Electron gain enthalpy

In the study of chemistry, electron gain enthalpy is an important concept when discussing the periodic trends of the elements. It describes the energy change when an electron is added to a neutral atom to form a negative ion. Understanding electron gain enthalpy is important because it helps predict how elements will react with each other, providing information about their chemical behavior.

What is electron gain enthalpy?

Electron gain enthalpy is essentially the amount of energy released or absorbed in the process of adding an electron to an atom. If energy is released, the process is exothermic, and electron gain enthalpy is negative. Conversely, if energy is absorbed, the process is endothermic, and electron gain enthalpy is positive.

Atom + e - → Ion - + energy (Released/Energy < 0) Atom + e - + energy → Ion - (Absorbed/Energy > 0)

The energy shift can vary considerably across the periodic table and depends on a number of factors, including atomic size, nuclear charge, and electron-electron interactions within the atom.

Factors affecting electron gain enthalpy

1. Atomic size

The size of the atom plays an important role. Generally, as the size of the atom decreases, the electron gain enthalpy becomes more negative. This occurs because smaller atoms have a greater effective nuclear charge, which attracts the extra electron more strongly. For example, consider halogens such as fluorine and chlorine.

Atomic Size & Down Arrow; → Electron Gain Enthalpy & Up Negative F > Cl > Br > I (General trend)

2. Nuclear charge

Nuclear charge also affects electron gain enthalpy. Higher nuclear charge usually means a stronger attraction for the incoming electron, resulting in a more negative electron gain enthalpy. As we move across a period, the nuclear charge increases, making the enthalpy more negative.

Period Trend (Across) & Right; → Nuclear Charge & Up Arrow; → Electron Gain Enthalpy & More Negative;

3. Electron-electron repulsion

When an electron enters an atom, it experiences repulsion from the electrons already present. These repulsions can reduce the energy released and thus make the electron gain enthalpy less negative or even positive in some cases. This is prevalent in atoms that have completely filled or half-filled p or d orbitals.

Ne, He and other noble gases often show positive electron gain enthalpy due to stable electronic configurations.

Trends in electron gain enthalpy

1. Periodic trends over a period

Moving from left to right across a period, elements generally become more electronegative, making electron gain enthalpy more negative. This trend is caused by increasing nuclear charge and decreasing atomic size, which together increase the affinity of the atom to attract additional electrons.

Example: In Period 2, Electron Gain Enthalpy for elements is more negative from Li to Cl.

2. Downward trend in the group

As one goes down a group, the electron gain enthalpy becomes less negative. This is due to the increase in atomic size and the shielding effect, where the inner electrons reduce the nuclear pull on the valence electrons. As a result, the ease of gaining an electron decreases.

Example: For halogens, Electron Gain Enthalpy goes from F (most negative) to I (least negative).

Visual representation of electron gain enthalpy in periodic trends

Exceptions in electron gain enthalpy

There are notable exceptions to the general trends of electron gain enthalpy across the periodic table. A prominent example is the relative electron gain enthalpy of fluorine and chlorine. Despite being smaller and having a higher nuclear charge than chlorine, fluorine exhibits a less negative electron gain enthalpy. This is due to the higher electron-electron repulsion in fluorine's compact 2p subshell, which makes an additional electron less energetically favorable.

F < Cl in terms of electron gain enthalpy (reflection of size and repulsion in F).

Applications of electron gain enthalpy

Understanding electron gain enthalpy helps chemists predict reactions and the formation of ions or compounds. It can help predict the following:

• Stability of ions in compounds.
• The possibility of an element to form anion-rich compounds.
• The reactivity of elements and their tendency to participate in specific types of reactions (such as redox reactions).

Example: formation of halides

Electron gain enthalpy is particularly useful in understanding why halogens such as chlorine or bromine readily form halide ions. These elements have the most negative electron gain enthalpies, reflecting their strong tendency to gain electrons and form stable negative ions (^X−).

Halogens (Large -ve electron gain enthalpy) → X + e - → X -

Summary

In conclusion, electron gain enthalpy is an essential aspect of understanding the chemical properties and behavior of elements. It provides important information about why certain reactions occur and helps predict the properties of newly formed compounds. By examining periodic trends, we understand patterns important for a deeper understanding of the fundamentals of chemistry.

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