Hydrogen (H)

Hydrogen is the first element in the periodic table with electronic configuration \(1s^1\). It is the most abundant element in the universe, present in stars and interstellar space, and on Earth it occurs mainly combined as water (H₂O) and in organic compounds.

In molecular form as \(\mathrm{H_2}\), it is a colorless, odorless, and highly flammable gas at STP.

Two H atoms share their 1s electrons to form a covalent \(\sigma\)-bond, achieving the helium-like configuration \(1s^2\). The bond dissociation energy is relatively high: \(D_{\mathrm{H-H}} \approx 436\,\text{kJ mol}^{-1}\).

This stable bonding explains why hydrogen is commonly found as \(\mathrm{H_2}\) rather than monatomic H under normal conditions.

Hydrogen has three isotopes:

• Protium (\(^1\!\mathrm{H}\)): \(^{1}\mathrm{H}\), nucleus contains 1 proton and 0 neutrons (most abundant).
• Deuterium (\(^2\!\mathrm{H}\) or D): has 1 proton and 1 neutron; heavy water is \(\mathrm{D_2O}\).
• Tritium (\(^3\!\mathrm{H}\) or T): radioactive, with 1 proton and 2 neutrons.

Hydrogen is placed in Group 1 by convention because it forms \(\mathrm{H^+}\) like alkali metals form \(\mathrm{M^+}\). However, it also shares properties with halogens (Group 17), such as forming diatomic molecules and the ability to gain an electron to form hydride \(\mathrm{H^-}\).

Due to this duality, hydrogen is often considered unique and sometimes shown separately.

Industrial methods:

• Steam Methane Reforming (SMR): \(\mathrm{CH_4 + H_2O \rightarrow CO + 3H_2}\)
• Water–gas shift (to increase H₂): \(\mathrm{CO + H_2O \rightarrow CO_2 + H_2}\)
• Electrolysis of water: \(\mathrm{2H_2O(l) \rightarrow 2H_2(g) + O_2(g)}\)

Laboratory: reaction of active metals with acids (e.g., \(\mathrm{Zn + 2HCl \rightarrow ZnCl_2 + H_2}\)).

Key uses include:

• Ammonia synthesis (Haber process): \(\mathrm{N_2 + 3H_2 \rightleftharpoons 2NH_3}\)
• Hydrogenation of oils and in petroleum refining (hydrocracking, desulfurization).
• Fuel cells for clean energy: overall reaction \(\mathrm{2H_2 + O_2 \rightarrow 2H_2O}\) releasing electricity and heat.
• Rocket fuel (liquid hydrogen with liquid oxygen).

Acids in aqueous solution generate hydronium/hydrogen ions. The pH is defined as \(\mathrm{pH = -\log_{10}[H^+]}\). Lower pH means higher \([\mathrm{H^+}]\) and stronger acidity.

Many common acids (HCl, HNO₃, H₂SO₄) contain ionizable hydrogen that dissociates in water.

Hydrides are classified as:

• Ionic (saline) hydrides: with s-block metals, e.g., \(\mathrm{NaH, CaH_2}\).
• Covalent (molecular) hydrides: with p-block elements, e.g., \(\mathrm{CH_4, NH_3, H_2O, HCl}\).
• Interstitial (metallic) hydrides: hydrogen dissolved in transition-metal lattices (e.g., Pd–H).

Hydrogen is highly flammable and forms explosive mixtures with air. It has a very low ignition energy and burns with a nearly invisible flame.

• Use proper ventilation and leak detection (H₂ diffuses rapidly upward).
• Avoid ignition sources; use spark-proof equipment.
• Store in approved cylinders and follow pressure/temperature guidelines.

\(\mathrm{H_2}\) exists as two spin isomers:

• Ortho-hydrogen: nuclear spins parallel (triplet), higher-energy form.
• Para-hydrogen: nuclear spins antiparallel (singlet), lower-energy form.

At room temperature, a mixture predominates; at very low temperatures, para-hydrogen is favored.