Periodic Trends - Electronegativity & Affinity
Chemistry: IB Diploma Higher Level, topic 3.1.4
Trends in Electronegativity
Electronegativity is a measure of the tendency of an atom to attract electrons (or electron density) towards itself.
Electronegativity
- In a bond between two unlike atoms, one atom will have a stronger attraction to the electron pair than the other.
- This means the electron pair will not be at the center of the bond.
- This causes the buildup of a partial charge on one atom.
- This is written as δ±
- The ability of an atom to attract the electron pair is called electronegativity.
Pauling electronegativities
- The values of electronegativity can be measured on several different scales.
- One example is the Pauling scale (named after Linus Pauling, its inventor).
- The most electronegative element is fluorine, with a value of 4.0.
- Other strongly electronegative elements are oxygen (3.4), chlorine (3.2), nitrogen (3.0)
- Carbon has an electronegativity of 2.5 and hydrogen has an electronegativity of 2.2.
Periodic trends
- In general, electronegativity increases from left to right across a period in the periodic table and decreases down a group.
- Thus, the nonmetals, which lie in the upper right, tend to have the highest electronegativities.
- As you may recall, fluorine the most electronegative element of all.
- Metals tend to be less electronegative elements, and the group 1 metals have the lowest electronegativities.
Trends in electronegativity
Noble gases
- Note that noble gases are excluded from this figure because these atoms usually do not share electrons with others atoms since they have a full valence shell.
- While noble gas compounds such as XeO2 do exist, they can only be formed under extreme conditions, and thus they do not fit neatly into the general model of electronegativity.
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Polarity
- The most electronegative atom in a bond will develop a partial negative charge (δ-) and vice versa.
- This charge difference is called a permanent dipole.
- A good example of a polar bond is the HCl molecule.
- A good example of a non-polar bond is the C-H bond.
- The electronegativities of hydrogen and carbon are very similar, so the bond is not polar.
Electron Affinity
The electron affinity of an element is the energy released or absorbed when an isolated gas-phase atom acquires an electron, measured in kJ/mol.
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Electron affinity vs electronegativity
- The electron affinity of an element is a measurable physical quantity.
- Electron affinity is the energy released or absorbed when an isolated gas-phase atom acquires an electron, measured in kJ/mol.
- Electronegativity describes how tightly an atom attracts electrons in a bond.
- It is a dimensionless quantity that is calculated by comparing the amounts of energy required to break different types of bonds.
Trends in electron affinity
- Electron affinity increases from left to right across a period in the periodic table and decreases down a group.
Trends - down a group
- As you go down a group, there is an increased number of electron shells.
- The greater the number of electrons between the nucleus and the outer electrons, the lower the effective nuclear charge.
- This is because the positive charge felt by the electron is reduced by the electrons in between.
- This means that the greater the number of electrons, the lower the electron affinity as it is more difficult to gain an electron.
Trends - across a period
- As you go across a period, the number of protons increases. This means nuclear charge increases across a period.
- As you go across a period, electrons are added to the same shell. For example, all elements in the second period have two electron shells.
- An increased nuclear charge across a period means a greater attraction between the nucleus and the electron to be gained.
- This means electron affinty increases across a period.
Test yourself on Periodic Trends - Electronegativity & Affinity
Electronegativity is labeled using which one of the following greek letters?