Electron Affinity is a measure of the attraction between an incoming electron and the nucleus of an atom or molecule. It is defined as the change in energy (in kJ/mole) of a neutral atom (in the gaseous phase) when an electron is added to the atom.

In this article, we will look into what electron affinity is, its periodic trends, the factors affecting electron affinity, and the values of electron affinity of different elements.

What is Electron Affinity?

Electron affinity is the energy change that occurs when an electron is added to a neutral atom to form a negatively charged ion. It measures the attraction between the incoming electron and the nucleus, with a more negative value indicating a higher affinity for electrons.

For example, fluorine has a high electron affinity, while noble gases have very low electron affinities. Atomic structure influences the electron affinity and the unit of measurement of electron affinity is in kilojoules per mole (kJ/mol) or electron volts (eV).

Definition of Electron Affinity

Electron affinity of an atom or molecule is defined as the amount of energy released when an electron is added to a neutral atom or molecule in the gaseous phase.

Electron Affinity Formula

The electron affinity is a quantitative measurement of the energy change that occurs when an electron is added to a neutral atom to form an anion. The general formula for electron affinity is given by:

EA = ( E_final – E_initial )

where,

• E_final is Energy of Atom after Electron is Added
• E_initial is Energy of Neutral Gaseous Atom

Trends in Electron Affinity

Trends in electron affinity refers to the change in value of electron affinity as we move across the periodic table. The electron affinity depends upon the nuclear charge and atomic size. Electron affinity is directly proportional to the nuclear charge on the atom and inversely proportional to its atomic size. Hence we can conclude:

• Electron affinity increases across the periodic table from left to right due to nuclear charge increases.
• Electron affinity generally decreases down a group of elements because each atom is larger than the atom above it.

Electron Affinity Table

The trend of electron affinity can be summarized by image of periodic table added below,

Types of Electron Affinity

Electron affinity can be classified into two types:

• First Electron Affinity
• Second Electron Affinity

First Electron Affinity

First electron affinity is the energy change that occurs when a neutral atom in the gaseous phase gains an electron to form a singly charged negative ion. It is the energy released (per mole of X) when this change happens.

First, electron affinities have negative values. For example, the first electron affinity of oxygen is -142 kJ mol^-1. By convention, the negative sign shows a release of energy.

Second Electron Affinity

Second electron affinity refers to the energy change that occurs when a singly charged negative ion in the gaseous phase gains an additional electron to form a doubly charged negative ion.

It is a positive value, indicating that energy is needed to add a second electron to an ion with a pre-existing negative charge. This process is endothermic, as the negative charge of the ion repels the incoming electron, requiring energy input.

For example, the second electron affinity of oxygen is +744 kJ/mol.

Electron Affinity Values

Electron affinity values defines the energy change due to addition of an electron to a gaseous atom. Each element has distinct electron affinity value. The general description of electron affinity value of each kind of element is given below:

• Electron affinities of noble gases are generally considered to be zero.
• Electron affinities of alkaline earth metals are negative.
• Nonmetals generally have higher electron affinities compared to metals.

Electron affinity of few of the common elements are described below:

Electron Affinity of Oxygen

Electron affinity of oxygen is the energy released when an electron is added to an isolated gaseous oxygen atom. The measurement of the electron affinity of oxygen is 1.46 eV. This value indicates the energy released when an electron is added to form O^–.

The first electron affinity of oxygen is -142 kJ mol^-1, which is lower than that of fluorine, indicating that oxygen has a lower tendency to accept an electron than fluorine.

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Electron Affinity of Chlorine

Electron affinity of Chlorine is -349 kJ/mol. This value indicates the energy released when an electron is added to form Cl^–. Chlorine’s high electron affinity is related to its position in the halogen group, which has the highest electron affinities among all elements.

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Electron Affinity of Fluorine

Electron affinity of fluorine is -328 kJ/mol. This negative value indicates that energy is released when an electron is gained by a fluorine atom, as there is an attraction between the protons in the nucleus and the added electron.

Fluorine’s high electron affinity is due to its small size, which brings the incoming electron closer to the nucleus, resulting in a strong attraction.

Read More, Fluorine

Factors Affecting Electron Affinity

Various factors that affect the Electron affinity are as follows:

• Atomic Size: Smaller atoms have a higher electron affinity due to the stronger attraction between the incoming electron and the nucleus. This results in a more significant release of energy when an electron is added to the atom.

• Nuclear Charge: The greater the nuclear charge, the stronger the attraction between the nucleus and the incoming electron, leading to a higher electron affinity.

• Screening Effect: The screening effect, also known as shielding, occurs when inner electrons repel the outer electrons from the full force of the nuclear charge. A higher screening effect reduces the effective nuclear charge experienced by the outermost electrons, leading to a lower electron affinity.

• Reactivity of Non-metals: Non-metals with higher electron affinities tend to be more reactive.

• Oxidizing Power of the Element: The electron affinity is related to the oxidizing power of an element. Elements with higher electron affinities are more likely to gain electrons and act as oxidizing agents in chemical reactions.

Electron Affinity of Halogens

Electron affinity of halogens decreases as you move down the group, with fluorine having the highest electron affinity and astatine having the lowest. The trends in electron affinity for halogens can be summarized as follows:

• Electron affinity generally decreases as you move down the group, with atomic size increasing.

• Fluorine has a higher electron affinity than chlorine due to its smaller size, which results in a stronger attraction between the incoming electron and the nucleus.

• High electron affinities of halogens is due to their almost complete outer shell of electrons.

• Electron affinity of halogens is related to their electronegativity, which decreases as you move down the group. This is because the electrons are progressively farther from the nucleus, resulting in a weaker attraction between the nucleus and the electrons.

Electron Affinity of Metals and Non-Metals

Metals have a lower electron affinity than non-metals, as they lose valence electrons to form cations and require energy to gain electrons. On the other hand, non-metals have a higher electron affinity, indicating a greater tendency to gain electrons to form anions and release energy in the process.

Electron affinity of non-metals is generally higher than metals, decreasing down a group in the periodic table. This difference in electron affinity is related to the stability achieved by attaining a fully stable octet, with non-metals being more likely to gain electrons to achieve this stable configuration.

Electron Affinity of Nobel gases

Electron affinities of noble gases are generally considered to be zero. Noble gases have complete valence electron shells, making them very stable and unreactive. Noble gases, such as Helium, Neon, and Argon, have an electron affinity nearly zero because they have a very stable octet configuration.

Electron Affinity Vs Ionization Energy

The difference between electron affinity and ionization energy is given below: