Difference Between Electronegativity and Electron Affinity

Main Difference – Electronegativity vs Electron Affinity

An electron is a subatomic particle of an atom. Electrons are found everywhere since every matter is made up of atoms. However, electrons are very important in some chemical reactions because the exchange of electrons is the only difference between reactants and products in these reactions. Electronegativity and electron affinity are two terms that explain the behavior of elements due to the presence of electrons. The main difference between electronegativity and electron affinity is that electronegativity is the ability of an atom to attract electrons from outside whereas electron affinity is the amount of energy released when an atom gains an electron.

Key Areas Covered

1. What is Electronegativity
      – Definition, Units of Measurement, Relationship with Atomic Number, Bonding
2. What is Electron Affinity
      – Definition, Units of Measurement, Relationship with Atomic Number
3. What is the Difference Between Electronegativity and Electron Affinity
      – Comparison of Key Differences

Key Terms: Atom, Electron, Electron Affinity, Electronegativity, Endothermic Reaction, Exothermic Reaction, Pauling Scale

What is Electronegativity

Electronegativity is the ability of an atom to attract electrons from outside. This is a qualitative property of an atom, and in order to compare the electronegativities of atoms in each element, a scale where relative electronegativity values reside is used. This scale is called “Pauling scale.” According to this scale, the highest electronegativity value that an atom can have is 4.0. The electronegativities of other atoms are given a value considering their capabilities of attracting electrons.

Electronegativity depends on the atomic number and the size of the atom in an element. When considering the periodic table, Fluorine (F) is given the value 4.0 for its electronegativity since it is a small atom and the valence electrons are located near the nucleus. Thus, it can attract electrons from the outside easily. In addition, the atomic number of Fluorine is 9; it has a vacant orbital for one more electron, in order to obey the octet rule. Therefore, Fluorine readily attracts electrons from outside.

Electronegativity causes a bond between two atoms to be polar. If one atom is more electronegative than the other atom, the atom with the higher electronegativity can attract electrons of the bond. This cause the other atom to have a partial positive charge due to lack of electrons around it. Therefore, electronegativity is the key to classify chemical bonds as polar covalent, nonpolar covalent and ionic bonds. Ionic bonds occur between two atoms with a huge difference in electronegativity between them whereas covalent bonds occur between atoms with a slight difference in electronegativity between the atoms.

The electronegativity of elements varies periodically. The periodic table of elements has a better arrangement of elements according to their electronegativity values.

Figure 1: Periodic Table of Elements along with Electronegativity of Elements

When considering a period in the periodic table, the atomic size of each element decreases from left to right of the period. This is because the number of electrons present in the valence shell and the number of protons in the nucleus are increased, and thus, the attraction between electrons and the nucleus is increased gradually. Therefore, the electronegativity is also increased along the same period because the attraction that comes from the nucleus is increased. Then the atoms can easily attract electrons from the outside.

Figure 02: Electronegativity (XP) from top to bottom of each group

The group 17 has the smallest atoms of each period, so it has the highest electronegativity. But the electronegativity decreases down the group because the atomic size increases down the group due to increasing the number of orbitals.

What is Electron Affinity

Electron affinity is the amount of energy released when a neutral atom or molecule (in the gaseous phase) gains an electron from outside. This electron addition causes the formation of a negatively charged chemical species. This can be represented by symbols as follows.

X       +       e^–       →     X^–      +       energy

The addition of an electron to a neutral atom or a molecule releases energy. This is called exothermic reaction. This reaction results in a negative ion. But if another electron is going to be added to this negative ion, energy should be given in order to proceed with that reaction. This is because the incoming electron is repelled by the other electrons. This phenomenon is called endothermic reaction.

Therefore, the first electron affinities are negative values and the second electron affinity values of the same species are positive values.

First Electron Affinity:               X_(g)   +   e^–        →         X^–_(g)    

Second Electron Affinity:         X^–_(g)    +   e^–                →         X^-2_(g)    

Same as electronegativity, electron affinity also shows periodic variation in the periodic table. This is because the incoming electron is added to the outermost orbital of an atom. The elements of the periodic table are arranged according to the ascending order of their atomic number. When the atomic number increases, the number of electrons they have in their outermost orbitals increases.

Figure 3: The General Pattern of Increasing the Electron Affinity along a Period

In general, the electron affinity should increase along the period from left to right because the number of electrons increases along a period; thus, it is difficult to add a new electron. When experimentally analyzed, the electron affinity values show a zig-zag pattern rather than a pattern that shows a gradual increase.

Figure 4: Variations of Electron Affinity of Elements

The above image shows that the period starting from Lithium (Li) shows a varying pattern rather than a gradual increase of electron affinity. Beryllium (Be) comes after Lithium (Li) in the periodic table, but the electron affinity of Beryllium is lower than Lithium. This is because the incoming electron is taken to the s orbital of Lithium where a single electron is already present. This electron can repel the incoming electron, resulting in a high electron affinity.  But in Beryllium, the incoming electron is filled to a free p orbital where no repulsion exists. Therefore the electron affinity has a slightly lesser value.

Difference Between Electronegativity and Electron Affinity

Definition

Electronegativity: Electronegativity is the ability of an atom to attract electrons from outside.

Electron Affinity: Electron affinity is the amount of energy released when a neutral atom or molecule (in the gaseous phase) gains an electron from outside.

Nature

Electronegativity: Electronegativity is a qualitative property where a scale is used to compare the property.

Electron Affinity: Electron affinity is a quantitative measurement.

Units of Measurement

Electronegativity: Electronegativity is measured from Pauling units.

Electron Affinity: Electron affinity is measured from either eV or kj/mol.

Application

Electronegativity: Electronegativity is applied for a single atom.

Electron Affinity: Electron affinity can be applied for either an atom or a molecule. 

Conclusion

The main difference between electronegativity and electron affinity is that electronegativity is the ability of an atom to attract electrons from the outside whereas electron affinity is the amount of energy released when an atom gains an electron.

References:

1. “Electron Affinity.” Chemistry LibreTexts. Libretexts, 11 Dec. 2016. Web. Available here. 30 June 2017. 
2. “Electronegativity.” Chemistry LibreTexts. Libretexts, 13 Nov. 2016. Web. Available here. 30 June 2017. 

Image Courtesy:

1.”Taula periòdica electronegativitat” By Joanjoc at Catalan Wikipedia – Transferred from ca.wikipedia to Commons., (Public Domain) via Commons Wikimedia
2. “Periodic variation of Pauling electronegativities” By Physchim62 – Own work (CC BY-SA 3.0) Commons Wikimedia
3. “Electron affinity periodic table” By Cdang and Adrignola (CC BY-SA 3.0) via Commons Wikimedia
4. “Electron affinity of the elements” By DePiep – Own work, Based on Electron affinities of the elements 2.png by Sandbh. (CC BY-SA 3.0) via Commons Wikimedia