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Chem1 Virtual Textbook - Electronegativity
Nice treatment of electronegativity, an extremely important topic.

HyperPhysics - Electronegativity



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Special points of emphasis

Electricity

Atomic Theory

Periodic Properties

The Chemical Bond

Functional Groups in Organic Chemistry

Electronegativity reflects the strength of attraction that an atom has for the electrons it shares with other atoms in chemical bonds. Atoms with strong nuclei that bind electrons tightly have high electronegativity values. Because knowing these will help you understand their behavior, memorize the values of the electronegativity of the common atoms of organic chemistry- carbon (2.5), hydrogen (2.1), oxygen (3.5), nitrogen (3.0), fluorine (4.0), chlorine (3.0), bromine (2.8), sulfur (2.5), phosphorus (2.1). Knowing these values is key to understanding many aspects of the chemical behavior of the organic functional groups.



Periodic Properties

The Chemical Bond

Intermolecular Forces

The electronegativity of an element is an incredibly helpful thumbnail signifier of its chemical 'personality', second only in importance to outer shell electron configuration. The greater the electronegativity of an element, the stronger its pull on the electrons it shares with other atoms in chemical bonds. By the time you walk into the MCAT, you really do want to have taught yourself a habit of being automatically conscious of the electronegativies of the atoms at play in any chemical situation. Such a habit makes chemistry more coherent and intuitive. Electronegativity has so many aspects of importance that it is impossible to create an exhaustive list.

One very significant role for electronegativity is in determining the polarity of covalent bonds, which then determines the degree of intermolecular force. The greater the electronegativity difference between the bonded atoms, the more polar the bond because the shared electrons spend more time around the more electronegative atom, giving it a partial negative charge and leaving the other atom partially positive. The bonded atoms form a permanent dipole. Furthermore, because there are discrete positive and negative charge densities, the dipole can be attracted by other dipoles leading to strong intermolecular forces.

This is especially true in the case of one subtype of polar bonds, which are strong enough to earn their own name, the hydrogen bond. Hydrogen bonds are intermolecular forces that arise in substances where hydrogen is bound to a polar element, such as oxygen, and the positive pole is a bare hydrogen nucleus. Especially strong interactions result.




Periodic Properties

The Chemical Bond

Intermolecular Forces

Functional Groups in Organic Chemistry

The States of Matter

The Physical Properties of Organic Compounds

A preoccupation in organic chemistry is how the electronegativity difference of bonded atoms leads to polarities which in turn determine the degree of intermolecular force. The carbonyl group of aldehydes and ketones is a prototypical organic functional group with dipole-dipole intermolecular forces leading to respectably strong intermolecular attractions.

Even stronger intermolecular forces are produced, however, with hydroxyl group or amine group, where the positive pole is hydrogen. With large electronegativity differences between bonded atoms, the resulting dipoles lead to strong intermolecular forces, especially if the positive pole is hydrogen, in which case the resulting intermolecular force is called a hydrogen bond.

Because the intermolecular forces corresponding to hydrogen bonding are stronger, a primary or secondary alcohol will be higher boiling than a similar aldehyde or ketone, respectively, which will in term be higher boiling than a similarly sized alkane, which will have only weak Van der Waals intermolecular forces (caused by temporary induced dipoles).

The stronger the intermolecular forces, the more energy is required to overcome the deep potential energy well of mutual attraction and lead to vaporizaton. In summary, electronegativity difference leads to bond polarity, which, in turn, leads to intermolecular force. Intermolecular force, in turn, plays a role in determining physical properties such as boiling point.




Periodic Properties

The Chemical Bond

Intermolecular Forces

Functional Groups in Organic Chemistry

The Physical Properties of Organic Compounds

Solutions

In addition to playing a role in determining physical properties such as boiling point, the electronegativity differences of a molecule’s atoms determine its solubility properties. The electronegativity difference among bonded atoms determines whether a bond is polar or nonpolar. The polarity of a molecule's chemical bonds determines the nature of the its attraction to other molecules.

In other words, whether a molecule contains polar or nonpolar bonds determines the kinds of intermolecular force it can participate in, whether simply Van der Waals forces (also called London dispersion forces) for nonpolar molecules, dipole-dipole interaction, or hydrogen bonding.

As we discussed earlier, the nature of intermolecular force plays a crucial role in determining physical properties such as boiling point. With boiling point, we are concerned with the intermolecular attraction between a molecule and other molecules of its own type.

With solution chemistry, we are also concerned with the intermolecular attraction between the molecule and the molecules of a potential solvent. The kind of intermolecular force relationships that a substance will engage in plays a crucial role in determing the solubility properties of the substance. For the solution process, the rule is 'like dissolves like'. Polar molecules dissolve in polar solvents while nonpolar molecules dissolve in nonpolar solvents.




Work, Energy, and Power

Electricity

Periodic Properties

The Chemical Bond

Thermochemistry

Chemical Thermodynamics and the Equilibrium State

Oxidation-Reduction

Electrochemistry

The topic of oxidation-reduction is another central domain of chemistry (and central MCAT topic) in which electronegativity is a crucial underlying concept. In fact, it is the manner in which electronegativity differences play out in chemical reactions that provides the underlying coherence of the oxidation-reduction system of chemistry. This is a somewhat complicated idea which we are going to be approaching from a number of directions as this MCAT course progresses. Let us summarize the basic gist here.

Oxidation-reduction is a systematic accounting procedure to reflect the changes in the bonding environment of electrons between products and reagents. Electronegativity reflects the strength of attraction an atom has for the electrons it shares in chemical bonds. When two atoms form a covalent bond, the more electronegative atom is assigned 'electron control' in the oxidation-reduction system. If an atom gains electron control through a chemical process, it is said to be 'reduced, ' while the atom that has lost electron control is said to be 'oxidized'.

The key to the system is that when a strongly electronegative atom is reduced, it draws the new electrons inwards towards its strongly attracting nucleus, and the bond becomes polarized. This closing of separation between like charges represents a potential energy decrease above and beyond the typical energy decrease that accompanies the formation of an ordinary bonding, molecular orbital. Think about that for a moment.

In other words, the formation of polar bonds corresponds to large potential energy decreases (tending toward negative internal energy change, negative enthalpy, negative free energy change), and as a general rule, polar bonds are stronger than nonpolar bonds (more energy is required to break them because the electrons have to be wrenched away from the oxidant).

Oxidation-reduction provides a systematic way to account for the tendency of polar bonds to be strong, low energy bonds. For this reason, the graph of reduction potentials of the various elements and their electronegativities virtually overlap. Having read the above discussion, could you explain why?

Oxidation-reduction gives a systematic way to solve problems, but remember what the meaning is below the surface. In general, the chemical system decreases in energy when electronegative elements gain electron control through a chemical reaction. In the case of redox reactions where covalent bonds are being broken and formed, the guiding narrative behind reaction potential is that electronegative elements form stronger covalent bonds. This is a lot to digest. Read it again, and remember that you will see these ideas quite a few times more as this MCAT course progresses.








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