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The Phosphorous acid Lewis structure is P–O single bond, P–O single bond and P–O single bond; the central P has 4 electron domains (1 lone pair), giving a tetrahedral electron-domain arrangement and a trigonal pyramidal molecular geometry with a ≈107° bond angle. P is best described as sp3-hybridized, and Phosphorous acid is polar because the trigonal pyramidal shape (and/or unequal terminal atoms) prevents cancellation of the bond dipoles.
Figure 1: Lewis Structure of Phosphorous acid (H3O3P)
| Chemical Formula | H3O3P |
| CAS Number | 13598-36-2 |
| Total Valence Electrons | 26 |
| Lewis Structure | P–O single bond, P–O single bond and P–O single bond |
| Electron-Domain Geometry | tetrahedral |
| Molecular Geometry | trigonal pyramidal |
| Hybridization Model | sp3 (central P) |
| Bond Angle | ≈107° |
| Polarity | Polar molecule |
As a hydride of P, Phosphorous acid is typically a strong reducing agent, a potential fuel component, or a precursor in inorganic and organometallic synthesis. Many pnictogen/sulfur hydrides are toxic, foul-smelling and pyrophoric or flammable, and are handled with appropriate ventilation and inert-atmosphere technique.
To draw the Phosphorous acid Lewis structure we first count the total valence electrons. H3O3P contains 3×1 (H) + 3×6 (O) + 5 (P); the net charge of 0 gives a total of 26 valence electrons to place.
The central atom is P. 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 = 3×1 (H) + 3×6 (O) + 5 (P) = 26 electrons.
Place P 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: P–O single bond, P–O single bond and P–O single bond; one lone pair on P.
Counting bonding and nonbonding electrons around P therefore satisfies the octet/expanded-octet requirement for the drawn structure.
VSEPR theory states that the 4 electron domains around P 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 P 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 P, 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, P uses sp3 hybrid orbitals to form the 4 equivalent electron-domain directions (3 bonding + 1 lone pair).
Formal charge on central P = V − N − ½B = 5 − 2 − ½×6 = 0. The preferred Lewis structure carries zero formal charge on the central atom (and on the terminals), so it is the dominant contributor.
Phosphorous acid 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 Phosphorous acid 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 |
|---|---|---|---|---|---|
| Phosphorous acid | H3O3P | P | 4 | trigonal pyramidal | Polar |
| Phosphine | H3P | P | 4 | trigonal pyramidal | Polar |
| Phosphorus trichloride | Cl3P | P | 4 | trigonal pyramidal | Polar |
| Phosphorus pentachloride | Cl5P | P | 5 | trigonal bipyramidal | Nonpolar |
| Phosphorus trifluoride | F3P | P | 4 | trigonal pyramidal | Polar |
| Phosphorus pentafluoride | F5P | P | 5 | trigonal bipyramidal | Nonpolar |
Phosphorous acid is polar. Its trigonal pyramidal geometry (and/or unequal terminal atoms) stops the bond dipoles from cancelling.
P–O single bond, P–O single bond and P–O single bond; one lone pair on P. It uses all 26 valence electrons and the preferred structure carries zero formal charge on the central atom.
The P angle is about ≈107°. Lone-pair repulsion compresses the angle from the ideal tetrahedral value.
The conventional model is sp3. Phosphorous acid has 4 electron domains around the central P.
The CAS number is 13598-36-2. PubChem (CID 107909) and NIST identify this with H3O3P; 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 13598-36-2 identifies the substance, not a universal purity specification.
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