Hydrogen, the very first element in the periodic table, earns its position due to its singular proton and electron, making it the simplest and lightest atom. Often called the most abundant element in the universe, it is a key building block of stars like our sun. However, its presence on Earth is almost entirely in combined forms, most famously as water (H₂O), which covers a majority of our planet’s surface. This chapter establishes that while hydrogen gas itself is rare in our atmosphere, its compounds are fundamental to the environment and life itself. The element is primarily prepared in laboratories through reactions such as acids with active metals like zinc or the electrolysis of water, processes that demonstrate its reactive nature.
The chemical behavior of hydrogen is dual-natured, earning it a unique spot in the periodic table. It can both donate and accept an electron, allowing it to form compounds with a vast range of elements. As a strong reducing agent, it readily gives up its electron to non-metals like oxygen, with its combustion reaction being highly exothermic. This reaction with oxygen to form water is a key area of study, highlighting its energy potential. Furthermore, its ability to act somewhat like a metal allows it to form ionic hydrides with highly reactive s-block elements, while its covalent bonding with non-metals like carbon and nitrogen is the foundation of all organic chemistry.
Beyond its chemical behavior, the biological and industrial significance of hydrogen is immense. Its role in water makes it a non-negotiable component for all known life forms, acting as the universal solvent for biological processes. In industry, hydrogen is crucial for the Haber process in manufacturing ammonia-based fertilizers, which are vital for global agriculture. It is also used in the hydrogenation of oils to produce fats like vanaspati ghee. Looking forward, hydrogen is championed as a clean fuel of the future, as its combustion produces only water vapor, offering a potential solution to fossil fuel pollution and positioning it as a cornerstone of sustainable energy research.
Exercise 6 (A)
Question 1.
Justify the position of hydrogen in the periodic table.
Ans:
Hydrogen’s position is unique and debated because it exhibits properties of both Group 1 (Alkali Metals) and Group 17 (Halogens).
Justification for Group 1:
- Like alkali metals, hydrogen has one electron in its valence shell.
- It can lose one electron to form a positive ion (H⁺), which is why it is commonly placed above Lithium.
Justification for Group 17:
- Like halogens, hydrogen is diatomic (H₂), similar to F₂ or Cl₂.
- It needs one more electron to complete its valence shell and can form a negative ion (H⁻), called a hydride, similar to halides (F⁻, Cl⁻).
- It forms covalent compounds with non-metals (e.g., H₂O, CH₄), just as halogens do.
Conclusion:
Due to its dual behavior, hydrogen is best placed separately at the top of the periodic table, acting as a bridge between the electropositive alkali metals and the electronegative halogens. Its common placement in Group 1 is primarily based on its electron configuration, not its chemical resemblance.
Question 2.
Why does hydrogen show dual nature?
Ans:
Hydrogen exhibits a dual nature because it is a quantum-scale object. This behavior is not unique to hydrogen but applies to all matter. However, it becomes significant and observable for tiny particles like the electron in a hydrogen atom.
At this small scale, the electron does not behave like a miniature planet orbiting a sun. Instead, its position and momentum are described by a wavefunction—a mathematical expression representing a probability wave. This wave-like nature explains the fixed energy levels and the distinct orbital shapes within the atom.
In experiments, we sometimes detect the electron as a single, localized particle (particle nature), but the probability of where we will find it is governed by its wave-like character. Therefore, hydrogen’s electron, and by extension hydrogen itself, demonstrates both wave-like and particle-like properties, a fundamental principle of quantum mechanics.
Question 3.
1. Compare hydrogen with alkali metals on the basis of: Ion formation
2. Compare hydrogen with alkali metals on the basis of: Reducing power
3. Compare hydrogen with alkali metals on the basis of: Reaction with oxygen
4. Compare hydrogen with alkali metals on the basis of: Oxide formation
Ans:
1. Ion Formation
- Hydrogen: It can form a positive ion (H⁺) by losing its single electron, similar to alkali metals. However, the H⁺ ion is just a bare proton, making it unstable and non-existent in a free state. It immediately associates with other molecules.
- Alkali Metals: They readily and stably form positive ions (M⁺) by losing their single valence electron. These ions are stable in solutions and crystalline structures.
2. Reducing Power
- Hydrogen: It acts as a mild reducing agent, especially when heated. Its ability to donate an electron is relatively weak.
- Alkali Metals: These are among the strongest known reducing agents. They violently donate their valence electron, reacting vigorously with many substances.
3. Reaction with Oxygen
- Hydrogen: It reacts with oxygen to form a neutral oxide—water (H₂O). The reaction is highly exothermic but typically requires initiation.
- Alkali Metals: They react with oxygen to form various ionic oxides (like M₂O, M₂O₂, MO₂). The reaction is spontaneous and often vigorous or explosive, especially for heavier members like Potassium and Rubidium.
4. Oxide Formation
- Hydrogen: Its oxide is water (H₂O), a neutral, covalent compound. It is a liquid at room temperature.
- Alkali Metals: Their oxides (e.g., Li₂O, Na₂O₂) are solid, ionic compounds. When dissolved in water, they form strong alkaline (basic) solutions.
Question 4.
1. In what respect does hydrogen differ from: alkali metals
2. In what respect does hydrogen differ from: halogens?
Ans:
1. In what respect does hydrogen differ from Alkali Metals?
While alkali metals have one valence electron like hydrogen, they differ significantly. Alkali metals are solid, readily lose their single electron to form positive ions (cations), and are highly reactive metals. In contrast, hydrogen is a diatomic gas, it can both lose and gain an electron, and it does not possess metallic characteristics like luster or malleability under standard conditions.
2. In what respect does hydrogen differ from Halogens?
Hydrogen differs from halogens in its electron behavior and ionic form. Halogens need only one electron to complete their octet, which they gain easily to form stable negative ions (anions). Hydrogen, while it can gain an electron to form a hydride ion (H⁻), does so very rarely and is a weak tendency. Furthermore, hydrogen exists as a diatomic gas but lacks the characteristic color, high reactivity with metals, and disinfectant properties typical of halogens like chlorine.
