Question

Hydrogen is an unusual element because it behaves in some ways like the alkali metal elements and in other ways like nonmetals. Its properties can be explained in part by its electron configuration and by the values for its ionization energy and electron affinity. (a) Explain why the electron affinity of hydrogen is much closer to the values for the alkali elements than for the halogens. (b) Is the following statement true? "Hydrogen has the smallest bonding atomic radius of any element that forms chemical compounds. If not, correct it. If it is, explain in terms of electron configurations. (c) Explain why the ionization energy of hydrogen is closer to the values for the halogens than for the alkali metals. (d) The hydride ion is \(\mathrm{H}\). Write out the process corresponding to the first ionization energy of the hydride ion. (e) How does the process in part (d) compare to the process for the electron affinity of a neutral hydrogen atom?

Short answer

In summary, hydrogen has an electron affinity closer to alkali metals than halogens due to its 1s^1 electron configuration, which allows both hydrogen and alkali metals to accept an electron easily. Hydrogen has the smallest bonding atomic radius, attributed to its 1s^1 electron configuration. The ionization energy of hydrogen is closer to the halogens due to the similarities in electron configuration and the energy required to remove an electron. The first ionization energy of the hydride ion (\(\mathrm{H}^-\)) involves the removal of an electron from an ion, while the electron affinity of a neutral hydrogen atom involves the addition of an electron. These processes have opposite directions but are related to the change in the number of electrons in a hydrogen atom/ion.

Step-by-step solution

a) Electron affinity comparison

The electron affinity of an element is the energy change when an electron is added to a neutral atom to form a negatively charged ion. Hydrogen has an electron configuration of 1s^1, just like alkali metals, which have one electron in their outermost shell. Both hydrogen and alkali metals can easily accept an electron to complete their outer shell, resulting in similar electron affinities. In comparison, halogens have much higher electron affinities due to their 1 electron less than the full outer shell, making it easier for the halogens to gain an electron.

b) Smallest bonding atomic radius

The statement is true. Hydrogen has the smallest bonding atomic radius of any element that forms chemical compounds. This can be explained by its electron configuration, 1s^1. Due to having only one electron orbiting the nucleus, the atomic radius of hydrogen is smaller than that of any other element.

c) Ionization energy comparison

Ionization energy is the energy required to remove an electron from an atom. Hydrogen has an ionization energy of 1312 kJ/mol, which is closer to the values for the halogens than for the alkali metals. This is because hydrogen and halogens have similar electron configurations, with only one electron needed to complete their outer shell. As a result, it requires more energy to remove an electron from these elements compared to the alkali metals, which have one outer shell electron that can be easily lost.

d) First ionization energy of the hydride ion

The first ionization energy of the hydride ion (\(\mathrm{H}^-\)) corresponds to the energy required to remove an electron, forming a neutral hydrogen atom. The process can be written as: \(\mathrm{H}^- \rightarrow \mathrm{H} + e^-\)

e) Comparing electron affinity and ionization energy processes

The process corresponding to the first ionization energy of the hydride ion (\(\mathrm{H}^-\) turning into \(\mathrm{H}+e^-\)) involves the removal of an electron from an ion. On the other hand, the process for the electron affinity of a neutral hydrogen atom (turning \(\mathrm{H}\) into \(\mathrm{H}^-\)) involves the addition of an electron to the neutral atom. Both processes are related to the change in the number of electrons in a hydrogen atom/ion, but their directions are opposite. The ionization energy represents the energy required to remove an electron, while the electron affinity represents the energy change when an electron is added.

Key concepts

Electron Affinity

Electron affinity refers to the energy change when an electron is added to a neutral atom, forming a negatively charged ion. In simple terms, it's how much an atom "wants" an extra electron. Hydrogen has a similar electron affinity to alkali metals because they both have one electron in their outermost shell, which makes them comparable in how they attract additional electrons.
On the other hand, halogens, like fluorine and chlorine, have much higher electron affinities. This is because they are just one electron short of a full outer shell, making them more eager to gain an electron and achieve stability.
This difference helps explain why the properties of hydrogen align more closely with alkali metals rather than with halogens.

Ionization Energy

Ionization energy is the energy needed to remove an electron from an atom. For hydrogen, this value is 1312 kJ/mol, which is closer to the halogens than to alkali metals.
Both hydrogen and halogens have electronic configurations that make them less likely to give up an electron since they are closer to having a complete outer shell. This similarity requires more energy to remove an electron, unlike alkali metals, which have a single outer electron that can be easily removed.
Thus, the ionization energy data supports hydrogen's resemblance to halogens regarding electron removal energy.

Bonding Atomic Radius

The bonding atomic radius is a measure of the size of an atom when it is part of a chemical compound. Hydrogen, with its electron configuration of 1s¹, has the smallest bonding atomic radius among elements that form chemical compounds.
Why is that? Simply because hydrogen atom has just one electron orbiting its nucleus, meaning that the space it occupies is minimal.
This gives hydrogen a unique place in chemistry, contributing to its interesting behavior in compounds.

Hydride Ion

The hydride ion (\( \mathrm{H}^- \)) happens when a hydrogen atom gains an additional electron, forming a negative ion. It's essentially the opposite of what happens when hydrogen loses an electron.
The process of removing an electron from a hydride ion to form a neutral hydrogen atom is written as:\( \mathrm{H}^- \rightarrow \mathrm{H} + e^- \).
This illustrates a fundamental property of hydrogen in its ion form, showing how it can both gain and lose an electron quite flexibly.

Alkali Metals

Alkali metals are found in the first column of the periodic table and include elements like lithium, sodium, and potassium. They have a single electron in their outermost shell, just like hydrogen.
This makes them ready to lose that electron to achieve a stable electron arrangement, which is similar to how hydrogen behaves under certain conditions.
The similarity in electron configurations between hydrogen and alkali metals results in comparable chemical behaviors, particularly regarding electron affinity and bonding tendencies.

Halogens

Halogens are elements found in the second-to-last column of the periodic table, including fluorine and chlorine. These elements have seven electrons in their outer shell, just one short of a full set of eight.
This makes them very reactive as they seek to gain one electron to complete their outer shell, leading to high electron affinity values.
Unlike alkali metals, halogens share a closer ionization energy relationship with hydrogen, explaining why hydrogen sometimes behaves similarly to these nonmetals in terms of electron removal.