1. Hydrogen (Position & Properties)

Hydrogen ($\text{H}$), the lightest and most abundant element, occupies a unique position in the periodic table. It can be placed in Group 1 due to its tendency to lose an electron to form $\text{H}^+$, or in Group 17 due to its ability to gain an electron to form $\text{H}^-$. Hydrogen exhibits properties of both metals and non-metals. It is a highly flammable gas, diatomic ($\text{H}_2$), and its isotopes are protium ($\text{H}$), deuterium ($\text{D}$), and tritium ($\text{T}$). Its elemental form is crucial for various industrial processes, including ammonia synthesis.

2. Hydrogen (Hydrides)

Hydrogen forms compounds with most elements known as hydrides. These are classified based on the nature of the bond formed: ionic or saline hydrides (formed with alkali and some alkaline earth metals, e.g., $\text{NaH}$), covalent or molecular hydrides (formed with non-metals and metalloids, e.g., $\text{CH}_4$, $\text{NH}_3$, $\text{H}_2\text{O}$), and metallic or non-stoichiometric hydrides (formed with transition metals, often showing variable composition). The properties of hydrides vary widely depending on the type of bonding.

3. Hydrogen (Water)

Water ($\text{H}_2\text{O}$) is a covalent compound formed by hydrogen and oxygen. Its unique properties, such as high boiling point, high heat capacity, and excellent solvent capabilities for polar and ionic compounds, are largely due to strong hydrogen bonding between water molecules. Water is essential for life and plays a vital role in many geological, chemical, and biological processes. It can act as both a weak acid and a weak base, and its solid form (ice) is less dense than its liquid form.

4. Hydrogen (Hydrogen Peroxide And Heavy Water)

Hydrogen peroxide ($\text{H}_2\text{O}_2$) is an unstable compound that acts as a mild oxidizing and reducing agent. It is used as a disinfectant and bleaching agent. Heavy water ($\text{D}_2\text{O}$) is water in which the hydrogen atoms are replaced by deuterium. It is used as a moderator in nuclear reactors, helping to control the chain reaction by slowing down fast neutrons. Both compounds highlight the diverse chemistry of hydrogen and its isotopes.