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Electronegativity

Written by tutor Kathie Z.

On this page you will find information related to electronegativty in physical chemistry. Links to helpful pages will also be at the end. Remember that your textbook publisher may have help links as well.

Definition: Electronegativity is an atomic property which has to do with the tendency to attract the electrons of another atom to form an ionic bond.

Discoverer: Linus Pauling defined this concept and the “Pauling” scale was named for him.

Pauling's is the most commonly used scale for describing an atom's electronegativity, hydrogen has a very low electronegativity and chlorine has a very high one.

Remember: when the highest occupied energy level of an atom is filled with electrons, the atom is stable and not likely to react. The noble gases have stable electron configurations and are considered inert, that is they have low reactivity.

So as you can see elements on the left hand (low electronegativity) of the Period Table of Elements, those in columns 1 and 2 have one or two more electrons than those in the most stable column 18 (the noble gases). Fluorine is considered to have the highest electronegativity. The noble gases have no electronegativity because they don't form bonds.

When two atoms have equal electronegativity they bond covalently, that is they share the electrons.

Some elements on the other hand will seek the stable configuration by moving electrons from one atom to another. Each element becomes an ion. An atom from the left side of the Periodic Table tends to lose an electron thereby becoming a positive ion (cation) because it now has more protons than electrons and will have a positive charge. At the same time the atom from the right side of the Periodic Table (but not a noble gas which are stable and don't need any electrons) will have gained one or two electrons and become a negative ion (anion) because it now has more electrons than protons.

The classic example of ionic bonding is the formation of salt (NaCl) from sodium and chlorine.

Na --> Na+ + e-

Sodium is from column I so it has only one valence electron. Sodium has very low electronegativity. This loose electron is attracted to chlorine which has a very high electronegativity because it has seven electrons in its valence orbital and needs one more to become stable like a noble gas.

e- + Cl -->Cl-

Now we have two oppositely charged ions which are attracted to each other:

Na+ + Cl- --> NaCl

Another way to look at what just happened:

Sodium (Na) has atomic number 11. This means that an atom of sodium always has 11 protons. As a neutral atom, it will have an equal number of electrons to balance the protons (11 e- = 11 p+ ). And since sodium is one column after Neon (Ne) with a full valence shell of 8 electrons, sodium needs to lose one electron to be stable.

Na --> Na+ + e-   This sodium cation has 11 protons > 10 electrons and therefore has the plus sign beside the Na.

Now when the electron is transferred to the neutral chlorine atom it is also changed: Chlorine has atomic number 17. This means that an atom of chlorine always has 17 protons. As a neutral atom, it will have an equal number of electrons to balance the protons (17 e- = 17 p+ ).And since chlorine is one column before Argon (Ar) with a full valence shell of 8 electrons, chlorine needs to gain one electron to be stable.

e- + Cl --> Cl-   This chloride ion has 17 protons < 18 electrons and therefore has a negative sign beside the Cl.

Now we have two oppositely charged ions which are attracted to each other (remember the general rule: Like charges repel each other, and unlike charges attract.)

Na+ + Cl- --> NaCl forming an ionic bond.

We have formed an ionic compound by:

Step 1: making an ion based on the electronegativity of the atoms
Step 2: transferring the electrons
Step 3: making an ionic compound with ionic bonds.

Ionization energy – it takes more energy as you move from left to right across the Periodic Table. As the electronegativity increases it takes more energy to remove an electron from the valence orbital. This makes sense, because it going to be harder to take one of seven electrons from the outer orbital of chlorine than to take one of one from sodium. Think about you and your friends, seven of you sit at a table for eight with one empty chair. Is it easier for all seven of you to get up and move to another table where one of your other friends is or to yell at him/her to come join you? You try to get the one person to move right? Same with electrons. This energy to make an ion is called ionization energy. So the higher the electronegativity the higher the ionization energy.

Polar covalent bonds are what happens when a one element is just slightly more electronegative than another. The result is that one end of the molecule has more electrons than the other making one side of the molecule slightly positive compared to the other which is now slightly negative.

The classic example is water (H2O). Oxygen (atomic number 8, with 2 electrons in the inner orbital and 6 in the outer valence orbital) has two sites which need electrons to be stable. Hydrogen has one proton and one electron in each atom. So what happens is that each of the hydrogen atoms go into the oxygen valence orbital leaving the two hydrogen protons hanging out alone.

The hydrogen side (often drawn like ears on a mickey mouse head) becomes the positive side while the larger oxygen atom side is slightly negative. The resulting polarity because of the water molecule's asymmetry are the peculiar characteristics of water.

Summary of Electronegativity

1. large difference in electronegativity results in ionic bonds
2. no difference in electronegativity results in covalent bonds
3. a slight difference in electronegativity results in polar covalent bonds.

Patterns of increasing electronegativity in the Periodic Table of Elements

On the Pauling scale, Fluorine (element 9) has been assigned a value of 4.0 and is considered the most highly reactive element with the highest electronegativity value. As we have said before, the electronegativity increases left to right across the Periodic Table. It also increases from the bottom of the chart to the top. So, Astatine and Iodine which are in the same column as Fluorine are less electronegative than Fluorine. (For more details on this and other chemistry concepts you may want to go to www.chemguide.co.uk/atoms/bonding/electroneg.html by Jim Clark it is an excellent site with very clear explanations.)


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