Chemistry Chemical Bonds and Lewis Structure When atoms transfer or share electrons: Dr. Fred Omega Garces.

Transcription

1 1001 Chemical Bonds and Lewis Structure When atoms transfer or share electrons: Gilbert N.Lewis ( ) An American chemist, many considered him one of the greatest of all chemists. e laid the foundations of our current understanding of bond formation. Dr. red mega Garces Chemistry 152 Miramar College Covalent Bonds and Lewis Structure

2 Compound classification by electron interactions The diagram below shows how electrons interact between atoms to form compounds. When electrons are transferred, the compounds are ionic. The class of compounds called salts are in this category. When electrons are shared, covalent bonds are formed and this class of compound are called molecular compounds. When electrons are not evenly shared, the compounds are called polar covalent molecules. If the electrons from atoms making up the compound are mutually shared, these compounds are called Covalent Compounds. Pure Substances Compounds If the electrons from the atoms making up the compound are transferred between atoms, these compounds are called ionic compounds. Molecules Covalent Ionic Compounds Sharing electrons non-metals If the electrons are unequally shared, then these are called polar covalent compounds. Polar- Covalent unequal sharing electrons metals or non-metals transfer electrons metal + nonmetal Covalent Bonds and Lewis Structure

3 ctet Rule Atoms and ions are most stable when they have complete outer shell filled with e -. (or when they have eight electrons in their valence shell). Many exceptions: i. (Some compounds can accommodate more than 8 e - ) transition metals & 3rd - lower row elements ii. (Some compounds will not lose or accept e - ) Boron will not form B +3 because a small atom (B) can t stabilize the concentrate charge, likewise C and Si hardly forms +4 ions. Carbon, Silicon is not stable by donating or accept electrons Covalent Bonds and Lewis Structure

4 orming Compounds from Elements Principle of electrical neutrality allows prediction of formulas of ionic compounds. Na Na + [Ne] 3s 1 e - [Ne] orce of attraction between ions lead to ionic bond formation. Na + + Cl - NaCl Cl Cl - [Ne] 3s 2 3p 5 [Ne]3s 2 3p 6 Ionic bond forms between Na + and Cl - due to electrostatic attraction between cation and anion. LATTICE ENERGY Covalent Bonds and Lewis Structure

5 Covalent Compounds Atoms mutually share their electrons Consider 2 luorine combining through e- transfer p 10 n 9 p 10 n 9 p 10 n 9 p 10 n The + is very unstable and will not sacrifice its e- to satisfy the other. When electrons are shred by two nonmetals (fluorine), each atom is able to obtain an octet. The preferred combination is through e - sharing 9 p 10 n 9 p 10 n 9 p 10 n 2 9 p 10 n Covalent Bonds and Lewis Structure

6 Covalent Molecules with multiple bonds Consider two oxygen atoms. Eact oxygen atom has six valence electrons and requires two more valence electrons each to possess eight electrons in its outer most orbital (ctet Rule). By sharing two electrons a piece four electrons are shared between the two oxygen atom. This results in a double bond and each oxygen completing the octet. 8 p 8 n Consider 2 oxygen atoms 8 p 8 n 8 p 8 n 2 8p 8n N 7 p 7n Consider 2 Nitrogen atoms N N 2 7p 7 n 7 p 7 n 7p 7 n N N The same strategy can be applied to two nitrogen atoms. Three electrons from each nitrogen atom are shared between the two nitrogen atoms resulting in a total of six valence electrons being shared forming a triple bond between the nitrogen atoms Covalent Bonds and Lewis Structure

7 Lewis Dot ormulas of Molecules All elements in the periodic table possesses valence electron which can be represented by dots. The representation is known as "Lewis Dot Symbol. The Lewis Dot symbol for the selected elements are shown below. The valence electrons for each of these elements correspond to the group number above each family. VALENCE ELECTRNS Covalent Bonds and Lewis Structure

8 Valence electrons: Number of bonds Non-metallic elements tend to form covalent bonds. Covalent compounds are formed by nonmetallic elements. In general there is a pattern on how many bonds each element from of family will possess based on its valence electrons. The table below summarizes the number of covalent bonds from certain elements. In general, these are the number of bonds formed by these atoms Covalent Bonds and Lewis Structure

9 Using the ctet Rule, molecular Lewis structure can be derived. Molecular Lewis structures are chemical representation showing how valence electrons are arranged in a chemical substance. When compounds are formed they tend to follow the ctet Rule (R). ctet Rule: Atoms will share e - until it is surrounded by eight valence electrons. Rules of the game- i) ctet Rule works mostly for second period elements. ii) Lewis Structure...according to the ctet Rule Many exceptions especially with 3rd period elements (d-orbitals) prefers 2 e - (electron deficient) iii) :C: N: :: :: 4 u.p 3u.p 2u.p. 1u.p. up = unpaired e- 4 bonds 3 bonds 2 bonds 1 bond =C= N N = - iv) & are terminal in the structural formula (Never central) Covalent Bonds and Lewis Structure

10 Lewis Structure by Periodic Group Examples of Lewis Structures according to Periodic Group Compound Total Valence Electrons Draw Single Bonds Calculate Number of Electrons Remaining a) = 8-6 b) N = 10 N-N 8 Use Remaining Electrons to Achieve Noble Gas Configuration N N Check Number of Electrons, 2, 8 N,8 c) N (1) = 8 2 N N, 2 N, 8 d) C (1) = 8 C 0 C, 2 C, 8 e) C (7) = 32 C 24 f) N = 10 N- 8 C N +, 8 C, 8 N, 8, Covalent Bonds and Lewis Structure

11 Covalent Compound: Lewis Structure Consider the following chemicals- C N ormaldehyde Difluoromoethyne Nitroglycerine Covalent Bonds and Lewis Structure

12 Covalent Compound: Lewis Structure Consider the following chemicals- ormaldehyde Difluoromoethyne Nitroglycerine Covalent Bonds and Lewis Structure

13 .. Lewis Structure dot structure Consider the following carbon dioxide: C C C via inspection Elements/Atomic sequence.... C.... Lewis dot symbol Applying ctet rule Carbon dioxide Lewis Structure Covalent Bonds and Lewis Structure

14 Lewis Structure by Bond Determination 1. (Connectivity) rom the Chemical ormula, determine the atom connectivity for the structure. i. Given a chemical formula, AB n, A is the central atom and B flanks the A atom. i.e., N 3, NCl 3, N 2. In these examples, N is central in the structure. ii. and are never central atoms. 2. (# of Bond) Determine the number of bonds in the compound, by calculating the theoretical ctet electrons (e) minus the total valence electrons (TVe). e is the theoretical number of electrons necessary for each atom in the structure to obtain a Noble Gas electron configuration, while TVe is the actual number of total valence electron for each atom in the structure. 3. (Remaining e-) Calculate the number of remaining electrons in the compound by taking the total valence electron (TVe) minus the number of electrons that was used to form bonds. Remaining e- divide by 2 = lone pairs. Complete Lewis structure by drawing atomic connectivity. Write bonds in the structure and the place remaining electrons to selected atoms in the structure to give each atom an octet. Keep in mind that the -atom is satisfied with 2 electrons (duet) Covalent Bonds and Lewis Structure

15 Lewis Dot Structure of C 2 by Bonds Table A. Calculate ctet electrons (e-) and Total Valence electrons to determine number of bonds C 2 e TVe 1 C 1 (8)= 8 1 (4) = (8)= 16 2 (6)= 12 Chg B. Calculate the number of bonds in compound structure. # bonds = (e - TVe) 2 = (24-16) = 8 = 4 bonds 2 2 C. Calculate the remaining electrons to add to structure to complete Lewis dot structure. Remaining e- = TVe - e - used in bonding. = 16-8 = 8 e - Remaining Writing Lewis Structure: irst determine atom connectivity keeping in mind that and can never be central atoms. Generally when given the formula, ABn, A is the central atom in the structure (but not always), and B atoms flank the central atom. Next use information from the above calculations. Total of 16e- in C 2, of which 8 electrons are used to form 4 bonds and 8 remaining electrons are used to complete Lewis structure. C 1. Write atom connectivity for C 2. C 2. Draw the four bonds in the structure C 3. Place the remaining 8 electrons in the structure to complete the Lewis Structure Covalent Bonds and Lewis Structure

16 Lewis Dot Structure of S 2 by Bonds Table A. Calculate (e-) and (TVe) S 2 e Tve 1 S 1 (8)= 8 1 (6) = (8)= 16 2 (6)= Writing Lewis Structure: 1. Write atom connectivity for S s. 2. Draw the four bonds in the structure. 3. Place the remaining 12 electrons in the structure such that each atom has an octet to complete the Lewis Structure Covalent Bonds and Lewis Structure

17 Lewis Dot Structure of S 2 by Bonds Table A. Calculate (e-) and (TVe) S 2 e TV e- 1 S 1 (8)= 8 1 (6) = (8)= 16 2 (6)= B. Number of Bonds. Bonding # bonds Electrons = (24-18) (24- = 18) 6 = = 3 bonds C. Remaining electrons. Remaining e- = 18-6 = 12 e - Remaining D. Lone Pairs. Remaining e- / 2 = 12/2 = 6 LP Writing Lewis Structure: 1. Write atom connectivity for S s. 2. Draw the three bonds in the structure. 3. Place the remaining 12 electrons in the structure such that each atom has an octet to complete the Lewis Structure S S S Covalent Bonds and Lewis Structure

18 Lewis Dot Structure of Cl 4 - by Bonds Table A. Calculate (e-) and (TVe) Cl - 4 e TV e 1 Cl 1 (8)= 8 1 (7) = (8)= 32 4 (6)= 24 Chg B. Number of Bonds. # bonds = (e- TVe) = # bonds 2 # bonds = (40-32) = 8 = 4 bonds 2 2 C. Remaining electrons. Remaining e- = 32-8 = 24 e - Remaining Writing Lewis Structure: 1. Write atom connectivity for Cl Draw the four bonds in the structure. 3. Place the remaining 24 electrons in the structure such that each atom has an octet to complete the Lewis Structure Cl Cl Cl Covalent Bonds and Lewis Structure

19 Lewis Dot Structure of P 3-3 by Bonds Table A. Calculate (e-) and (TVe) P -3 3 e T Ve 1 P 1 (8)= 8 1 (5) = (8)= 24 3 (6)= 18 Chg B. Number of Bonds. # bonds = (e- TVe) = # bonds 2 # bonds = (32-26) = 6 = 3 bonds 2 2 C. Remaining electrons. Remaining e- = 26-6 = 20 e - Remaining Writing Lewis Structure: 1. Write atom connectivity for P 3-3. P 2. Draw the three bonds in the structure. P 3. Place the remaining 20 electrons in the structure such that each atom has an octet to complete the Lewis Structure Cl P Covalent Bonds and Lewis Structure

20 : : Lewis Dot Structure of 2 P 3 - by Bonds Table A. Calculate (e-) and (TVe) 2 P - 3 e Tve 1 P 1 (8)= 8 1 (5) = (8)= 24 3 (6)= (2)= 4 2 (1)= 2 Chg B. Number of Bonds. # bonds = (e- TVe) = # bonds 2 # bonds = (36-26) = 10 = 5 bonds 2 2 C. Remaining electrons. Remaining e- = = 16 e - Remaining Writing Lewis Structure: 1. Write atom connectivity for 2 P Draw the five bonds in the structure. 3. Place the remaining 16 electrons in the structure such that each atom has an octet to complete the Lewis Structure P P P Covalent Bonds and Lewis Structure

21 Lewis Structures: Examples Example i) C 2 Cl Cl ii) Si 2 C Si N iii) N 3 iv) S 2-4 S 2- a) Linear b) bent (trigonal) c) trigonal d) tetrahedral e) pyramidal f) bent (tetrahedral) Covalent Bonds and Lewis Structure

22 Lewis Structures: Examples Example i) C 2 Cl Cl ii) Si 2 C Si N iii) N 3 iv) S 2-4 S 2- a) Linear b) bent (trigonal) c) trigonal d) tetrahedral e) pyramidal f) bent (tetrahedral) Covalent Bonds and Lewis Structure

23 Exception to ctet Rule There are generally three type of exception to the ctet Rule 1. dd e - : Compounds with odd number of electrons (generally considered radicals which tend to be very reactive) N N 2. e - deficient : Compounds with a central atom with less than eight electrons in its valence shell. (Usually the central atom is B or Be) B 3. Valence shell expansion: Compounds with a central atom with more than eight electrons in its valence shell. (Usually the central atom is in the third, fourth, fifth etc period) P Covalent Bonds and Lewis Structure

24 Exception: Valence shell expansion Some atoms can accommodate more than an octet especially if the central atom is from the 3rd, 4th,... period. i.e., PCl 6, S 4, As 6 - Modification of Bond Table: Determine the total valence electron. Determine the minimum # of bonds. Determine remaining electrons Re = (TVe - ) - (# e - in bonding) Assign remaining electrons to outer most atoms to satisfy octet rule (Except -atom). Assign all remaining electrons to central atom Covalent Bonds and Lewis Structure

25 Summary Compounds, elements coming together: i) electrons are shared between elements if there is mutually sharing, covalent compounds forms if there is unequal sharing, polar covalent compounds forms. ii) electron transfer occurs, ionic compounds forms (next section). Lewis Structure Determination: i) Molecular ormula ii) Atomic Sequence ( and are terminal) iii) Determine the # of bonds e - and TVe - # of Bonds = (e - TVe - ) / 2 iv) Determine remaining electrons Re = (TVe - ) - (# e - in bonding) v) Make sure all atoms satisfy octet rule (Except which is satisfied with 2 electrons) Covalent Bonds and Lewis Structure