Physical Properties of Hydrogen
Atomic Extremes, The Strongest Single Bond, and Liquid Hydrogen.
To master Hydrogen for competitive exams, you must strictly separate the properties of the Hydrogen Atom ($H$) from the properties of the Dihydrogen Molecule ($H_2$). Mixing up the ionization energy of the atom with the bond dissociation energy of the molecule is one of the most common mistakes made by students.
1. Physical Properties of the Hydrogen Atom ($H$)
The hydrogen atom consists of a single proton and a single electron ($1s^1$). Due to the lack of inner shielding electrons, the nucleus exerts a massive pull on the valence electron.
- Atomic Radius: It is the smallest atom in the periodic table. The atomic radius is approximately $37\text{ pm}$ ($0.37\text{ \AA}$).
- Ionization Enthalpy ($\Delta_i H$): The energy required to remove the electron to form $H^+$ is incredibly high: $1312\text{ kJ/mol}$. Because this is so high, $H^+$ ions do not exist freely under normal chemical conditions.
- Electron Gain Enthalpy: The energy released when adding an electron to form the Hydride ion ($H^-$) is $-73\text{ kJ/mol}$. While it can gain an electron, its affinity is much lower than halogens (e.g., F is $-328\text{ kJ/mol}$).
2. Physical State of Dihydrogen ($H_2$)
Under standard conditions, hydrogen atoms pair up to form a diatomic, covalently bonded molecule ($H_2$).
| State at STP | Colorless, Odorless, Tasteless Gas |
|---|---|
| Combustibility | Highly combustible (Burns with a pale blue flame) |
| Density | $0.089\text{ g/L}$ (The lightest known substance; approx 1/14th the density of air) |
| Solubility in Water | Extremely low ($\sim 2\text{ mL}$ per $100\text{ mL}$ of water) |
Why are its Melting and Boiling Points so low?
The melting point of $H_2$ is $13.96\text{ K}$ and its boiling point is $20.39\text{ K}$. These are the lowest of any substance except Helium.
Because the $H-H$ bond is perfectly non-polar, and the molecule is incredibly small with only two electrons, its electron cloud is extremely rigid and non-polarizable.
Therefore, the only intermolecular forces holding separate $H_2$ molecules together are exceptionally weak London Dispersion Forces (van der Waals forces). It requires very little thermal energy to break these forces, keeping it a gas down to $\sim 20\text{ K}$.
3. The Extremes of the $H-H$ Bond
The covalent bond between the two hydrogen atoms in $H_2$ is the most distinct feature of the molecule and dictates all its chemical reactivity.
Figure 1: Potential Energy Curve for the formation of the H₂ molecule.
- Bond Length ($74\text{ pm}$): The $H-H$ bond is incredibly short because the 1s orbitals are very close to the nuclei, allowing for excellent overlap.
- Bond Dissociation Enthalpy ($435.88\text{ kJ/mol}$): This is the highest bond dissociation energy for a single bond between any two homonuclear atoms in the entire periodic table.
Because the $H-H$ bond is so incredibly strong, Dihydrogen ($H_2$) is relatively inert at room temperature. Most of its reactions require very high temperatures (to break the bond), electric arcs, or catalysts (like finely divided $Pt$, $Pd$, or $Ni$) which adsorb the $H_2$ molecule and weaken the bond.
4. Atomic Hydrogen & The Atomic Hydrogen Torch
Because of the massive bond dissociation enthalpy, obtaining free Atomic Hydrogen ($H$) is difficult.
If you heat $H_2$ gas to $2000\text{ K}$, only about 0.081% of it dissociates into atoms. To get a significant amount (e.g., 95% dissociation), you must heat it to over $5000\text{ K}$ or pass an electric arc through it under low pressure.
The Atomic Hydrogen Torch:
Molecular hydrogen ($H_2$) is passed through an electric arc between tungsten electrodes, dissociating into highly reactive Atomic Hydrogen ($H$). This process absorbs massive amounts of electrical energy.
When these individual atoms hit the surface to be welded, they immediately recombine to form $H_2$. Because the formation of the $H-H$ bond is wildly exothermic, it releases all that stored energy onto the metal surface, generating temperatures close to $4000\text{ K}$. This is used to cut and weld high-melting metals.
Mastery Check: Hydrogen Physics
15 High-Yield Questions to test your JEE/NEET Preparation
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