Electron Affinity - Introduction, Definition, Trends, FAQs
Electron Affinity: Introduction
When an atom loses or gains energy through chemical reactions that result in the loss or gain of electrons, the atom's energy is defined. A chemical reaction that releases energy is referred to as an exothermic reaction, while one that absorbs energy is referred to as an endothermic reaction. Exothermic reactions produce negative energy, which is denoted by a negative sign, whereas endothermic reactions produce positive energy, which is denoted by a positive sign. When a person drops a book, for example, both processes are demonstrated. When he or she lifts a book, he or she is giving the book potential energy (energy absorbed).
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When he or she drops the book, however, the potential energy transforms into kinetic energy, which manifests as sound when it hits the ground (energy released).Energy is released when an electron is added to a neutral atom (first electron affinity); thus, the first electron affinities are negative. However, adding an electron to a negative ion requires more energy (i.e., second electron affinity), which outweighs any energy released during the electron attachment process, resulting in positive second electron affinities.
First Electron Affinity (energy released, negative energy):
X(g)+e−→X−(g)
Second Electron Affinity (positive energy as needed energy exceeds gained energy):
X−(g)+e−→X2−(g)
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Electron affinity definition:
Electron affinity is the change in energy (in kJ/mole) of a neutral atom (in the gaseous phase) when an electron is added to form a negative ion. In other words, the probability of a neutral atom gaining an electron.
Electron affinity of halogens.
On the periodic table, the halogens are to the left of the noble gases. Fluorine (F), chlorine (Cl), bromine (Br), iodine (I), and astatine (A) are the five toxic non-metallic elements that make up Group 17 of the periodic table (At). Even though astatine is radioactive and has only short-lived isotopes, it behaves similarly to iodine and is commonly categorized as a halogen. Because halogen elements have seven valence electrons, forming a full octet requires only one additional electron. Because of this, they are more reactive than other non-metal groups.
NCERT Chemistry Notes :
Electron Affinity (decreases down the group) As atomic size decreases electron affinity decreases (At<I<Br<F<Cl). The nucleus will be less attractive to an electron, resulting in a low electron affinity. Fluorine, on the other hand, has a lower electron affinity than chlorine. This is due to fluorine's smaller size when compared to chlorine.
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The greater the distance between two objects, the less attraction there is; thus, when an electron is added to the outside orbital, less energy is released. Furthermore, an element with more valence electrons is more likely to gain electrons and form a stable octet. An atom with very few valence electrons is less likely to gain electrons.
However, because the number of valence electrons increases as the group number decreases, one might assume that the element will be more stable and have a higher electron affinity. The shielding effect is not taken into account. As the period decreases, the shielding effect increases, causing electrons to repel one another. This is why, as one moves down the periodic table, the attraction between the electron and the nucleus decreases.
The first electron affinities become less as you move down the group (in the sense that less energy is evolved when the negative ions are formed). Fluorine deviates from this pattern and must be accounted for separately. The strength of the attraction between the incoming electron and the nucleus is measured by the electron affinity; the stronger the attraction, the more energy is released. Nuclear charge, distance, and screening are the same factors that influence this attraction as they are for ionization energies. Extra screening electrons offset the increased nuclear charge as you move down the group.
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