CHEM 1305 Introductory Chemistry

Transcription

1 CHEM 1305 Introductory Chemistry Introductory Chemistry: Concepts and Critical Thinking 7 th Edition, Charles H. Corwin Chapter 12. Chemical Bonding Modified by: Dr. Violeta F. Coarfa 1

2 Chemical Bond Concept Recall that an atom has core electrons and valence electrons. Core electrons are found close to the nucleus. Valence electrons are found in the most distant s and p energy subshells. Importance of Valence Electrons: they are important in determining how an element interacts chemically with other elements, by following the octet rule Number of valence electrons: given by the main group number of the elements * the count of valence electrons are not useful ** except for He, which has 2 electrons of valence

3 Learning Check Refer to the periodic table and predict the number of valence electrons for an atom of each of the following representative elements: a. Na b. Al c. S d. Xe Solution: Find the element in the periodic table, note the group number, and indicate the number of valence electrons. a. Sodium is in Group IA/1, and so Na has 1 valence electron. b. Aluminum is in Group IIIA/13, and so Al has 3 valence electrons. c. Sulfur is in Group VIA/16, and so S has 6 valence electrons. d. Xenon is in Group VIIIA/18, and so Xe has 8 valence electrons. Practice Exercise Refer to the periodic table and state the number of valence electrons for any element in each of the following groups: a. Group IIA b. Group VA c. Group 14 d. Group 17

4 Electron-Dot Symbols (Lewis) Definition: convenient representations of the valence electrons ns 1 ns 2 ns 2 np 2 ns 2 np 4 ns 2 np 6 ns 2 np 1 ns 2 np 3 ns 2 np 5 s-block and p-block Be careful - for Gr3A-7A (nonmetals): you must first draw a valence electron on each side of the element, then the dots are paired

5 Learning Check Draw the electron dot formula for each of the following elements: a. Si b. Xe Solution: Let s find the group number of the element in the periodic table and note the number of valence electrons. We write the symbol of the element and place the same number of dots around the symbol as there are valence electrons. In these examples, Si has 4 valence electrons and Xe has 8. he electron dot formulas are as follows: a. b. Practice Exercise: Draw the electron dot formula for each of the following elements: a. K b. I

6 Types of Bonds Octet Rule: Atoms bond in such a way that each atom acquires eight electrons in its outer shell. There are two ways in which an atom may achieve an octet: 1. Transfer of electrons from one atom to another 2. Sharing one or more pairs of electrons Positive ion (cation) from a metal atom Negative ion (anion) from a nonmetal atom Ionic bonds are formed when a cation is attracted to a anion Covalent bonds are formed when two nonmetals share valence electrons

7 Learning Check Bond Prediction Predict whether each of the following is held together by ionic or by covalent bonds: a. ammonia, NH 3 b. magnesium nitride, Mg 3 N 2 Solution: A metal ion and nonmetal ion are attracted in ionic bonds; two or more nonmetal atoms are attracted in covalent bonds. a. Ammonia contains the nonmetals nitrogen and hydrogen. It follows that NH3 has covalent bonds. b. Magnesium nitride contains a metal (Mg) and a nonmetal (N). It follows that Mg 3 N 2 has ionic bonds Practice Exercise Predict whether each of the following is held together by ionic or by covalent bonds: a. aluminum oxide, Al 2 O 3 b. sulfur dioxide, SO 2 7

8 Ionic Bonds An ionic bond is formed by the attraction between a positively charged cation and a negatively charged anion. This electrostatic attraction is similar to the attraction between opposite poles on two magnets. The ionic bonds are very strong and result in the formation of a rigid, crystalline structure. NaCl sodium chloride (table salt)

9 Ions - Ionic Charges Atoms lose or gain electron(s) and form positive or negative ions, which have 8 electrons on the outer shell cation ion with a positive charge If a neutral atom of a metal loses one or more electrons it becomes a cation. Metals lose the valence e - Na - 1 e- 11 protons Na + 11 protons 11 electrons 10 electrons Na + looks like the closest noble gas, Ne Na + is smaller than Na atom anion ion with a negative charge If a neutral atom of a nonmetal gains one or more electrons it becomes an anion. Nonmetals gain the (8 - # of group) e - Cl - 17 protons + 1 e 17 electrons Cl - 17 protons 18 electrons Cl - looks like the closest noble gas, Ar Cl - ion is bigger than Cl atom

10 Formation of Cations and Anions - Examples Metals lose valence e - cations Nonmetals gain ( 8 - # of group) e - anions 10

11 Monoatomic Ions - Ionic Charges METALS lose e - to become positive ions (cations) Main group Valence Electrons Lost electrons Ionic Charge(s) 1 A A A (main charge); +1 (secondary charge) 4 A (main charge); +2 (secondary charge) 5 A (main charge); +3 (secondary charge) Note: For transition metals - use Periodic Table NONMETALS gain e - to become negative ions (anions) Main group Valence Electrons Gained electrons Ionic Charge 4 A e - = 4 e A e - = 3 e A e - = 2 e A e - = 1 e - -1

12 Learning Check Which of the following statements are true regarding an ionic bond between an iron ion and a sulfide ion in an FeS formula unit? a. The iron atom loses electrons and the sulfur atom gains electrons. b. The iron atom has a larger radius than the iron ion. c. Iron and sulfide ions form a bond by electrostatic attraction. Solution: All of these statements are true regarding an ionic bond. a. An iron atom loses electrons and a sulfur atom gains electrons. b. The radius of an iron atom is greater than its ionic radius. c. An ionic bond is from the attraction of positive and negative ions. Practice Exercise Which of the following statements are true regarding an ionic bond between a zinc ion and an oxide ion in a ZnO formula unit? a. Zinc and oxygen form a bond by sharing electrons. b. The oxygen atom is larger in radius than the oxide ion. 12 c. Zinc and oxide ions form a bond by the repulsion of ions.

13 Common monatomic ions Ga +1 Ga +3

14 Covalent Bonds Covalent bonds are formed when two nonmetal atoms share electrons, and the shared electrons in the covalent bond belong to both atoms. When hydrogen chloride (HCl) is formed, the hydrogen atom shares its one valence electron with the chlorine. This gives the chlorine atom eight electrons in its valence shell, making it isoelectronic with argon. The chlorine atom shares one of its valence electrons with the hydrogen, giving it two electrons in its valence shell, and making it isoelectronic with helium. 1s 1 1s 1 3s2 3p 5 3s 2 3p 5 1s 1 3s 2 3p 5 e - are shared equally (non-polar covalent bond) e - are shared unequally (polar covalent bond) When a covalent bond is formed, the valence shells of the two atoms overlap with each other. In HCl, the 1s energy sublevel of the hydrogen atom overlaps with the 3p energy sublevel of the chlorine atom. The mixing of sublevels draws the atoms closer together.

15 Which of the following statements are true regarding a covalent bond between a hydrogen atom and an oxygen atom in an H 2 O molecule? a. Valence electrons are shared by hydrogen and oxygen atoms. b. The H O bond length is less than the sum of the two atomic radii. Solution: All of the previous statements regarding a covalent bond in a molecule are true. a. Valence electrons are shared by the two nonmetal atoms. b. The bond length is less than the sum of the two atomic radii. Practice Exercise Learning Check Which of the following statements are true regarding a covalent bond between a hydrogen atom and a sulfur atom in an H 2 S molecule? a. Valence electrons are transferred from hydrogen to sulfur. Answer: NO! b. The H S bond length equals the sum of the two atomic radii. Answer: NO! 15

16 EN increases Electronegativity and Bond Polarity EN increases F the highest electronegativity Fr the lowest electronegativity Nonmetals more electronegative than metals

17 Predicting the type of bond from Electronegativity If EN difference between bond atoms is big enough (2.0): electrons are exchanged between atoms ionic bond If EN difference between bond atoms is , electrons are equally shared nonpolar covalent bond structures EN difference type of electron sharing bond type H =0 Shared equally Nonpolar covalent Cl =0 Shared equally Nonpolar covalent HBr =0.7 < 2.0 Shared unequally Polar covalent HCl =0.9 < 2.0 Shared unequally Polar covalent NaCl =2.1 > 2.0 Electron transfer Ionic MgO =2.3 > 2.0 Electron transfer Ionic BeCl =1.5 <2.0 Shared unequally Polar covalent

18 Learning Check Classify each of the following as a polar or a nonpolar bond: a. Cl Cl b.cl N c. Cl Br d.cl H Solution: Practice Exercise Use Periodic Table to find the electronegativity values: a.the Cl Cl bond = 0 is nonpolar. b.the Cl N bond = 0 is nonpolar. c. The Cl Br bond = 0.1 is nonpolar d.the Cl H bond = 0.8 is polar. Classify each of the following as a polar or a nonpolar bond: a. C C b. C O c. C S d. C H 18

19 Number of Covalent Bonds - given by the number of electrons needed to form the electronic configuration of the closest noble gas. Examples: 1. H (Gr.1A) 1 bond 2. B (Gr. 3A) 3 bonds 3. C and Si (Gr 4A) 4 bonds 4. N and P (Gr 5A) 3 bonds 5. O and S (Gr 6A) 2 bonds 6. F, Cl, Br and I (Gr 7A) 1 bond

20 Review Chapter 12 - Core Electrons. Valence Electrons. Octet rule. - Metals positive ions/cations; Nonmetals negative ions/anions - Ionic Bond. - Covalent Bond. Nonpolar Covalent Bond vs. Polar Covalent Bond. - Number of covalent Bonds. Multiple covalent Bonds. - Electronegativity. How to predict a bond type by using Electronegativity.. 20