Electronegativity — Overview & Periodic Trend

Electronegativity and the Periodic Table

Electronegativity is defined as an atom's attraction to the electrons in a bond. The higher the electronegativity, the higher the electron affinity. All elements on the periodic table have assigned electronegativity values. The most electronegative atom is Fluorine (top right). Electronegativity increases up and to the right on the periodic table.

Electronegativity and Bonding

When atoms form bonds, the electronegativity difference influences the bond polarity. Bonds between atoms that have similar electronegativities are nonpolar covalent bonds. Bonds with a small difference in electronegativity on the atoms are polar covalent. When the electronegativity differs by more than two units, the bond is considered ionic.

Example:

The electronegativities of some atoms are listed below:

• H - 2.1

• C - 2.5

• O - 3.5

• Na - 0.9

• Cl - 3.0

Now let's consider some molecules!

• Methane ($CH_4$) - the $C-H$ bond has a difference of 0.4, so the bond is nonpolar covalent.

• Water ($H_2O$) - the $O-H$ bond has a difference of 1.4, so the bond is polar covalent.

• Sodium chloride salt ($NaCl$) - has a bond difference of 2.1, so the bond is ionic.

Electronegativity

(Video) Periodic Trends in Electronegativity

by Sciencepost

This video introduces the topic of electronegativity, more specifically, what electronegativity is, what elements are electronegative and why, as well as trends of electronegativity seen on the periodic table.

What is Electronegativity?

Electronegativity measures an element's ability to attract electrons to itself during bonding. What determines electronegativity? It is a balance between the atomic number and the atomic radius. Elements with higher atomic numbers have more protons. So, there is more electrostatic attraction between the nucleus and the electron shells. But, we also need to consider the atomic radius. A huge radius means that the valence electrons are further from the atom's nucleus. Also, the lower orbitals shield the valence shell. So, they experience less attraction. Remember, it's the valence electrons that participate in chemical bonding. Elements with larger atomic radii have lower electronegativities.

What Element is the Most Electronegative?

Fluorine is the most electronegative element. The atomic number is relatively small. But, its valence shell only consists of 2s and 2p electrons. It strikes the perfect balance! We also see that electronegativity correlates with the octet rule. Elements that are close to completing their octet are more electronegative. They want electrons to fill their valence shells. So, we expect the alkali and alkaline earth metals to have tiny electronegativities. And they do! These elements generally lose electrons. So, we also see that electronegativity is inversely related to ionization energy.

So, what's the general periodic trend? As we move down a period on the periodic table, electronegativity drops. Why? The atomic radius is expanding. As we move across a group, from the alkali metals to the non-metals, electronegativity increases. Why? The atomic number is increasing along with the effective nuclear charge. So, what's the least electronegative element? Francium! On the periodic table, it's at the bottom left corner. It has the largest atomic radii and the minimal effective nuclear charge. But what about the lanthanides and actinides? Remember, they should be inserted after barium and radium. We have them on the bottom of the periodic table to make it easier to print. So, francium really is at the bottom.

How do we Calculate Electronegativity?

The most common form of electronegativity is the Pauling scale. It was created by Linus Pauling in 1932. Pauling was an advocate of the valence bond theory. Valence bond theory is based on the quantum mechanical model. Remember, quantum mechanics uses probability. So, when two atoms bond, there is a probability the bond will be ionic. There is also a covalent probability. So, every bond has some ionic and some covalent character. How do we approximate the covalent character? By taking the average bond dissociation energies of the elements bonded to themselves. For example, let's say we have a compound AB. The covalent character is the average of the A-A and B-B bonds. The ionic character comes from the electronegativity difference. The sum of the covalent and ionic natures is the compound's bond dissociation energy.

Pauling used the bond dissociation energies to estimate the degree of ionic or covalent character. He formulated it as a scale with hydrogen as the standard. So Pauling gave hydrogen a value of 2.2. Then, he used the bond dissociation energy of an element bonded to hydrogen. Overall, the scale is approximate. But it's incredibly accurate.