Lecture 13. Professor Hicks Inorganic Chemistry (CHE151)

Transcription

1 Lecture 13 Professor icks Inorganic hemistry (E151) Lewis symbols aka Lewis Structures of atoms Visual representation of electron configuration 1 electron = dot 2 paired electrons = line ucleus + core electrons = element symbol Lewis symbol notation Electron onfiguration = 1s 2 2s 2 2p 2 4 valence electrons = 2 valence electrons or Valence electron = Element symbol = nucleus + core electrons All three represent a carbon atom G. Lewis 1

2 Main group atoms ions ctet rule Lewis symbols of three atoms all their ions all have the 8 valence e - s electron configuration [e] 2s 2 2p 6 8 = octet of valence electrons Gain/lose electrons noble gas electron configuration oble gases all have 8 valence electrons Ions of main group elements have an octet of valence electrons obey ctet Rule Lewis Structures Lewis symbols Atoms and monatomic ions - Atoms +/- electrons ions - ctet rule Lewis structures Molecules and polyatomic ions - Show bonding atom to atom (the skeletal structure of the molecule/ion) - Atoms molecules/polyatomic ions - Extend concept of ctet Rule from atoms forming monatomic ions 2

3 Lewis structures an be used to predict 1) If a compound / polyatomic ion will exist 2) Its solubility in oil or water (non-polar or polar liquids) 3) Three-dimensional shape 4) Intermolecular forces (tendency to stick to other molecules) Molecular compounds and polyatomic ions More numerous than ionic compounds Multiple compounds with same elements -, 2,, 2, 2, 2 4, - l -, l 2-, l 3-, l 4 - o crossing over rule to determine formulas If a valid Lewis structure can be drawn the compound/ion probably exists 3

4 Steps for Drawing Lewis Structures 1 alculate the total the number of valence electrons in the molecule or polyatomic ion. 2 Draw the skeletal structure for the molecule/polyatomic ion. 3 Give octets to all atoms in the skeletal structure (except hydrogen) by placing lone pairs on them. 4 ount the total number of valence electrons in this structure. 5 If the Lewis structure contains the correct number of valence electrons it is complete and has no resonance structures that obey the octet rule. 6 If the structure contains more valence electrons than the molecule or polyatomic ion has then reduce the number of valence electrons in the structure by erasing lone pairs on adjacent atoms and replacing them with a bonding pair between those atoms. Bonding in molecular compounds and polyatomic ions Based on attraction of nuclei for electrons Electrons act like glue to hold nuclei together z+ attraction e- attraction z+ e- nucleus nucleus repulsion 4

5 Two kinds of electron pairs Bonding electron pair on-bonding or Lone electron pair ydrogen Does not obey octet rule oble gas it becomes like is [e] = 1s 2 beys duet rule Like - ion nly 1 bond at end like a cap 5

6 ircle the octets on all the atoms that have octets. B ircle the octets on all the atoms that have octets. B 6

7 Draw Lewis structures that obey the ctet/duet rule for 2 2 S l 2 (skeletal structure --) Resonance structures More than one possible way to arrange electrons = resonance structures Atoms connected in the same way not different substances Molecules/ion is one structure that is a weighted average of all resonance structures Example: Draw the Lewis structure and resonance structures for 2. 7

8 Draw a Lewis structure and its resonance structures for 2 (skeletal structure --) Double ended arrows are placed between resonance structures Resonance structures are shown in brackets Isomers Different compounds with same formula Atoms arranged differently T resonance structures attached to attached to cyanic acid isocyanic acid same atoms arranged differently one structure cannot be changed into the other by moving electrons/double bonds 8

9 Draw Lewis structures and resonance structures for 3 S 2 (skeletal structure -S-) B 3 3- (skeletal structure has B in center) 3 2- (skeletal structure has in center) Isoelectronic structures Same arrangement of same # valence electrons on different atoms substitute elements in same group these three are isoelectronic S Se same # e- s has 1 less proton structure has negative charge these three are isoelectronic substitute elements from different groups - 9

10 ormal Valence The number of bonds an atom forms in order to achieve an octet is called its Valence The ormal Valence of an atom is the number of bonds it most often forms to obey the octet rule ormal valence is a periodic property ormal valence = ormal charge harge atoms would have if they got divorced (equitably) Each bond split 1 electron to each atom ormal charges add up to total charge If the octet rule is obeyed 1) Each bond over normal valence +1 in 2) Each bond under normal valence -1 in 10

11 Assigning formal charge atom = 0 with 5 valence e- s = +1 = +1 atom = 0 with 6 valence e- s = -1 = -1 1) Draw circles around each atom - split bonding pairs in half - give lone pairs to the atom they are on 2) Evaluate the charges of the circles as if they were Lewis symbols Write Lewis structures for these ions: (a) 2 2-, (b) 2 2-, (c) +, (d) 4+. Show formal charges. 11

12 ormal charge Resonance structures with lower formal charges dominate the observed structure S +1 0 S S Best structure b/c 1) Minimal charges and 2) harges consistent with electronegativity of atoms ormal charge Resonance structures with lower formal charges dominate the observed structure Best structure b/c 1) Minimal charges and 2) harges consistent with electronegativity of atoms 12

13 Exceptions to ctet rule Group three elements sometimes are stable with 6 electrons (sextet) Molecules with odd numbers of electrons cannot have octets - Some (especially neutral species) are stable enough to exist with less than an octet The Bl 3 molecule has an incomplete octet around B. Draw three resonance structures of the molecule in which the octet rule is satisfied for both the B and the l atoms. Also, draw one where the B has a sextet. Show formal charges on all structures. Based upon the formal charges which is the best structure? 13

14 Exceptions to the octet rule ctet = 8 = full s + p orbitals Elements with d orbitals can have more than octet 18 electrons = full s + p + d orbitals of same n value Many elements can lower s by exceeding octet More than 8 electrons = expanded octet ctet rule/expanded octets Rarely less than octet sometimes exceed octet Rarely less than octet never exceed octet 14

15 Draw a Lewis structure for 2 S 4 that obeys the octet rule and assign all formal charges. The skeletal structure has all bound to S and the s each bound to different s. Scramble these three maybe 9.43 The following Lewis structures for (a), (b) 2 2, (c) Sn 2, (d) B 3, (e), (f), and (g) 3 are incorrect. Explain what is wrong with each one and give a correct structure for the molecule. (Relative positions of atoms are shown correctly.) 15

16 Write three resonance structures for hydrazoic acid, 3. The atomic arrangement is. Show formal charges. Based upon the predict which resonance structure or structures dominates the observed structure. 16

17 Draw three reasonable resonance structures for the ion. Show formal charges. Write Lewis structures for Se 4 and Se 6. Is the octet rule satisfied for Se? 17

18 Draw Lewis Structures and any resonance structures for Draw Lewis Structures and any resonance structures for

19 Draw Lewis Structures and any resonance structures for 2 Draw Lewis Structures and any resonance structures for I 2 Bases upon this structure predict the charge of an ion formed from I 2. 19

20 Steps for Drawing Lewis Structures 1 alculate the total the number of valence electrons in the molecule or polyatomic ion. 2 Draw the skeletal structure for the molecule/polyatomic ion. 3 Give octets to all atoms in the skeletal structure (except hydrogen) by placing lone pairs on them. 4 ount the total number of valence electrons in this structure. 5 If the Lewis structure contains the correct number of valence electrons it is complete and has no resonance structures that obey the octet rule. 6 If the structure contains more valence electrons than the molecule or polyatomic ion has then reduce the number of valence electrons in the structure by erasing lone pairs on adjacent atoms and replacing them with a bonding pair between those atoms. 7 If the molecule or polyatomic ion contains less valence electrons then add electrons to the structure on any atom that can exceed the octet rule.