Electronegativity – all about it

Life can be unfair sometimes. The stronger often get more than the weaker. Chemical bonds are no different. In the course of this explanation you will find out which atoms are weak and which strong and how this can be determined.

Fundamentals of Electronegativity

Atoms can connect in different ways. Some atoms, such as sodium and chlorine, form an ionic bond. This forms the salt sodium chloride (NaCl). Other atoms, such as hydrogen and carbon, form an atomic bond or «covalent bond». The result of this is water (H2O). To find out what kind of bond other atomic compounds are, you need electronegativity.

Electronegativity indicates how strongly an element or atom can attract bonding electrons.

Every element has a specific electronegativity (EN). Chemists also like to use the symbol Χ (chi) for this. Electronegativity measures ability of an element/atom to attract the bonding electrons in a bond. You can find out the electronegativity of an element using the periodic table. The electronegativities are entered there on the element tables. They are always numbers between 0.7 and 4.

The numbers can vary slightly from periodic table to periodic table because there are different ways of determining electronegativity. The reason for this is that the electronegativity of an atom cannot be considered alone. Electronegativity always refers to the behavior of the atom when it forms a bond with another atom with a single bond. Electronegativity therefore depends on the type and number of binding partners in a molecule.

The distribution of electronegativity in the periodic table

The element fluorine (F) always has the highest electronegativity. It is always between 3.98 and 4.10. The alkali metal francium (Fr) has the lowest electronegativity. This is between 0.70 and 0.90. All other elements have electronegativity somewhere in between.

At the bottom left of the periodic table you will find the elements with the lowest electronegativity. The electronegativity in the periodic table increases steadily from bottom left (francium) to top right (fluorine). Within the groups, the electronegativity decreases from top to bottom.

Classification systems of electronegativity

The electronegativity model was introduced by Linus Pauling in 1932 and has been modified several times since then. Today, in addition to the Pauling scale, the Allred-Rochow and Mulliken scales are also used. These different scales result in different electronegativities for chemical elements.

Pauling scale

Pauling determined electronegativity based on two elements A and B bonded together. The elements have different electronegativity, so one pulls the bonding electrons more to its side. The difference is found using this formula:

DAB, DAA, and DBB are the bond dissociation energies. the Bond dissociation energy is the energy required to break chemical bonds into individual atoms. You can determine this energy experimentally. A value of ΧF = 3.98 was set for fluorine as a reference point. In this way, the difference in the dimensionless electronegativity values ​​of the chemical elements can be calculated.

electronegativity table

In this table you can see the resulting electronegativity values ​​for some main group elements.

element

EN to Pauling

fluorine

3.98

oxygen

3.44

chlorine

3:16

nitrogen

3.04

bromine

2.96

iodine

2.66

sulfur

2.58

carbon

2.55

selenium

2.55

hydrogen

2.20

phosphorus

2:19

Allred-Rochow scale

This scale assumes that electronegativity is proportional to electrostatic attraction f is. This attraction is due to the nuclear charge Z exerted on the bonding electrons.

right = atomic radius

e = elementary charge

Zeff = effective atomic number

This method has the advantage that it is easier to calculate. With the methods of Pauling and Mulliken it is difficult to determine the required values ​​experimentally.

Mulliken scale

According to Mulliken, the mean value is formed from the ionization energy EGG and the electron affinity Eea the electronegativity.

The unit of the result is electron volts (eV).

Calculation of the electronegativity difference

Electronegativity is used to find out whether compounds have an ionic or covalent (atomic) bond. To find out, you have to calculate the difference between the electronegativity of the two connection partners.

A simple example is sodium chloride. Sodium has an electronegativity of 0.9 and chlorine has an electronegativity of 3.2. Now you have to calculate the difference. The smaller value must always be subtracted from the larger one so that the difference is positive.

ΧCl – ΧNa = 3.2 – 0.9 = 2.3

You can remember that if the difference in electronegativity is more than 1.7, there is always an ionic bond. If the value is less than 1.7, there is always an atomic bond:

  • > 1.7 = ionic bond
  • < 1.7 = atomic bond

In the case of sodium chloride, there is therefore an ionic bond.

You can use this to calculate the electronegativity difference in water.

ΧO – ΧH = 3.4 – 2.2 = 1.2

1.2 is less than 1.7. Accordingly, it is an atomic bond or a covalent bond.

For sodium sulfide (Na2S), the electronegativity difference is 1.7 (ΧS – ΧNa = 2.6 – 0.9 = 1.7). In this example, electronegativity cannot be used to determine exactly what type of bond is involved. However, other research shows that sodium sulfide is ionic bonded. This example shows that 1.7 is not a hard limit. The transition is more fluid.

Polar and non-polar bonds

Nonpolar Bonds

First, let’s look at oxygen. Oxygen always occurs as O2. If you form the difference here, the result is: ΧO – ΧO = 3.4 – 3.4 = 0. This means that the oxygen atoms attract electrons equally strongly. This type of bond is called a non-polar atomic bond.

polar bonds

Again to our example sodium chloride: Χclass – ΧN / A = 3.2 – 0.9 = 2.3. In this example, we don’t get a difference of zero. Chlorine pulls the negative bonding electrons to its side much more than the very weak sodium. Such unbalanced bonds are called polar atomic bonds.

In a structural formula, such a polar atomic bond is denoted by a delta. The more electronegative atom gets a delta minus and the other gets a small delta +.

There is also a guide value for the transition from a non-polar to a polar atomic bond. If the difference in electronegativity is less than 0.5, the bond is said to be nonpolar. If it is greater than 0.5, one speaks of a polar atomic bond:

  • < 0.5 = non-polar atomic bond
  • > 0.5 = polar atomic bond

Electronegativity – The most important thing

  • Electronegativity measures ability of an element/atom to attract the bonding electrons in a bond.
  • The electronegativity in the periodic table increases steadily from bottom left (francium) to top right (fluorine).
  • The element fluorine has the greatest electronegativity.
  • In addition to the Pauling scale, the Allred-Rochow and Mulliken scales are also used to describe electronegativity.
  • Electronegativity difference above 1.7 indicates ionic bonding and below 1.7 indicates atomic bonding.
  • If the difference in electronegativity is less than 0.5, the bond is said to be nonpolar. If it is greater than 0.5, one speaks of a polar atomic bond.

proof

  1. Pauling (1960). The nature of the chemical bond and the structure of molecules and crystals. Mei Ya Publications Taipei.
  2. Allred (1961). Electronegativity values ​​from thermochemical data. Journal of Inorganic and Nuclear Chemistry.
  3. Mulliken (1934). A New Electroaffinity Scale. The Journal of Chemical Physics.