Physical Science Quarter 1 Lesson 2: Polarity of Elements PDF

PHYSICAL SCIENCE Quarter 1 Lesson 2: Polarity of Elements ELECTRONEGATIVITY According to Linus Carl Pauling, the electr...

PHYSICAL SCIENCE Quarter 1 Lesson 2: Polarity of Elements 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. The value of electronegativity of an element describes the ability of its atom to compete for electrons with another atom to which it is bonded. Since electronegativity is relative, it has no unit of measurement. Factors Affecting the Electronegativity of an Atom 1. Atomic Size or Radius Electronegativity of a bonded atom decreases as its size or radius increases. This is due to the shielding effect. Shielding effect of atomic or electron shielding is when electrons in the inner electron shells of an atom can shield the outer electrons from the pull of the nucleus. The nucleus can pull the outer electrons in tighter when the attraction is strong and less tight when the attraction is weakened. Therefore, the more shielding that occurs, the less attraction there is between the outer electrons and nucleus, so the further the electrons in the outer shell can spread out. This means the atomic radius will be larger. 2. Effective Nuclear Charge The higher the effective nuclear charge means that the electronegativity is also high. 3. Hybridization state of an atom Electronegativity increases with increasing s-character of the hybrid orbital. The happens because the s-orbital is nearer to the nucleus thus increasing electronegativity. 4. Bond order The number of covalent bonds present between to bonded atom is known as the bond order. Moreover, with the increase in bond order, the bond distance decreases thus increasing the effective nuclear charge which results to the increase of electronegativity. 5. Oxidation number The electronegativity value increases with the increase in oxidation number. This is because the radius of an atom decreases with the increasing oxidation number. Periodic Trends of Electronegativity Figure 1. Periodic Table of Elements with Electronegativity Values 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. Exemptions: 1. For elements in group II-B, the electronegativity increases from top to bottom. Therefore, Zn < Cd < Hg. 2. For elements in group III-A, the electronegativity of aluminum is greater than gallium. Therefore, Al < Ga. POLAR AND NON-POLAR MOLECULES Polarity of covalent bonding is described by the result of electronegativity difference. Polarity depends on the structure of the molecule. Molecules with uneven electronegativity have partial positive and negative charges which makes them polar. Otherwise, they are nonpolar. There are ways to determine the polarity of a molecule which requires your knowledge on electronegativity and VSEPR theory. In the last lesson, you study the electronegativity of elements. Before you proceed to the next lesson of this next module, take time to read the role of electronegativity difference in bonding of atoms.  PHYSICAL SCIENCE Quarter 1 Electronegativity Difference A molecule is made of atoms that are participating in chemical bonds with one another. There are different types of bonds that we will be able to describe and the key to their formation has to do with the difference in electronegativity between the two atoms participating in the bond. If the electronegativity difference is greater than or equal to 1.7 (ΔEN ≥ 1.7), the chemical bonding becomes ionic. For instance, EN of Sodium = 0.9 and EN of Chlorine = 3.0, the ΔEN = 2.1, with this, chlorine, has the higher electronegativity, will steal an electron from sodium. In this situation, chlorine really wants an electron while sodium really wants to get rid of one resulting to a positively charged sodium ion, and a negatively charged chloride ion. Particles of opposite charge have an electrostatic attraction so these two ions will form a compound through ionic bonding. This is a very strong attraction because it is happening between ions that have a formal charge If the electronegativity difference is less than to 1.7 (ΔEN < 1.7), no electrons will be stolen but instead the two atoms share the electrons, thus covalent bonding occurs. The electron from this atom feels the attraction from the other nucleus and so thus the electron from this atom, so the atoms share the two electrons to form a covalent bond. Covalent bonds can be polar or non-polar. a. If the electronegativity difference is greater than to 0.5 but not exceeding 1.7 (1.7 > ΔEN > 0.4), the covalent bonding becomes polar. For example, EN of Hydrogen = 2.1 and the EN of Chlorine = 3.0, the ΔEN = 0.9, instead of completely stealing the electrons, the chlorine will just sort of hug the electron in the resulting covalent bond. The more electronegative atom in a polar covalent bond pulls the electron density towards itself and as a result of slight electron excess, we call this atom partially negative, denoted by the symbol Delta minus (𝛿−). The atom whose electron is getting pulled away is slightly electron deficient and therefore partially positive of Delta plus. Thus, possessing a dipole moment. b. If the electronegativity difference is less than or equal to 0.5 (ΔEN ≤ 0.4), the covalent bonding becomes non-polar. In a non-polar covalent bonding, the electrons are shared more or less evenly or in the case of two atoms of the same elements, precisely evenly. The atoms will atoms will have no partial charges at all. Polarity of Molecules The outer shell of many atoms, Hydrogen being an exemption, holds eight electrons (Octet rule). Atoms that have an outer shell containing eight electrons are considered chemically stable. We often say they fulfill what is called the Octet rule, which is the tendency of atoms to prefer eight electrons in their outer shell. When atoms combine to form compounds, they can give up, accept, or share electrons in order to assemble an octet of electrons in their outer shells. A covalent bond is formed when atoms share electrons in order to satisfy the octet rule. For example, a carbon atom holds four electrons in its outer shell, but it would be much more stable with eight electrons. To gain the additional four electrons, carbon can share with other atoms, and thereby have a completed outer shell. If one carbon and four hydrogen atoms come together and share their electrons, carbon achieves an octet of electrons. Note that each hydrogen in this compound achieves two electrons in its outer shell. This is the stable configuration of the first electron shell. Covalent bonding can be polar or non-polar. As mentioned earlier, the polarity of bonds is based on the electronegativity difference of the two atoms involved in the bond formation. But simply having a polar bond present in a molecule does not guarantee molecular polarity. This is because of the possibility of the dipole moments being cancelled out. Polar covalent bond - Having electronegativity difference greater than 0.4 but not greater than 1.7 - Produce partial charges and dipole moment - More electronegative atom has a more pulling power - Less electronegative atom has a less pulling power Non-polar covalent bond - Having electronegativity difference less than or equal 0.4 - No partial charges and dipole moment - Equal sharing of electrons  PHYSICAL SCIENCE Quarter 1 Note: Even though carbon dioxide molecule has dipole moments, it will just cancel out since they are going towards opposite direction. Thus, the resultant dipole moment is zero. REFERENCES: Esguerra, J. H., Dapul, G. R., Salazar, M., & Bantang, J. Y. (2016). Teaching Guide for Senior High School Physical Science. Quezon City: Commission on Higher Education. Helmenstine, A. (2019, September 27). Electronegativity Definition and Trend. Retrieved from Science Notes - Learn Science Do Science: https://sciencenotes.org/electronegativity-definition-and-trend/ Santiago, K. S., & Silverio, A. A. (2016). Exploring Life Through Science Series - Senior High School Physical Science. Quezon City: Phoenix Publishing House, Inc.