We've been studying chemical bonding. We know that some elements want electrons. Others are willing to give up their electrons. But, for both, their goal is the same. They want to have a complete octet. Acquiring a full octet lowers the atom's total energy. So, it's the most stable electron configuration. An ionic bond arises when electrons get transferred between atoms. It creates ions with opposite charges. So, they are electrostatically attracted. A covalent bond occurs when nuclei share electrons.
What is Electronegativity?
What property determines the type of chemical bond? Electronegativity! It measures an atom's capacity to pull electrons to itself. On the periodic table, electronegativity increases across a period. So, the alkali and alkaline earth metals have the lowest values. The metalloids and early nonmetals are intermediate. The later nonmetals and halogens are the highest. The electronegativity decreases down a group. That's because the valence shell is further from the nucleus.
The Electronegativity Scale
We measure electronegativity on a unitless scale from 0 to 4. Technically, the scale is 0.79 to 3.98. That's because the least electronegative element is cesium (0.79). We would expect it to be francium. But, francium is unbelievably radioactive. Its most stable isotope has a half-life of 22 minutes! So, we've been unable to take adequate measurements. The most electronegative element is fluorine (3.98).
The electronegativity scale is also relative. That means we set one standard value. Then, we compare everything else to that standard. Hydrogen is the standard with a value of 2.20. So, any elements with larger values want electrons more than hydrogen. Ones with lesser values pull the electrons less.
What Determines Electronegativity?
Electronegativity is a balance between two competing factors. The more an atom can pull electrons, the greater its electronegativity. So, more protons create a stronger pull on the valence electrons. We would expect the atomic number to be the determining factor. But, we also have to consider the radius. The further the valence shell is from the nucleus, the less attraction it experiences. But, there are actually two factors. The most apparent facet is distance. Electrons that reside in the sixth or seventh energy level are very far from the nucleus. But, we must also consider shielding. A large atom has many electrons between the valence shell and its nucleus. These core electrons block the attraction between the nucleus and the outer shell. So, it's easier to lose those electrons.
Electronegativity, Periodic Trends, and Chemical Properties
Let's take a quick look at electronegativity versus the periodic trends and chemical properties.
Electron Affinity:
- The electron affinity is the energy released when a neutral atom gains an electron. Electronegativity directly correlates to electron affinity. The most electronegative elements are close to filling their octet. So, they become more stable when they gain an electron. Remember, there are a lot of exceptions for electron affinity.
Ionization Energy:
- Ionization energy and electronegativity are directly related. A higher electronegativity equals a stronger pull on the valence electrons. So, it takes more energy to ionize.
Bond Energy and Bond Length:
- The electronegativity difference is an excellent estimate of bond energy. The greater the difference, the stronger the bond. Remember, stronger bonds are also shorter.
