Why Arsenic Forms Arsine But Not Ammonia
In the fascinating world of chemistry, especially when studying elements from the periodic table, a common point of confusion among students preparing for NEET, JEE, or other competitive exams is the difference in chemical behavior between elements of the same group. One such notable example is the behavior of nitrogen and arsenic in forming hydrides. This article explores the science behind why arsenic forms arsine but not ammonia on this topic, despite both elements belonging to Group 15.
🔍 Understanding the Group 15 Elements
Group 15 of the periodic table, also known as the nitrogen family, includes the elements: nitrogen (N), phosphorus (P), arsenic (As), antimony (Sb), and bismuth (Bi). These elements share common valency and some chemical properties due to their similar electron configuration. However, their behavior, particularly in hydride formation, varies significantly. The hydrides include:
- NH3 – Ammonia
- PH3 – Phosphine
- AsH3 – Arsine
- SbH3 – Stibine
- BiH3 – Bismuthine
Though all of them have a general formula of MH3, only ammonia shows strong hydrogen bonding, basic nature, and widespread use. This leads us to ask: why doesn’t arsenic behave the same?
🧪 Key Differences Between Ammonia and Arsine
To understand why arsenic forms arsine but not ammonia on this topic, let’s compare key physical and chemical differences:
| Property | Ammonia (NH3) | Arsine (AsH3) |
|---|---|---|
| Electronegativity of central atom | 3.0 (high) | 2.0 (low) |
| Hydrogen bonding | Strong | Absent |
| Bond angle | 107.8° | 91.8° |
| Thermal stability | High | Low |
| Basicity | Strong base | Weak base |
📘 Why Arsenic Forms Arsine But Not Ammonia
Here are the scientific explanations for this phenomenon:
1. Size of the Atom
Arsenic has a much larger atomic radius compared to nitrogen. This results in less effective orbital overlap with hydrogen atoms, weakening As–H bonds in arsine. Nitrogen’s smaller size allows for stronger N–H bonds in ammonia.
2. Electronegativity
Nitrogen’s higher electronegativity leads to polar N–H bonds and stronger hydrogen bonding. Arsenic’s lower electronegativity results in weakly polar bonds and minimal hydrogen bonding in AsH3.
3. Hybridization and Bond Angles
Nitrogen in NH3 is sp3 hybridized, leading to bond angles of ~107°, ideal for stability. In contrast, arsenic’s larger orbitals distort the geometry, reducing the bond angle to ~91.8°, indicating poorer hybridization and less stable geometry.
4. Hydrogen Bonding Capability
Ammonia molecules exhibit strong intermolecular hydrogen bonding, giving rise to high boiling points, solubility in water, and basicity. Arsine, on the other hand, lacks this ability due to weak As–H polarity.
5. Thermal Stability and Reactivity
Arsine decomposes easily upon heating, releasing toxic arsenic. This makes it unsuitable for applications similar to ammonia, which is thermally stable and widely used in fertilizers and industrial chemistry.
🧠 Conceptual Understanding for NEET & JEE
This topic is frequently tested in competitive exams. Here’s how you can approach questions based on this concept:
- Statement-Based Questions: Analyze each statement based on electronegativity, bonding, and atomic size.
- Reason-Assertion: Evaluate if the reasoning correctly explains why one element behaves differently than another.
- Match the Following: Match hydrides with their properties: bond angle, basicity, boiling point, etc.
📚 Real-Life Applications of Ammonia vs Arsine
- Ammonia: Used in fertilizers, household cleaners, pharmaceuticals, and refrigeration systems.
- Arsine: Extremely toxic, used in semiconductor manufacturing under strict conditions.
🔗 Related Question on Learnlyfly
🔚 Final Thoughts
Understanding why arsenic forms arsine but not ammonia on this topic highlights the beauty of periodic trends and chemical logic. Though nitrogen and arsenic belong to the same group, subtle differences in size, electronegativity, and bonding capabilities change their chemistry completely.
For NEET and JEE aspirants, mastering such comparisons boosts conceptual clarity and helps score high in physical and inorganic chemistry sections.
📘 Published on Learnlyfly — Mastering chemistry, one concept at a time.