In chemistry, electronegativity is a measure of the amount of force with which an atom attracts electrons in a bond. An atom with a high electronegativity attracts electrons very strongly, while one with a low electronegativity attracts them weakly. Electronegativity values are used to predict how different atoms will behave when they bond with each other, making this measurement an important skill in basic chemistry.
Method 1 of 3: Electronegativity Basics
Step 1. Keep in mind that chemical bonds occur when atoms share electrons
To understand electronegativity, it is important to first understand what a "bond" is. Two atoms in a molecule that are "connected" to each other on a molecular diagram are said to have a bond between them. This means that they share a set of two electrons in which each atom contributes one electron to the bond.
The exact reasons why atoms share electrons and bonds are a bit beyond the scope of this article. If you know English and want to learn more, read this article that talks about basic links or look for related articles on wikiHow
Step 2. Understand how electronegativity affects the electrons in the bond
When two atoms share a set of two electrons in a bond, they do not always share it equally. When an atom has a higher electronegativity than the one to which it is bonded, it will attract the two electrons in the bond towards itself. An atom with a very high electronegativity can attract electrons completely to its side of the bond, only sharing them with the other atom.
- For example, in the NaCl (sodium chloride) molecule, the chlorine atom has a fairly high electronegativity and sodium has a fairly low one. Therefore, the electrons will be attracted towards chloride and away from sodium.
Step 3. Use an electronegativity table as a reference
An electronegativity table has the elements arranged exactly like a periodic table, except that each atom is labeled with its electronegativity. These can be found in a variety of chemistry books and technical articles, as well as on the Internet.
This is a link that will take you to an excellent electronegativity table. Note that this table uses the Pauling electronegativity scale, which is the most common. However, there are other ways to measure electronegativity, one of which will be noted later
Step 4. Remember that electronegativity is usually made up of simple calculations
So if you don't have an electronegativity table handy, you can still calculate the strength of the electronegativity of an atom compared to that of another element's atom based on its location on a conventional periodic table. You will not be able to calculate a numerical value, but you will be able to evaluate the difference between the electronegativities of 2 different elements. As a general rule:
- The electronegativity of an atom becomes higher as you scroll to the right on the periodic table.
- The electronegativity of an atom becomes higher as you move towards above on the periodic table.
- Therefore, the atoms located in the upper right have the highest electronegativity and those in the lower left the lowest.
- For example, in the example with the NaCl molecule mentioned above, you can know that chloride has a higher electronegativity than sodium because it is almost at the highest point in the upper right. On the other hand, sodium is far to the left, making it one of the lowest ranked atoms.
Method 2 of 3: Finding the Bonds with Electronegativity
Step 1. Find the difference in electronegativity between the two atoms
When two atoms are bonded, the difference between their electronegativities can tell you the qualities of their bond. Subtract the smallest electronegativity from the largest to find the difference.
- For example, if we look at the HF molecule, we should subtract the electronegativity of hydrogen (2, 1) from fluorine (4, 0). 4, 0 - 2, 1 = 1, 9.
Step 2. If the difference is less than about 0.5, the bond is nonpolar covalent
In this case, the electrons are shared almost evenly. These bonds do not form molecules that have large differences in charge at each end. Nonpolar bonds are often very difficult to break. This is because the atoms share electrons, which will stabilize their bond. It will take a lot of energy to break this link.
- For example, the molecule O2 has this kind of link. Since the two oxygen atoms have the same electronegativity, the difference between them is 0.
Step 3. If the difference is between 0.5 and 1.6, the bond is polar covalent
These bonds have more of the electrons at one end than at the other. This makes the molecule a little more negative at the end with electrons and a little more positive at the end without electrons. The imbalance of charge in these bonds can cause the molecule to participate in some special reactions, such as joining another atom or molecule, or splitting a molecule. This is because it will still be reactive.
- A good example of this is the molecule H2Or (water). The O atom is more electronegative than the two H atoms, so it keeps the electrons tighter and makes the entire molecule partially negative at the end with the O atom and partially positive at the ends with the H atoms.
Step 4. If the difference is greater than 2.0, the bond is ionic
In these bonds, the electrons will be completely at one end of the bond. The most electronegative atom will gain a negative charge while the least electronegative atom will gain a positive charge. These types of bonds allow their atoms to react well with each other and even to be separated by polar molecules.
- An example of this is the NaCl (sodium chloride or salt) molecule. Chlorine is so electronegative that it completely attracts both electrons in the bond to itself, leaving sodium with a positive charge.
- NaCl can be divided into a polar molecule, like H2O (water). In a water molecule, the hydrogen part of it will be positive, while the oxygen part will be negative. By adding salt to the water, the molecules in the water will break down the salt molecules, dissolving it.
Step 5. If the difference is between 1.6 and 2.0, check for any metals
If there is a metal in the link, this will be ionic. If there are only non-metals, the link will be polar covalent.
- Metals include most of the atoms on the left side and in the middle of the periodic table. This web page has a table showing which elements are metals.
- Our example with the HF molecule mentioned above falls within this range. Since H and F are not metals, they will have a bond polar covalent.
Method 3 of 3: Finding the Mulliken Electronegativity
Step 1. Find the first ionization energy of the atom
Mulliken electronegativity is a slightly different way of measuring electronegativity than that used in the Pauling table above. If you want to find the Mulliken electronegativity for a given atom, find the first ionization energy of that atom. This is the energy that is required to make the atom's discharge a single electron.
- This is something you should probably look for in chemistry reference materials. This site has a good table that you might want to use (scroll down to find it).
- As an example, suppose we try to find the electronegativity of lithium (Li). In the table found on the website above, we can see that its first ionization energy is 520 kJ / mol.
Step 2. Find the electron affinity of the atom
This is the measure 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 the reference material. This website has resources that you may want to investigate.
- The electron affinity of lithium is 60 kJ mol-1.
Step 3. Solve the Mulliken electronegativity equation
Using kJ / mol as units for the energies, the equation for Mulliken electronegativity will be ONMulliken = (1, 97×10−3)(ANDi+ Eea) + 0, 19. Plug the values into the equation and solve for ENMulliken.
In our example, we would solve it as follows:
ONMulliken = (1, 97×10−3)(ANDi+ Eea) + 0, 19
ONMulliken = (1, 97×10−3)(520 + 60) + 0, 19
ONMulliken = 1, 143 + 0, 19 = 1, 333
- In addition to the Pauling and Mulliken scales, there are other electronegativity scales such as the Allred – Rochow scale, the Sanderson scale, and the Allen scale. They all have their own equations for calculating electronegativity (some of which can be very complex).
- Electronegativity is unitless.