PhySci - 1Q - Week 2_Polarity of Elements.pdf

 Department of Education MAKATI HIGH SCHOOL Gen. Luna St. Brgy. Poblacion, Makati City Week 2 – Module 2 POLARITY OF ELEMENTS  Essential Questions 1. What is electronegativity among atoms and molecules? 2. What is polarity of molecules? 3. How are polar and non-p...

 Department of Education MAKATI HIGH SCHOOL Gen. Luna St. Brgy. Poblacion, Makati City Week 2 – Module 2 POLARITY OF ELEMENTS  Essential Questions 1. What is electronegativity among atoms and molecules? 2. What is polarity of molecules? 3. How are polar and non-polar molecules differ from each other? Learning Objective 1. define electronegativity and familiarize its trend in the periodic table of elements 2. determine if a molecule is polar or nonpolar given its structure  Chemical bonds are formed when an atom lose or share electrons a) Ionic bond occurs when there is a transfer of one or more valence electrons from one atom to another. It exist between metal (loses electrons) and nonmetal (accepts electrons) atoms. Ex. NaCl, CaCl2, Fe2O3 and KBr b) Covalent bond occurs when two nonmetals shared electrons together Ex. CO2, CCl4 and N2O  Electronegativity According to Linus Carl Pauling, the electronegativity of an element is the tendency for the nucleus of the atoms of elements to attract electrons when they are chemically combined with the atoms of another element.  Across a period, the electronegativity of elements increases from left to right. This is due to an increase in nuclear charge. Alkali metals such as Li, Na, K, Rb, Cs, and Fr have the lowest electronegativities, while halogens such as F, Cl, Br, I, and Ts have the highest. Since most noble gases do not form compounds, they do not have electronegativities.  Down the group, the electronegativity of elements decreases. This is due to other trends. Elements down the group become bigger, thus shielding effect increases. Therefore, the capacity of the nucleus to attract bonding electrons decreases.  Polarity of Molecules Polarity is the measure of the degree of inequality in the attraction of electrons between atoms in a molecule. Polarity means having dipoles​, a positive and a negative end.  Based on polarity, bonds can either be polar or nonpolar​. In determining the polarity of a bond, it is important to know the electronegativity values of the atoms constituting it.  Polar Bonds Polar molecules​, are molecules that have unequal distribution of charges resulting in dipoles. Water (H​2O) and ammonia (NH​3 ) are examples of polar molecules.  The atom with a higher electronegativity has greater electron-attracting ability. Thus, if an atom in a molecule has a greater electronegativity than other atoms, that atom will pull its surrounding electrons closer to it, leaving it bearing a partial negative charge. And since its surrounding atoms will partially be stripped off of an electron, from a neutral charge, these other atoms will have a partial positive charge. This bond is said to be polar because of the “poles” of partially negative and positive charges. Hence, polar covalent bonds are characterized by asymmetrical electron clouds ​between the nuclei of each atom. The poles in a polar bond are represented by the lowercase Greek letter delta, δ​. The partially negative end is designated with δ- while the partially positive end is designated with δ+.  Non-Polar Bonds Nonpolar bonds, on the other hand, are described by symmetrical electron clouds between the nuclei. This results when the atoms forming the bond have the same or relatively the same electronegativity values. The electrons feel equal or relatively equal attraction from each nuclei, which results in their symmetrical distribution in the bond.  Electronegativity Difference The polarity of bonds is based on the electronegativity difference (Δδ) of the two atoms involved in the bond formation. This could differentiate whether covalent bonds are polar or nonpolar. Non-Polar bonds Non-polar bond Polar bond Polar bond  Dipole Moments Dipole moments are vector quantities. Vector quantities have directions and magnitudes. They are represented by arrows. The magnitude depends on the length of the arrow, while the direction depends on where does the arrow point in space.  The arrowhead points to the more electronegative atom, while the arrowtail is located to the less electronegative one (or the more electropositive atom). As a convention, the arrowtail resembles a “+” symbol, which represents the δ+ end.  Using this convention, you can represent the HCl bond according to the figure below. Learning Objective 3. relate the polarity of a molecule to its properties  Essential Question What are the different properties of molecules based on polarity? Explain each.  Properties of Molecules based on Polarity The properties of the molecules are related to their polarity. To understand this relationship, you must be able to describe the forces of attraction between the molecules. When substances undergo phase changes, these forces of attraction are broken or formed. These forces play significant roles to almost all properties of substances, including solubility, melting point and boiling point. Polar molecules have partially positive and partially negative ends. When two polar molecules are near each other, the negative end of one molecule is attracted to the positive end of the other. This type of interaction is known as dipole-dipole interaction and is present between any polar molecules.  Solubility Solubility refers to the maximum amount of solute that dissolves in a given quantity of solvent. This follows the rule of thumb, “like dissolves like.” Polar solutes dissolve in polar solvents. Likewise, nonpolar solutes dissolve in nonpolar solvents.  Melting Point Melting point is the temperature at which a solid turns into liquid​. Heat is needed to break the forces of attraction between the molecules. Because polar molecules have relatively stronger forces of attraction compared with nonpolar ones, a greater amount of heat must be applied to break these forces. Therefore, in general, polar molecules have higher melting points than nonpolar molecules​.  Boiling Point Boiling point is the temperature at which a liquid turns into gas​. Similar to melting point, a greater amount of heat is needed to break the forces of attraction of polar molecules compared to nonpolar ones. Generally, polar molecules have higher boiling points than nonpolar molecules​.  Water is called the "universal solvent" because it is capable of dissolving more substances than any other liquid. This is important to every living thing on earth. It means that wherever water goes, either through the air, the ground, or through our bodies, it takes along valuable chemicals, minerals, and nutrients.