The number of hydrogen atom(s) attached to phosphorus atom in hypophosphorous acid is
(a) three
(b)

Q: Why doesn't hypophosphorous acid release all three of its hydrogens in water?

For a hydrogen atom to be released as an $H^+$ ion (proton), it must be attached to a highly electronegative atom like oxygen. In $H_3PO_2$, only one hydrogen is attached to oxygen (forming an $-OH$ group). The other two are attached directly to phosphorus, which is not electronegative enough to allow the hydrogen to leave as an ion.

Q: How do I know how many $P=O$ and $P-OH$ bonds to draw?

As a rule of thumb for phosphorus oxoacids, always start by drawing one $P=O$ double bond and one $P-OH$ single bond. Then, distribute the remaining oxygen and hydrogen atoms. Any leftover oxygens will form more $-OH$ groups, and any leftover hydrogens will bond directly to the phosphorus.

Q: What makes $H_3PO_2$ a good reducing agent?

The reducing nature of phosphorus oxoacids is directly proportional to the number of direct $P-H$ bonds. Since $H_3PO_2$ has two $P-H$ bonds, it can easily undergo oxidation (lose those hydrogens), making it a strong reducing agent.

Q: Is the basicity of an acid always equal to the number of hydrogen atoms in its formula?

No, this is a common trap! Basicity is determined by the number of *replaceable* or *ionizable* hydrogen atoms. In oxoacids, this is strictly the number of $-OH$ groups. $H_3PO_2$ has 3 hydrogens but only 1 $-OH$ group, so its basicity is 1.

Q: What is the oxidation state of phosphorus in $H_3PO_2$?

Let the oxidation state of P be $x$. Hydrogen is $+1$ and Oxygen is $-2$. The equation is $3(+1) + x + 2(-2) = 0$. Solving this gives $3 + x - 4 = 0$, which means $x = +1$. So, the oxidation state of phosphorus is $+1$.

Question

The number of hydrogen atom(s) attached to phosphorus atom in hypophosphorous acid is

Options:

(A) three

(B) one

(C) two

(D) zero

Sigma Percentile · Accepted Answer

### Decoding the Structure of Phosphorus Oxoacids

When dealing with the oxoacids of phosphorus, students often get confused between the number of hydrogen atoms present in the chemical formula and the actual basicity of the acid. The key to unlocking these mysteries lies in drawing the correct Lewis structure. Let's take a deep dive into **hypophosphorous acid**, which has the chemical formula $H_3PO_2$.

### The Master Blueprint of Phosphorus Oxoacids

All oxoacids of phosphorus follow a strict structural blueprint. The central atom is always Phosphorus ($P$), which belongs to Group 15 and has 5 valence electrons.

To satisfy its valency and form a stable structure, phosphorus will always form at least one strong double bond with an oxygen atom, creating a **$P=O$ (oxo) group**. Additionally, it will form at least one single bond with an $-OH$ group, creating a **$P-OH$ (hydroxy) group**.

### Constructing Hypophosphorous Acid ($H_3PO_2$)

Let's apply this blueprint to $H_3PO_2$:

1. **The Central Atom:** We place Phosphorus ($P$) in the center.
2. **The Oxo Group:** We use one oxygen atom to form the mandatory $P=O$ double bond.
3. **The Hydroxy Group:** We use the remaining one oxygen atom and one hydrogen atom to form a $P-OH$ single bond.

At this point, we have used up the central $P$ atom, both $O$ atoms, and one $H$ atom. The remaining inventory consists of exactly **two hydrogen atoms**.

### The Fate of the Remaining Hydrogens

Since there are no more oxygen atoms left to form hydroxyl groups, these two remaining hydrogen atoms have no choice but to bond directly to the central phosphorus atom.

This results in the formation of two **$P-H$ bonds**.

### Chemical Implications: Basicity vs. Reducing Power

The structural arrangement of these hydrogen atoms dictates the chemical behavior of the acid:

*   **Basicity:** Only the hydrogen atom attached to the highly electronegative oxygen atom in the $-OH$ group can be released as an $H^+$ ion in an aqueous solution. Since $H_3PO_2$ has only one $-OH$ group, it is a **monobasic acid**, despite having three hydrogen atoms in its formula.
*   **Reducing Power:** The hydrogen atoms directly attached to the phosphorus atom ($P-H$ bonds) are non-ionizable. However, they are responsible for the strong reducing properties of the acid. Because $H_3PO_2$ has two $P-H$ bonds, it acts as a powerful reducing agent.

Therefore, by simply drawing the structure, we can confidently conclude that there are exactly **two** hydrogen atoms directly attached to the phosphorus atom in hypophosphorous acid.

Animated Solution for Chemistry - s and p-Block Elements: The number of hydrogen atom(s) attached to phosphorus atom in hypophosphorous acid is

Select Answer:

Visualized Solution

Formula of Hypophosphorous Acid

Central Atom

Oxo Group Formation

Hydroxy Group Formation

Direct P-H Bonds

Conclusion

The Sigma Insight: Group 15 Elements

Decoding the Structure of Phosphorus Oxoacids

The Master Blueprint of Phosphorus Oxoacids

Constructing Hypophosphorous Acid ( $H_{3} P O_{2}$ )

The Fate of the Remaining Hydrogens

Chemical Implications

Similar Questions

In a molecule of pyrophosphoric acid, the number of $P—OH$ , $P = O$ and $P—O—P$ bonds/moiety(ies) respectively are

The number of ionisable hydrogens present in the product obtained from a reaction of phosphorus trichloride and phosphonic acid is

The compound(s) with P–H bond(s) is(are)

The order of the oxidation state of the phosphorus atom in $H_{3} PO_{2}$ , $H_{3} PO_{4}$ , $H_{3} PO_{3}$ and $H_{4} P_{2} O_{6}$ is

The correct statement(s) related to oxoacids of phosphorous is(are)

The oxidation states of 'P' in $H_{4} P_{2} O_{7}$ , $H_{4} P_{2} O_{5}$ and $H_{4} P_{2} O_{6}$ , respectively are

The pair that contains two P—H bonds in each of the oxoacids is

The pair in which phosphorus atoms have a formal oxidation state of +3 is

Good reducing nature of $H_{3} P O_{2}$ is attributed to the presence of

Which of the following statements is true ?