In chemistry, electronegativity is a measurement of the degree to which an atom attracts electrons in a bond. Atoms with high electronegativity attract electrons strongly, while atoms with low electronegativity attract electrons weakly. Electronegativity values are used to predict the behavior of different atoms when bonded to each other, making it an important skill in basic chemistry.
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Method 1 of 3: Electronegativity Fundamentals
Step 1. Understand that chemical bonds occur when atoms share electrons
To understand electronegativity, it is important to first understand the meaning of bonding. Any two atoms in a molecule that are related to each other in a molecular diagram, have bonds. Basically, this means that both atoms have a pool of two electrons - each atom contributes one atom to the bond.
The exact reasons why atoms share electrons and bonds are beyond the scope of this article. If you want to learn more, try reading the following articles on bonding basics or other articles
Step 2. Understand how electronegativity affects the electrons in a bond
When both atoms have a pool of two electrons in a bond, the atoms do not always share fairly. When one atom has a higher electronegativity than the atom to which it is bonded, it attracts the two electrons in the bond closer to itself. Atoms with high electronegativity can attract electrons to the side of the bond, sharing them with all other atoms.
For example, in the NaCl (sodium chloride) molecule, the chloride atom has a fairly high electronegativity and sodium has a fairly low electronegativity. Thus, the electrons will be attracted close to chloride and stay away from sodium.
Step 3. Use the electronegativity table as a reference
The electronegativity table of the elements has the elements arranged exactly as in the periodic table, except that each atom is labeled with its own electronegativity. These tables can be found in a variety of chemistry textbooks and engineering articles as well as online.
This is a link to an excellent table of electronegativity. Note that this table uses the most commonly used Pauling electronegativity scale. However, there are other ways to measure electronegativity, one of which is shown below
Step 4. Keep in mind the electronegativity tendencies for an easy estimate
If you don't have a handy electronegativity table yet, you can still estimate the electronegativity of an atom based on its location on the regular periodic table. As a general rule:
- The electronegativity of the atom increases tall the more you move to right in the periodic table.
- The electronegativity of the atom increases tall the more you move ride in the periodic table.
- Thus, the atoms on the top right have the highest electronegativity and the atoms on the bottom left have the lowest electronegativities.
- For example, in the NaCl example above, you can tell that chlorine has a higher electronegativity than sodium because it's almost to the top right. On the other hand, sodium is far to the left, making it one of the lowest atomic levels.
Method 2 of 3: Finding Bonds by Electronegativity
Step 1. Find the difference in electronegativity between the two atoms
When two atoms are bonded, the difference between the electronegativities of the two can tell you about the quality of the bond between them. Subtract the smaller electronegativity from the larger one to find the difference.
For example, if we look at the HF molecule, we will subtract the electronegativity of hydrogen (2, 1) from fluorine (4, 0). 4, 0 – 2, 1 = 1, 9
Step 2. If the difference is below 0.5, the bond is non-polar covalent
In this bond, the electrons are fairly shared. This bond does not form a molecule that has a large difference in charge between the two atoms. Non-polar bonds tend to be very difficult to break.
For example, the O. molecule2 have this type of bond. Since both oxygens have the same electronegativity, the difference between their electronegativities is 0.
Step 3. If the difference is between 0.5-1, 6, the bond is polar covalent
This bond has more electrons in one atom. This makes the molecule slightly more negative at the end of the atom with more electrons, and slightly more positive at the end of the atom with fewer electrons. The imbalance of charge in these bonds allows molecules to take part in certain special reactions.
A good example of this bond is the H. molecule2O (water). O is more electronegative than the two H's, so O has more electrons and makes the whole molecule partially negative at the O end and partially positive at the H end.
Step 4. If the difference is more than 2.0, the bond is ionic
In this bond, all the electrons are at one end of the bond. The more electronegative atom gets a negative charge and the less electronegative atom gets a positive charge. Such bonds allow the atoms to react well with other atoms and even be separated by polar atoms.
An example of this bond is NaCl (sodium chloride). Chlorine is so electronegative that it attracts both electrons in the bond towards itself, leaving sodium with a positive charge
Step 5. If the difference is between 1.6-2, 0, find the metal
If there is metal in the bond, the bond is ionic. If there are only non-metals, the bond is polar covalent
- Metals comprise most of the atoms on the left and center of the periodic table. This page has a table showing the elements that are metals.
- Our HF example from above, is included in this tie. Since H and F are not metals, they have bonds polar covalent.
Method 3 of 3: Finding Mulliken Electronegativity
Step 1. Find the first ionization energy of your atom
Mulliken's electronegativity is slightly different from the method of measuring electronegativity used in Pauling's table above. To find the Mulliken electronegativity for a given atom, find the atom's first ionization energy. This is the energy required to make an atom give up a single electron.
- This is something you may need to look for in chemistry reference materials. This site has a good table, which you may want to use (scroll down to find it).
- For example, suppose we look for the electronegativity of lithium (Li). In the table on the above site, we can see that the first ionization energy is 520 kJ/mol.
Step 2. Find the electron affinity of the atom
Affinity is a measurement of the energy obtained when an electron is added to an atom to form a negative ion. Again, this is something you should look for in reference materials. This site has resources you may want to look up.
The electron affinity of lithium is 60 KJ mol-1.
Step 3. Solve the Mulliken electronegativity equation
When you use kJ/mol as the unit for your energy, the equation for the Mulliken electronegativity is ENMulliken = (1, 97×10−3)(Ei+Eea) + 0, 19. Plug your values into the equation and solve for ENMulliken.
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In our example, we'll solve it like this:
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- ENMulliken = (1, 97×10−3)(Ei+Eea) + 0, 19
- ENMulliken = (1, 97×10−3)(520 + 60) + 0, 19
- ENMulliken = 1, 143 + 0, 19 = 1, 333
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Tips
- In addition to the Pauling and Mulliken scales, other electronegativity scales include the Allred–Rochow scale, the Sanderson scale, and the Allen scale. All of these scales have their own equations for calculating electronegativity (some of those equations can get quite complicated).
- Electronegativity has no units.
