The decisive structural fact is not simply that the individual bonds are polar; it is whether molecular symmetry allows their vector sum to vanish. In Selenite ion the trigonal pyramidal arrangement prevents cancellation, so a permanent dipole remains.
Interpretation consistent with standard VSEPR / molecular-geometry references.This polarity profile underpins its behavior as a solvent, reactant or process intermediate; when sourcing, specify grade and container compatibility.
| Species | Formula | Central Atom | Electron Domains | Molecular Geometry | Polarity |
|---|---|---|---|---|---|
| Selenite ion | O3Se-2 | Se | 4 | trigonal pyramidal | Polar |
| Selenium dioxide | O2Se | Se | 3 | bent | Polar |
| Hydrogen selenide | H2Se | Se | 4 | bent | Polar |
Selenite ion is polar. Its trigonal pyramidal geometry (and/or unequal terminal atoms) stops the bond dipoles from cancelling.
Se–O single bond, Se–O double bond and Se–O single bond; one lone pair on Se. It uses all 26 valence electrons and the preferred structure carries zero formal charge on the central atom.
The Se angle is about ≈107°. Lone-pair repulsion compresses the angle from the ideal tetrahedral value.
The conventional model is sp3. Selenite ion has 4 electron domains around the central Se.
The CAS number is 14124-68-6. PubChem (CID 1090) and NIST identify this with O3Se-2; use it to locate suppliers and database records.
Technical scope: Lewis structures, VSEPR and hybridization are model-based descriptions for chemical communication and structure prediction; actual molecular electronic structure is more accurately represented by molecular-orbital and quantum-chemical methods. Commercial grades vary substantially by supplier and intended use; CAS 14124-68-6 identifies the substance, not a universal purity specification.
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