Phosphine, PH3 is a chemical compound that is a colorless, flammable, and very toxic compound. Though pure PH3 is an odorless, technical sample of phosphine that smells like garlic or decaying fish because of the presence of substituted phosphine and diphosphane (P2H4).
Phosphine gas is used to manage a wide range of insects in sealed containers or structures for non-food/non-feed goods. Phosphine is only employed in industry on rare occasions, and exposure usually occurs as a byproduct of several procedures. When acid or water comes into contact with metallic phosphide, it can cause exposure.
In 1783, Philippe Gengembre (1764–1838), a Lavoisier student, produced phosphine for the first time by heating white phosphorus in an aqueous solution of potash (potassium carbonate). Organophosphorus compounds were given the name “phosphine” for the first time in 1857, as they were comparable to organic amines. By 1865, the gas PH3 had been given the name “phosphine.”
Properties of PH3
- It is a colorless organic compound with highly toxic properties and has a clove of strong garlic or decaying fish like odor.
- It causes an explosion upon contact with water or acid.
- The melting point and boiling point of PH3 are -132.8°C and -87.7°C respectively.
- It has a molar mass of 33.99758 g/mol and a density of 1.379 g/l at 25°C.
- It has a vapor pressure of 41.3 atm at 20°C
- It is soluble in alcohol, ether and slightly soluble in benzene, chloroform, and ethanol.
Preparation of phosphine
Phosphine can be made by different methods. It can be produced industrially by reacting white phosphorus with sodium or potassium hydroxide, yielding potassium or sodium hypophosphite as a by-product.
3 KOH + P4 + 3 H2O → 3 KH2PO2 + PH3
It is prepared in the laboratory by disproportionation of phosphorous acid
4 H3PO3 → PH3 + 3 H3PO4
| Compound name | Phosphine |
| Chemical notation | PH3 |
| PH3 Valence electrons | 8 |
| PH3 Formal charges | 0 |
| Molecular geometry of Ph3 | Tetrahedral |
| Electron geometry of PH3 | Tetrahedral |
| PH3 Bond angle | 107° |
| PH3 Hybridization | sp3 type |
| PH3 Dipole moment | 0.58D |
| Is PH3 Polar or Nonpolar? | Polar |
PH3 Lewis Structure
In almost all cases, chemical bonds are formed by interactions of valence electrons in atoms. In 1916 Gilbert N. Lewis introduced a simple way of representing the valence electrons to understand how those valence electrons interact.
A Lewis structure is a structural representation of a molecule where dots are used to show electron positions around the atoms and lines or dot pairs represent covalent bonds between atoms. It helps to identify the lone pairs of electrons in molecules and to determine chemical bond formation. Lewis structures can be made for molecules that contain covalent bonds and for coordination compounds.
Lewis structures are also called Lewis dot diagrams, electron dot diagrams, Lewis dot formulas, or electron dot formulas. Technically, Lewis structures and electron dot structures are different because electron dot structures show all electrons as dots, while Lewis structures indicate shared pairs in a chemical bond by drawing a line.
Valence electrons
If the outer shell is not closed, an outer shell electron associated with an atom can participate in the formation of a chemical bond. Valence (or valency) is an atom or group of atoms’ ability to chemically unite with other atoms or groups.
In the modern periodic table, Hydrogen is situated in a group IA element while phosphorus is situated in a group VA element. Therefore, the valance electrons of hydrogen are one while phosphorus has five valance electrons in its outermost shell.
Octet Rule
By forming covalent bonds and gaining or losing electrons from its outermost valence shell, an atom tends to have eight electrons in its outermost valence shell, to fulfill the octet rule. The main group elements, oxygen, carbon, and nitrogen, follow the octet laws. Except for hydrogen, helium, and lithium, all s-block and p-block elements follow the octet rule.
The octet rule was proposed by American scientist Gilbert Newton Lewis in 1917. Lewis observed that the atoms aim to acquire the structural configuration of the Noble gas, which is closer to the periodic table of elements, by mixing with one another.
Formal charge
An atom’s formal charge is a parameter that shows whether the atom is electrostatically balanced or imbalanced.
The formal charge of any chemical is determined using the formula below.
Formal Charge = Valence electrons – Unbonded electrons – Half of the bonded electrons
Steps for drawing Lewis dot structure of PH3
Step 1. Count the total number of valence electrons present on each atom of the PH3 molecule
In the modern periodic table, Hydrogen is situated in a group IA element while phosphorus is situated in a group VA element. Therefore, the valance electrons of hydrogen are one while phosphorus has five valance electrons in its outermost shell.
Hence, the valance electrons of PH3 are the sum of valance electrons of one phosphorous and three hydrogen atoms.
Mathematically,
Valance electrons of PH3= 5+3*1=8
Therefore, the valance electrons present in the PH3 compounds are 8.
Step 2. Determine the total number of valence electrons pairs for PH3 molecules
The total valence electrons of the molecules are the sum of sigma bonds, pi bonds, and the lone pairs present at the valence shells
i.e. Mathematically,
Total valence electrons= sigma bonds+pi bonds+lone pairs at valence shell
However, we can easily calculate it by dividing the total valance electrons by two, yielding valence electron pairs.
In the case of the PH3 compound, the total number of valance electrons pairs is 8/2 i.e 4.
Step 3. Identify the Central atom and draw a simple skeleton diagram.
Rarely, The ability to have a big valence is required for an atom to be central. But, more crucially, in order to select which atom should be the center atom, both the number of atoms and their electronegativity must be considered.
Based on the Linus Pauling scale, the electronegativity of phosphorous(2.19) is lesser compared with the electronegativity of hydrogen( 2.20). So, phosphorus acts as a central atom while three hydrogens are situated in terminal ends.
Step 4. Put lone pairs of electrons on each atom.
Since the valance electrons of phosphorous are five and that of hydrogen is one. Three hydrogen atoms are bonded with the phosphorous. Three valence electrons of phosphorous form pairs with three valence electrons from the hydrogen atoms. The remaining two unpaired electrons of phosphorous are placed on the 4th side that forms a lone pair. There is only one lone pair of phosphorous in the PH3 compound.
Step 5. Complete the octet of all the atoms and make a covalent bond if necessary.
In the lewis structure, it is very necessary for atoms of the compound to fulfill their octet state to acquire a stable state except for some exceptional elements like Boron, Beryllium, Aluminum, Hydrogen, Helium, and Lithium.
In the modern periodic table, Hydrogen is situated in a group IA element while phosphorus is situated in a group VA element. Therefore, the valance electrons of hydrogen are one while phosphorus has five valance electrons in its outermost shell. In the case of the PH3 compound, The electron configuration of hydrogen is 1s1. As a result, it only has one electron on top. In a covalent bond, each of the three hydrogens will fill one of the phosphorus atom’s vacant spaces. This will provide phosphorous eight electrons, completing the octet rule.
Step 6. Calculate the formal charge distribution on all atoms and check the stability.
Now, the formal charge distribution on all-atom molecules is calculated to check whether atoms are electrostatically balanced or unbalanced.
Formal charge (FC) = [Valence electrons (V) – Lone pair electrons (L) -Bonded pair electrons (B)/2]
Formal charge of phosphorous= 5-2-6/2=5-2-3=0
The formal charge of Hydrogen= 1-0-(½)*2=0
since, both the charge of phosphorous and hydrogen in the PH3 compound is found to be zero. This means that the atoms in compounds are electrostatically balanced and stable.
Also Read:
- NH3 Lewis Structure, Molecular Geometry, Hybridization, Polar or Nonpolar
- SO3 Lewis Dot Structure, Shape, Hybridization, Polarity
- XeF4 Lewis Structure, Electron Geometry & Shape, Hybridization, Bond Angle, Polarity
PH3 Molecular Geometry & Shape
In VSEPR theory each atom in the compound is arranged such that the compound becomes stable in nature giving a compound a unique shape.
Basically, lone pairs, bonding electrons determine the molecular geometry and the shape of the compound. The molecular geometry and shape of any compound depend on the VSEPR theory.
The central atom i.e phosphorous has one lone pair of electrons.
AXN Notation for PH3 molecule:
where A represents the number of the central atoms. In this case, phosphorous is the central atom which means A is phosphorous.
X represents the number of atoms bonded to the central atom. 3 hydrogen atoms are bonded to the central atom phosphorous. So, X= 3
and N represents the non-bonding electrons (lone pair) of the central atom. In this case, phosphorus has one lone pair of electrons. So N =1
Therefore, from the AXN notation of the PH3compound, the AXN generic formula of PH3 is AX3N1.
According to the VSEPR theory, if the PH3 molecule has an AX3N1 generic formula, the molecular geometry and electron geometry will both be tetrahedral forms.
Bond angle of PH3
Bond angles mean the angle formed between two covalent bonds. If the electronegativity of the central atom increases, bond pairs of electrons are pulled towards the central atom due to which repulsion between bond pairs increases and bond angles increases. Since the electron geometry of PH3 forms a tetrahedral shape, we can predict that the bond angle of PH3 is approximately 107 degrees.
Hybridization of PH3
The hybridization of PH3 is sp3 hybridization.
The process of intermixing of atomic orbitals of an atom of nearby equal energy to form entirely new orbitals which are equal in number to mixing orbitals and having an identical shape and same energy content is known as hybridization. Hybridization can be determined by calculating the bond pairs and lone pairs of atoms present in compounds.
Or simply, we can predict the hybridization of any compound from the given steps.
If the total number of valence electrons is less or equal to 8, divide it by 2.
(V.E. < 8 divided by 2)
If the total number of valence electrons of a molecule is between 8 to 56, divide it by 8.
(8 < V.E < 56 divided by 8)
Similarly, if the valence electrons of a given molecule are more than 56, divided by 18.
(V.E. > 56 divided by 18)
In the case of the PH3 compound, the valance electron is 8. Then, it is divided by 2 which leaves the quotient of 4 and the remainder 0. since the remainder is 0, there is no need to further divide it by 2. Hence, the number of orbitals are 4. Therefore, the hybridization of PH3 is Sp3 hybridization.
Is PH3 Polar or Nonpolar?
The polarity of the molecule results from the non-symmetrical sharing of the valence electron, creating a region of unequal charges in the molecule. The polarity of any molecule depends upon the electronegativity difference, dipole moment, and molecular geometry shape.
Based on the Linus Pauling scale, the electronegativity of phosphorous is 2.19 while the electronegativity of hydrogen is 2.20. Their electronegativity difference and the molecular geometry and electron geometry of PH3 both are of tetrahedral forms that result in the charge distribution as a non-uniform across the whole molecule. This indicates that PH3 is polar in nature.
Electronegativity of PH3
As we know, Electronegativity is the dimensionless property of atoms in compounds to attract the shared pairs of electrons towards themselves. Below is the property of how electronegativity affects the polarity of compounds.
- If the electronegativity difference is greater than 1.7, the bond will have an ionic character.
- If the electronegativity difference is between 0.4 and 1.7, the bond will have a polar covalent character.
- If the electronegativity difference is less than 0.4, the bond will have a nonpolar covalent character.
Based on the Linus Pauling scale, the electronegativity of phosphorous is 2.19 while the electronegativity of hydrogen is 2.20. Due to the electronegativity difference, phosphorus and hydrogen atoms are slightly polarized. Phosphorus is negatively polarized and hydrogen is positively polarized.
PH3 MO Diagram
Summary
- PH3 is a colorless organic compound with highly toxic properties and has strong garlic or decaying fish-like odor.
- When acid or water comes into contact with metallic phosphide, it can cause exposure.
- The valance electrons present in the PH3 compounds are 8.
- The bond angle of PH3 is approximately 107 degrees.
- The hybridization of PH3 is Sp3 hybridization.
- PH3 is polar in nature.
