Question

The interior of the planets Jupiter and Saturn are believed to contain metallic hydrogen: hydrogen that is put under such tremendous pressure that it no longer exists as ${\mathrm{H}}_2$, molecules, but instead exists as an extended metallic solid. Predict what properties metallic hydrogen might have compared to "normal" hydrogen in terms of first ionization energy, atomic size, and reactivity.

Short answer

In metallic hydrogen, compared to normal hydrogen: 1. First Ionization Energy: Metallic hydrogen would likely have a lower first ionization energy due to the delocalized electrons in the sea of electrons formed by metallic bonding. 2. Atomic Size: Metallic hydrogen would likely have a smaller atomic size since the atoms are compressed in a metallic lattice. 3. Reactivity: Metallic hydrogen would likely be less reactive due to the electrostatic attractions between the positive hydrogen nuclei and the delocalized electrons making it harder for other species to react with the hydrogen atoms present in the metallic solid.

Step-by-step solution

Understanding Normal Hydrogen Properties

Normal hydrogen exists in molecular form (H2) and is a nonmetal. 1. First Ionization Energy: Hydrogen gas has a low first ionization energy because it only has one electron and one proton, making it relatively easy to remove the electron. 2. Atomic Size: Hydrogen is the smallest atom since it has only one electron and one proton. 3. Reactivity: Hydrogen gas is reactive because it readily forms a covalent bond with other atoms to achieve a stable electron configuration (e.g., combining with oxygen to form water).

Effects of Metallic Hydrogen on Atomic Properties

Now, let's consider how hydrogen in a metallic state might affect its properties. 1. First Ionization Energy: In metallic hydrogen, the electrons would be delocalized in a "sea of electrons," which is typical for metallic bonding. This could lead to easier removal of electrons, causing a decrease in first ionization energy compared to hydrogen in its molecular form. 2. Atomic Size: In a metallic lattice, hydrogen atoms are packed closely together. Since there's a large force compressing the atoms, the atomic size of metallic hydrogen would likely be smaller than that of normal hydrogen. 3. Reactivity: As metallic hydrogen forms an extended metallic solid, its reactivity would likely decrease compared to normal hydrogen gas. This decreased reactivity may be due to the electrostatic attractions between the positive hydrogen nuclei and the delocalized electrons making it harder for other species to react with the hydrogen atoms present in the metallic solid.

Conclusions

In metallic hydrogen, compared to normal hydrogen: 1. First Ionization Energy: Metallic hydrogen would likely have a lower first ionization energy than normal hydrogen due to the delocalized electrons in the sea of electrons formed by metallic bonding. 2. Atomic Size: Metallic hydrogen would likely have a smaller atomic size than normal hydrogen since the atoms are compressed in a metallic lattice. 3. Reactivity: Metallic hydrogen would likely be less reactive than normal hydrogen due to the electrostatic attractions between the positive hydrogen nuclei and the delocalized electrons making it harder for other species to react with the hydrogen atoms present in the metallic solid.

Key concepts

Ionization Energy

Ionization energy is the energy required to remove an electron from an atom. In normal hydrogen, which exists as a diatomic molecule (${\mathrm{H}}_2$), the first ionization energy is relatively low. This is because each hydrogen atom has only one electron, which is weakly attracted to its single proton.

For metallic hydrogen, the scenario is quite different. When hydrogen becomes metallic due to extreme pressure conditions, its electrons are not bound to individual atoms. Instead, they form a delocalized "sea of electrons." This means the electrons are free to move throughout the metallic structure, making them easier to remove. Hence, metallic hydrogen is predicted to have a lower first ionization energy than molecular hydrogen.

• Normal hydrogen: High ionization energy due to localized electrons.
• Metallic hydrogen: Lower ionization energy due to delocalized electronic structure.

Atomic Size

Atomic size refers to the volume occupied by an atom. Normal hydrogen has the smallest atomic size, characterized by its single electron orbiting a single proton.

In the case of metallic hydrogen, the atoms are compressed into a tightly packed lattice because of the high pressures needed to transition hydrogen into its metallic state. When atoms are squeezed into this structure, they pack closer, reducing the atomic size further. This compression leads to a smaller atomic size in metallic hydrogen compared to its diatomic gaseous form.

• Normal hydrogen: Larger atomic size due to individual molecules.
• Metallic hydrogen: Smaller atomic size due to tight packing in metallic lattice.

Reactivity

Reactivity is how easily an element undergoes chemical reactions. Normal hydrogen (${\mathrm{H}}_2$) is quite reactive, readily forming bonds with other elements to create compounds like water.

Metallic hydrogen, however, behaves differently. In this form, hydrogen atoms and electrons are tightly bound in a stable metallic lattice. This binding, due to electrostatic attractions within the lattice, makes metallic hydrogen less likely to react with other elements or compounds. Consequently, the reactivity of metallic hydrogen is expected to be lower than that of its gaseous counterpart.

• Normal hydrogen: High reactivity due to electron accessibility for bonding.
• Metallic hydrogen: Low reactivity due to stable metallic bonding and delocalized electrons.