Question

Why does hydrogen have the minimum ionization energy and maximum electron affinity of all the elements? What are the reasons for this, both physically and quantitatively (in mV)?

Answer 1

Hydrogen has the lowest ionization energy and the highest electron affinity among all the elements due to its unique electronic structure and the nature of its atomic orbitals. Let's explore the reasons for these characteristics:

1. Ionization energy: The ionization energy is the energy required to remove an electron from an atom, forming a positively charged ion. Hydrogen has the lowest ionization energy because it has only one electron in its 1s orbital. This electron experiences a relatively weak attractive force from the nucleus since there is only one proton. Thus, it is easier to remove the electron from hydrogen compared to any other element with more electrons and a stronger nuclear charge.

Quantitatively, the ionization energy of hydrogen is approximately 1312 millielectron volts (meV). This value corresponds to the energy required to remove one electron from a hydrogen atom in its ground state.

2. Electron affinity: Electron affinity is the energy change associated with the addition of an electron to a neutral atom, forming a negatively charged ion. Hydrogen has the highest electron affinity because of its electronic structure. When a hydrogen atom gains an electron, it fills its 1s orbital and achieves a stable, closed-shell configuration, similar to helium. This stability leads to a favorable energy change upon the addition of an electron.

Quantitatively, the electron affinity of hydrogen is approximately -72.8 meV. This value represents the energy released when a hydrogen atom gains one electron to form a hydrogen anion (H-).

It's worth noting that while hydrogen has the lowest ionization energy and the highest electron affinity among the elements, these values are still relatively small compared to other elements due to hydrogen's small nuclear charge and size. As we move across the periodic table, ionization energy generally increases, and electron affinity decreases due to the increasing nuclear charge and electron-electron repulsion.

In summary, hydrogen's minimum ionization energy and maximum electron affinity are a consequence of its single electron configuration in the 1s orbital and the resulting low attractive force between the nucleus and electron. These characteristics make hydrogen unique among the elements in terms of its ionization energy and electron affinity.