What is electronegativity? Electronegativity is a measure of the tendency of an atom to attract a bonding pair of electrons. The Pauling scale is the most commonly used. Fluorine (the most electronegative element) is given a value of 4.0, and values range down to caesium and francium which are the least electronegative at 0.7. What happens if two atoms of equal electronegativity bond together? The most obvious example of this is the bond between two carbon atoms. Both atoms will attract the bonding pair to exactly the same extent. That means that on average the electron pair will be found half way between the two nuclei, and you could draw a picture of the bond like this:
It is important to realise that this is an average picture. The electrons are actually in a sigma orbital, and are moving constantly within that orbital. | |||||
Help! A sigma orbital is a molecular orbital formed by end-to-end overlap between two atomic orbitals. | |||||
The carbon-fluorine bond Fluorine is much more electronegative than carbon. The actual values on the Pauling scale are
That means that fluorine attracts the bonding pair much more strongly than carbon does. The bond – on average – will look like this:
Why is fluorine more electronegative than carbon? A simple dots-and-crosses diagram of a C-F bond is perfectly adequate to explain it.
The bonding pair is in the second energy level of both carbon and fluorine, so in the absence of any other effect, the distance of the pair from both nuclei would be the same. The electron pair is shielded from the full force of both nuclei by the 1s electrons – again there is nothing to pull it closer to one atom than the other. BUT, the fluorine nucleus has 9 protons whereas the carbon nucleus has only 6. Allowing for the shielding effect of the 1s electrons, the bonding pair feels a net pull of about 4+ from the carbon, but about 7+ from the fluorine. It is this extra nuclear charge which pulls the bonding pair (on average) closer to the fluorine than the carbon. | |||||
Help! You have to imagine what the bonding pair “sees” if it looks in towards the nucleus. In the carbon case, it sees 6 positive protons, and 2 negative electrons. That means that there will be a net pull from the carbon of about 4+. The shielding wouldn’t actually be quite as high as 2-, because the 1s electrons spend some of their time on the far side of the carbon nucleus – and so aren’t always between the bonding pair and the nucleus. Incidentally, thinking about electrons looking towards the nucleus may be helpful in picturing what is going on, but avoid using terms like this in exams. | |||||
The carbon-chlorine bond The electronegativities are:
The bonding pair of electrons will be dragged towards the chlorine but not as much as in the fluorine case. Chlorine isn’t as electronegative as fluorine. Why isn’t chlorine as electronegative as fluorine? Chlorine is a bigger atom than fluorine.
| |||||
In the chlorine case, the bonding pair will be shielded by all the 1-level and 2-level electrons. The 17 protons on the nucleus will be shielded by a total of 10 electrons, giving a net pull from the chlorine of about 7+. That is the same as the pull from the fluorine, but with chlorine the bonding pair starts off further away from the nucleus because it is in the 3-level. Since it is further away, it feels the pull from the nucleus less strongly. Bond polarity and inductive effects Polar bonds Think about the carbon-fluorine bond again. Because the bonding pair is pulled towards the fluorine end of the bond, that end is left rather more negative than it would otherwise be. The carbon end is left rather short of electrons and so becomes slightly positive.
The symbols We describe a bond having one end slightly positive and the other end slightly negative as being polar. Inductive effects An atom like fluorine which can pull the bonding pair away from the atom it is attached to is said to have a negative inductive effect. Most atoms that you will come across have a negative inductive effect when they are attached to a carbon atom, because they are mostly more electronegative than carbon. You will come across some groups of atoms which have a slight positive inductive effect – they “push” electrons towards the carbon they are attached to, making it slightly negative. Inductive effects are sometimes given symbols: -I (a negative inductive effect) and +I (a positive inductive effect). | |||||
Note: You should be aware of terms like “negative inductive effect”, but don’t get bogged down in them. Provided that you understand what happens when electronegative atoms like fluorine or chlorine are attached to carbon atoms in terms of the polarity of the bonds, that’s really all you need for most purposes. | |||||
Some important examples of polar bonds Hydrogen bromide (and other hydrogen halides)
Bromine (and the other halogens) are all more electronegative than hydrogen, and so all the hydrogen halides have polar bonds with the hydrogen end slightly positive and the halogen end slightly negative. | |||||
Help! Halogen: a member of group VII of the Periodic Table – fluorine, chlorine, bromine and iodine. Halide: a compound of one of these – e.g. hydrogen chloride, hydrogen bromide, etc. | |||||
The polarity of these molecules is important in their reactions with alkenes. | |||||
The carbon-bromine bond in halogenoalkanes | |||||
Note: You may come across halogenoalkanes under the names “haloalkanes” or “alkyl halides”. | |||||
Bromine is more electronegative than carbon and so the bond is polarised in the way that we have already described with C-F and C-Cl.
The polarity of the carbon-halogen bonds is important in the reactions of the halogenoalkanes. | |||||
The carbon-oxygen double bond An orbital model of the C=O bond in methanal, HCHO, looks like this:
| |||||
The very electronegative oxygen atom pulls both bonding pairs towards itself – in the sigma bond and the pi bond. That leaves the oxygen fairly negative and the carbon fairly positive.
| |||||
+ and 
Frequently Asked Questions About Electronegativity
What is electronegativity?
Electronegativity is a chemical property that describes the ability of an atom to attract a shared pair of electrons towards itself when forming a covalent bond with another atom.
What does electronegativity mean?
It signifies how strongly an atom pulls on the electrons involved in a chemical bond. A higher electronegativity value means a stronger pull.
How is electronegativity determined, calculated, or found?
Electronegativity values are typically looked up using established scales (like the Pauling or Mulliken scale) which are derived from experimental data such as bond energies, ionization energies, and electron affinities. For common elements, you usually reference a periodic table that lists these values rather than calculating them from scratch.
How to find the electronegativity of an element?
The easiest way is to consult a periodic table that includes electronegativity values for each element. Different tables may use slightly different scales (most commonly the Pauling scale).
What is the trend of electronegativity on the periodic table?
Electronegativity generally increases as you move from left to right across a period (due to increasing nuclear charge) and decreases as you move from top to bottom down a group (due to increasing atomic size and shielding effects). The highest values are found in the upper right corner.
Why does electronegativity increase across a period?
Across a period, the number of protons in the nucleus increases, leading to a higher effective nuclear charge. The valence electrons are in the same energy level (shell), so this stronger positive pull from the nucleus attracts bonding electrons more strongly.
Why does electronegativity decrease down a group?
Down a group, the atomic radius increases as electrons occupy higher energy levels (shells). The valence electrons are further from the nucleus, and inner electron shells shield the valence electrons from the full nuclear charge. This weaker attraction to the nucleus results in a lower ability to attract bonding electrons.
What is the most electronegative element?
Fluorine (F) is the most electronegative element on the Pauling scale, with a value of 3.98.
Why is Fluorine the most electronegative element?
Fluorine is in the upper right corner of the periodic table. It is small, has a high effective nuclear charge, and its valence electrons are relatively close to the nucleus, allowing it to exert the strongest pull on bonding electrons.
What is the least electronegative element?
Francium (Fr) is theoretically the least electronegative, but Cesium (Cs) is the element with the lowest experimentally determined electronegativity value (0.79 on the Pauling scale), due to its large size and low effective nuclear charge felt by its valence electron.
Is oxygen electronegative?
Yes, oxygen is a highly electronegative element, with a Pauling value of 3.44.
Is nitrogen electronegative?
Yes, nitrogen is electronegative, with a Pauling value of 3.04. It is slightly less electronegative than oxygen.
Is carbon electronegative?
Yes, carbon is moderately electronegative, with a Pauling value of 2.55.
Is hydrogen electronegative?
Hydrogen's electronegativity (2.20) is often considered intermediate. It is similar to carbon and falls near the middle of the scale, allowing it to form polar bonds with both more and less electronegative elements.
Is oxygen more electronegative than nitrogen?
Yes, oxygen (3.44) is more electronegative than nitrogen (3.04).
Is oxygen more electronegative than chlorine?
Yes, oxygen (3.44) is slightly more electronegative than chlorine (3.16).
Is oxygen more electronegative than hydrogen?
Yes, oxygen (3.44) is significantly more electronegative than hydrogen (2.20).
Is nitrogen more electronegative than carbon?
Yes, nitrogen (3.04) is more electronegative than carbon (2.55).
Is carbon more electronegative than hydrogen?
Yes, carbon (2.55) is slightly more electronegative than hydrogen (2.20).
Is sulfur more electronegative than oxygen?
No, oxygen (3.44) is significantly more electronegative than sulfur (2.58).
Do noble gases have electronegativity?
Generally, standard electronegativity scales like Pauling's do not assign values to noble gases (Group 18) because they are typically very stable and do not readily form chemical bonds under normal conditions, so they have little to no tendency to attract bonding electrons.
Why do noble gases not have electronegativity values?
Noble gases have a full valence electron shell (octet rule satisfied), making them chemically inert and stable. They have very high ionization energies and electron affinities close to zero, meaning they neither readily lose nor gain electrons to form bonds, which is the basis for electronegativity.
How does electronegativity relate to polarity?
The difference in electronegativity between two bonded atoms determines the polarity of the bond. A large difference means the more electronegative atom pulls the shared electrons closer, creating a polar covalent bond with partial positive and negative charges. A zero or very small difference results in a nonpolar covalent bond.
What does electronegativity have to do with polarity?
Electronegativity is the fundamental property that causes bonds (and often molecules) to be polar. The greater the difference in electronegativity between bonded atoms, the more polar the bond.
What electronegativity difference is polar?
There isn't a single strict threshold, but a general guideline on the Pauling scale is: a difference of less than ~0.4 is nonpolar covalent, between ~0.4 and ~1.7 is polar covalent, and greater than ~1.7 is often considered ionic (or very polar).
How to find electronegativity difference?
To find the electronegativity difference between two atoms in a bond, subtract the electronegativity value of the less electronegative atom from the value of the more electronegative atom.
How does electronegativity affect acidity?
For certain types of acids, particularly binary acids (like HX), increased electronegativity of the nonmetal (X) across a period weakens the H-X bond and stabilizes the resulting anion, making the acid stronger. In oxyacids, increased electronegativity of the central atom or adding more electronegative oxygen atoms also increases acidity by polarizing O-H bonds and stabilizing the conjugate base.
How does electronegativity affect the interactions between water molecules?
Oxygen is much more electronegative than hydrogen in water (H₂O). This large difference causes unequal sharing of electrons, giving oxygen a partial negative charge and hydrogen partial positive charges. These partial charges lead to strong attractive forces between water molecules called hydrogen bonds, which are crucial for water's properties like high boiling point and solvent capabilities.
How is electronegativity measured?
Electronegativity is not a quantity that is directly measured like mass or temperature. It's a derived value calculated from other measurable properties, such as the energy required to break chemical bonds or the energy involved in adding/removing electrons from an isolated atom.
How is electronegativity related to covalent bonding?
In covalent bonding, atoms share electrons. Electronegativity determines how equally or unequally these shared electrons are distributed between the bonded atoms, thereby determining if the covalent bond is nonpolar or polar.
How are reactivity and electronegativity related?
Elements with very high electronegativity (e.g., halogens) are highly reactive because they strongly attract electrons to achieve a stable configuration. Similarly, elements with very low electronegativity (e.g., alkali metals) are highly reactive because they easily lose their valence electron to achieve stability. Elements in the middle of the scale tend to be less reactive.
What determines electronegativity?
The primary factors determining an atom's electronegativity are its effective nuclear charge (the positive charge felt by the valence electrons) and its atomic radius. A higher effective nuclear charge and a smaller atomic radius generally result in higher electronegativity because the nucleus can pull bonding electrons more strongly.
Is electron affinity the same as electronegativity?
No, they are related but distinct concepts. Electron affinity is the energy change when an electron is added to a *neutral isolated atom* in the gaseous state. Electronegativity is the tendency of an atom to attract electrons *when it is part of a chemical bond*.
What is an electronegative atom?
An electronegative atom is an atom that has a relatively high value on an electronegativity scale, indicating a strong tendency to attract shared electrons when it forms a chemical bond.
Where is the highest electronegativity found?
The highest electronegativity is found in the upper right corner of the periodic table, specifically with the element Fluorine (F).
