Hydrogen is the most abundant element in the universe (70% of the universe's total mass). It is not found in the atmosphere due to its lightweight In a free state; it occurs in traces in volcanic gases and the outer atmosphere of sun and stars of the universe. . It is present in combined states water, coal, animal and vegetable matter. All organic compounds contain hydrogen as an essential constituent.
From the above discussion, we can imagine how important is hydrogen for us.
• Electronic Configuration of Hydrogen 1s¹
Position of hydrogen in the periodic table: The position of hydrogen in the periodic table is not justified because it resembles both alkali metals as well as halogens.
• Resemblance of Hydrogen with Alkali Metals
(i) Electronic Configuration: Hydrogen has one electron in its valence shell-like alkali metals.
(ii) Both hydrogen and alkali metals form unipositive ions.
For example,
Na ———–> Na+ + e–
H ———-> H+ + e–
(iii) Hydrogen and alkali metals both show a +1 oxidation state.
(iv) Hydrogen, as well as other alkali metals, act as reducing agents.
(v)Both have an affinity for electro-negative elements, For example, Na²O, NaCl, H²0, HCl.
• Resemblance with Halogens
(i) Electronic configuration: The hydrogen and halogen family both require one electron to fulfill the inert gas configuration.
(ii) Ionisation energy of hydrogen is almost similar to halogens.
(iii) Hydrogen, as well as halogens, are Diatomic in nature.
(iv) Many compounds of hydrogen as well as halogens are of covalent nature
For example, CH⁴, SiH⁴, CCl⁴, SiCl⁴.
• Occurrence of Hydrogen
Hydrogen is the most abundant element in the universe. It is present in combined state as water, coal, animal, and vegetable matter. All organic compounds contain hydrogen as an essential constituent.
• Isotopes of Hydrogen
Hydrogen has three isotopes. Like - Protium, Deuterium, Tritium.
• Preparation of Dihydrogen, H²
Laboratory Preparation of Dihydrogen
(i) It is prepared by the reaction of granulated zinc with dil HCl.
Zn + 2HCl ——–> ZnCl² + H²
• Properties of Dihydrogen
Physical properties
(i) Dihydrogen is a colorless, odorless, and tasteless gas.
(ii) It is a combustible gas.
(iii) It is insoluble in water.
(iv) It is lighter than air.
• Chemical Properties
Reaction with halogens: It reacts with halogens, X² to give hydrogen halides HX.
• Hydrides
The hydrides are classified into three types:
(i) Ionic or saline or salt-like hydrides
(ii) Covalent or molecular hydrides (iii) Metallic or non-stoichiometric hydrides.
• Ionic or Saline Hydrides
Hydrides formed between hydrogen and electropositive element of group I and II belonging to s-block. These are known as stoichiometric compounds.
Properties of saline or ionic hydrides:
(i) The hydrides of lighter elements like Li, Be, Mg, etc., have significant covalent character.
(ii) Ionic hydrides are crystalline, non-volatile, and non-conducting in the solid-state.
(iii) They conduct electricity in the molten state and liberate hydrogen at anode.
• Covalent or Molecular Hydrides
These are binary compounds of hydrogen with non-metals belonging top-block.
For example, NH³, CH⁴, H²O, HF are mostly volatile compounds with low boiling points. They are classified as:
(i) Electron-Deficient Molecular Hydride: Molecular hydrides in which the central atom does not have octet are called electron-deficient hydrides
e.g., BH³, MgH², BeH².
(ii) Electron precise hydrides: Those hydrides in which the central atom has its octet complete, e.g., group 14 hydrides. They are tetrahedral in geometry.
(iii) Electron-rich hydrides: Those metal hydrides which contain lone pair of electrons are called electron-rich hydrides, e.g., NH³, PH³, H²0, and H²S.
NH³ and PH³ have 1 lone pair, and H²O and H²S have 2 lone pairs of electrons.
• Metallic or Non-Stoichiometric Hydrides
These hydrides are also known as interstitial hydrides. Transition metals groups 3, 4, and 5 form metallic hydrides. In group 6, chromium alone tends to form CRH. Metals of 7, 8, and 9 do not form hydrides. This is called a hydride gap.
The latest study shows that only Ni, Pd, Ce, and Ac are interstitial in nature, that means they can occupy hydrogen atom in the interstitial sides. The hydrides are generally non-stoichiometric, and their composition varies with temperature and pressure, for example, Ti H 1.73, CeH2.7′, LaH2.8, etc.
These hydrides have a metallic lock, and their properties are closely related to those of the parent metal. They are strong reducing agents in most cases due to free hydrogen atoms in the metal lattice.
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