Polarity of Molecules Handout

When atoms join to form molecules, they are held together by bonds. These bonds can be classified by the extent of transfer or sharing of electrons between atoms. A bond formed between two nonmetals that share electrons are called a **covalent bond**. It can be subdivided into two general types: po...

When atoms join to form molecules, they are held together by bonds. These bonds can be classified by the extent of transfer or sharing of electrons between atoms. A bond formed between two nonmetals that share electrons are called a **covalent bond**. It can be subdivided into two general types: polar and non-polar bond. In **polar bond**, the electrons are unequally shared between two bonded atoms which result in the development of partial positive (δ+) and partial negative (δ--) charges. The direction toward which shared electrons are pulled is indicated by a polar arrow ( ˫ ). In a **nonpolar bond**, electrons are equally shared, and a charge is evenly distributed over the two bonded atoms. **Polarity** determines whether electrons are shared equally or not. A **polar molecule** results when a molecule contains polar bonds in an unsymmetrical arrangement. **Nonpolar molecules** are of two types. Molecules whose atoms have equal or nearly equal electronegativities have zero or very small dipole moments. The second type of nonpolar molecule has polar bonds, but the molecular geometry is symmetrical allowing the bond dipoles to cancel each other. Take note that in a diatomic molecule, the polarity of the bond is the same with the polarity of the molecule, while in a polyatomic molecule, the polarity of the bonds and the polarity of the molecule may not be the same. The following steps and examples will help us in determining whether a molecule is polar or nonpolar. 1. Draw the Lewis structure of a given molecule. 2. In a **diatomic molecule**, if atoms are the same, the molecule is nonpolar. If atoms are different, the molecule is polar. (See figure 1.) - If the difference in electronegativity of the bond is greater than 0.4, it is polar and if it is equal to 0.4 or lesser, it is nonpolar. If ALL of the bonds are nonpolar, the molecule is nonpolar. However, if the molecule has polar bonds, continue with the steps. 3. If the central atom has no lone pairs with all the atoms bonded to the central atom are the same, the molecule is symmetrical, therefore, it is considered to be nonpolar. If the central atom has lone pairs or the atoms bonded to the central atom are different, the molecule is asymmetrical, therefore, it is polar. (See figure 2) 1. **Molecules** - are particles made up of two or more atoms that are chemically bonded together 2. **Diatomic molecule** -- a type of molecule composed of only two atoms 3. **Polyatomic molecule** - a type of molecule with three or more atoms +-----------------------------------------------------------------------+ | **Properties of Molecules According to their Polarity** | | ======================================================= | +-----------------------------------------------------------------------+ The properties of molecules to be discussed in this section are solubility, melting point, and boiling point. **[Solubility]** - When we say solubility, it is the ability of a molecule to be dissolved in a solvent. - Solubility uses the \"like dissolves like\" rule which means that molecules with the same type of polarity (polar to polar; nonpolar to nonpolar) will be soluble in one another while molecules with differing polarities (polar to nonpolar) will be insoluble in one another. Polar molecules are soluble in water, while nonpolar molecules are soluble in oils and fats. - An example is an alcohol (containing oxygen and hydrogen) and water (H~2~O). Have you noticed that alcohol can easily be mixed in the water? This is because both substances are polar. Another example is oil (containing carbon and hydrogen only) and water. What happens when they are mixed? They separate. Since oil is nonpolar and water is polar, oil cannot be dissolved. **[Melting and Boiling Point]** - The melting point is the temperature at which a molecule changes from a solid-state to a liquid, or melts while the boiling point of a substance is the temperature at which it can change its state from a liquid to a gas. - The polarity of the molecules determines the forces of attraction between the molecules. Polar molecules are attracted by the opposite charge effect (the positive end of one molecule is attracted to the negative end of another molecule). Therefore, it has a stronger attraction as compared to nonpolar molecules. So, how does it relate to the boiling and melting point? The stronger the forces of attraction, the higher the boiling and melting point, or the greater the polarity, the higher the boiling and melting point of a substance with similar sizes. For you to have a better understanding of the properties of molecules according to their polarity, let us do a simple experiment. +-----------------------------------------------------------------------+ | **Intermolecular Forces** | | ========================= | +-----------------------------------------------------------------------+ Bonding forces (ionic, covalent, and metallic bonds) are classified as **intramolecular forces,** forces that hold atoms together in a molecule. In contrast, **intermolecular forces*** *are attractions that occur between molecules. Intramolecular forces are many times stronger than intermolecular forces of attraction. Intermolecular forces are responsible for the condensed phases of substances. **General Types of Intermolecular Forces** **Intermolecular forces** are attractive forces that operate between molecules. They arise from the interaction of positive and negative charges. Intermolecular forces are much weaker than intramolecular in terms of energy involved. However, intermolecular forces are responsible for the properties of molecules. These explain why substance exists as solid, liquid, or gas at room temperature. The following are the types of intermolecular forces. Intermolecular Forces of Attraction \| ChemistryBytes.com **[Ion-Dipole Forces]** **Ion-dipole forces** exist between an ion (charged particles) and a dipole (polar) molecule. A positive ion will be attracted to the negative pole of the polar molecule, while a negative ion will be attracted to the positive pole of the polar molecule. This can be seen when NaCl dissolves in water. The ions of NaCl (Na^+^ and Cl^-^) become separated. Since water is a polar molecule, it has a partially positive end and a partial negative end. Thus, the positive sodium ion (Na^+^) will be attracted to the partially negative end of the water molecule, while the negative chloride ion (Cl^-^) will also be attracted to the partially positive end of the water molecule (See figure 1). **[Dipole-Dipole Forces]** **Dipole-dipole forces** are attractive forces that occur between polar molecules. The partially positive end of one molecule attracts the partially negative ends of other molecules. An example is hydrogen chloride (HCl) which has a partially positive hydrogen atom and a partially negative chlorine atom. A collection of many hydrogen chloride molecules will align themselves so that the oppositely charged regions of neighboring molecules are near each other (see figure 2). **[Hydrogen Bonds]** A **hydrogen bond** is a special type of dipole-dipole force between polar molecules having an H atom covalently bonded to a highly electronegative atom (O, N, or F) with lone electron pairs. Hydrogen tends to be strongly positive due to the strong tendencies of F, O, or N to attract the electron towards it. The highly electronegative elements make hydrogen strongly positive. The ability of water to form H-bond relates to its ability as a universal solvent. H-bond prevents the water from evaporating quickly into the atmosphere. It also causes ice to float in water since, at freezing temperature, water molecules tend to form a crystal lattice as it expands. **[London Dispersion Forces]** **London Dispersion Force** is present in all molecules. It is the only force present in nonpolar molecules. Although it is very weak and acts in very small distances, they are strong enough to cause substances normally found as gases, to liquefy at high pressures or low temperatures. London dispersion forces tend to be stronger in a larger atom or molecule. Dispersion force is formed due to the attraction between the positively charged nucleus of an atom with the negatively charged electron cloud of a nearby atom. For example, the electron cloud of a helium atom contains two electrons, and, when averaged over time, these electrons will distribute themselves evenly around the nucleus. However, at any given moment, the electron distribution may be uneven, resulting in an *instantaneous dipole*. This weak and temporary dipole can subsequently influence neighboring helium atoms through electrostatic attraction and repulsion resulting in the formation of another temporary dipole called an *induced dipole*.  The types of intermolecular forces that occur in a substance will affect its properties due to its varying strengths. You should remember from the kinetic theory of matter that the phase* *of a substance is determined by how strong the forces are between its particles. The weaker the forces, the more likely the substance is to exist as a gas. This is because the particles can move far apart since they are not held together very strongly. If the forces are very strong, the particles are held closely together in a solid structure. The relative strength of intermolecular forces is illustrated in Fig. 5. **Properties of Substances in Relation to Intermolecular Forces** Enumerated and discussed below are the properties of substances that are related to the intermolecular forces. **[Surface Tension]** **Surface tension** is a phenomenon caused by cohesive forces (intermolecular forces) between molecules allowing liquids to create a thin film on its surface. This causes liquids to acquire a certain shape when put on a container or dropped on surfaces. Molecules within a liquid experience force of attraction equally in all directions. However, there are no forces above the surface of the liquid, but there are normal forces below. Because of this imbalance, forces of attraction tend to pull molecules toward the interior of the liquid. This creates surface tension. Stronger intermolecular force equates to stronger surface tension. **[Boiling and Melting Point]** **Boiling Point** is the temperature at which the vapor pressure of the liquid is equal to the applied pressure on the liquid. **Melting Point** is the temperature at which a substance begins to change from solid to liquid. In general, substances with weak intermolecular forces will have low melting and boiling points while those with strong intermolecular forces will have high melting and boiling points. The strong intermolecular force of the substance holds the molecules tightly thus making the bond hard to evaporate and boil. Remember also that the temperature of a material affects the energy of its particles. The more energy the particles have, the more likely they are to be able to overcome the forces that are holding them together. This can cause a change in phase **[Vapor Pressure]** **Vapor pressure** is a measure of the tendency of a material to change into the gaseous or vapor state, and it increases with temperature. A liquid with weak intermolecular forces will evaporate easily thus, has a higher vapor pressure while liquid with strong IMF will not easily evaporate thus will have a lower vapor pressure. **[Viscosity]** **Viscosity** is the resistance of a liquid to flow. The more viscous a liquid is the thicker its consistency. In layman\'s term, it is the measure of the thickness of a liquid. In general, stronger intermolecular forces leads to higher viscosity.