Lewis Dot Structures and Chemical Bonding PDF

Lewis Dot Structures and Chemical BONDING Section 3. Lewis Dot Structures 2  An Atom Refresher An atom has three parts: Proton = positive Neutron = no charge Electron = negative The proton & neutron are found in the center of the atom, a plac...

Lewis Dot Structures and Chemical BONDING Section 3. Lewis Dot Structures 2  An Atom Refresher An atom has three parts: Proton = positive Neutron = no charge Electron = negative The proton & neutron are found in the center of the atom, a place called the nucleus. The electrons orbit the nucleus. Copyright © Houghton Mifflin Company.All rights reserved. Presentation of Lecture Outlines, 9–4  Atom Models There are two models of the atoms we will be using in class. Bohr Model Lewis Dot Structure  Bohr Model The Bohr Model shows all of the particles in the atom. In the center are circles. Each circle represents a single neutron - or proton. Protons should have a plus or “p” written on them. Neutrons should be blank or have + an “n.” + In a circle around the nucleus are - the electrons. Electrons should have a minus sign or an e.  Rules for Electrons You can’t just shove all of the electrons into the first orbit of an electron. Electrons exist in shells or energy levels. – Also called orbitals Only so many can be in any certain shell. The electrons in the outer most shell of any element are called valance electrons. Nucleus 1st shell 2nd shell 3rd shell 8  Electron Shells Only two electrons (e-) will fit in the first shell. Only eight electrons will fit in the second and third shells.  Rules for Drawing an Atom: 1. Determine the number of protons 3 (atomic number) 2. Determine the number of Li neutrons (Atomic mass – atomic Lithium number) 7 3. Determine the number of electrons (atomic number)  So let’s try it…. Protons = 3 - 3 + Li + + - Lithium - 7 Electrons = 3 Atomic Mass = 7 2 in the 1 shell, 1 in the st 2nd shell Neutrons = 4 (7-3=4)  Stable Atoms (Magic Number 8!) Atoms with a full outer shell (8 e-) are stable – They don’t react or bond with any other element Ex: He = 2 valence electrons in the first shell Ne = 8 valence electrons in the second shell Ar = 8 valence electrons in the third shell These are the Noble Gasses! Created by G.Baker www.thesciencequeen.net Created by G.Baker www.thesciencequeen.net  Know how to determine the valence electron for all elements. Definition: Valence Electrons An electron in an outer shell of an atom that can participate in forming chemical bonds with other atoms. Electrons directly involved in Example: forming bonds to form compounds, H has 1 valence electron F has 7 valence electrons Carbon has 4 valence electrons  Bond Polarity Ionic and covalent bonding is not black and white Sharing is not usually equal – It’s an electron tug of war Must look at electronegativity to determine how equally electrons are shared  What Electronegativity Means? Is a measure of tendency of an atom to attract electrons, the higher its value, the higher its tendency to attract electrons. - Metals have low electronegativity and non-metal have high electronegativity. This property plays an important role in forming compounds Copyright © Houghton Mifflin Company.All rights reserved. Presentation of Lecture Outlines, 9–17  Electronegativity The ability of an atom in a molecule to attract electrons to itself Developed by Linus Pauling – Gave values for all elements based on thermochemical data Linus Carl Pauling (1901-1994)  Electronegativity: On the periodic chart, electronegativity increases as you go… – …from left to right across a row. – …from the bottom to the top of a column.  Electronegativities of the Elements Cs (EN = 0.7) is least F with EN = 4.0 is electronegative most electronegative element element Au is at the peak of an island of electronegativity, and is most electronegative metal  How About Ionization Energy? Is the needed energy to pull or remove one or more electron/s from a neutral atom. The lower the ionization energy the easier it is to remove its valence electrons. Copyright © Houghton Mifflin Company.All rights reserved. Presentation of Lecture Outlines, 9–21  Lewis Dot Structure Bohr Model The Lewis Dot - - - + Structure is a bit - + + + - different from the + + + + Bohr model. - - - It only shows the element symbol and O it’s valence electrons. Lewis Dot Diagram  Lewis Structures A molecular formula shows the number and identity of all of the atoms in a compound, but not which atoms are bonded to each other. A Lewis structure shows the connectivity between atoms, as well as the location of all bonding and nonbonding valence electrons. 23  Drawing Lewis Diagrams for Molecules Step 1 Draw each element separately, showing the valence electrons. (Use different symbols for different atoms) Step 2 Choose the least occurring atom as the central atom (or the least electronegative). Except hydrogen or Oxygen. Step 3 Add next frequently occurring atoms around the central atom keeping symmetry in mind. You cannot put two of one atom next to each other Ie/ OOAl must be O Al O Step 4 Add all the electrons and then put Hydrogen on LAST,5oxygen Step Never atoms leave anonunpaired 2nd to last. electron Step 6 Circle your 8’s starting with the central atom Group 1 = 2e- H; Group 2 = 2e- or 4e- :Mg ;Mg;  (Connectivity) From the Chemical Formula, determine the atom connectivity for the structure. Find the central atom. Examples: i. Given a chemical formula, ABn, A is the central atom and B flanks the A atom. i.e., NH3, NCl3, NO2. In these examples, N is central in the structure. H H N ii. H and F Hare never central atoms. Valence electrons and number of bonds (# of Bond) Determine the number of bonds in the compound Recall the number of bonds at atom prefers depending on the number of valence electrons Fam i ly # Covalent Bonds* Halogens F, Br, C l, I X 1 bond often Calcogens O, S O 2 bond often N itrogen N, P N 3 bond often Carbon C , Si C 4 bond always  Lewis Structure, Octet Rule Guidelines When compounds are formed they tend to follow the Octet Rule. –Octet Rule: Atoms will share e- until it is surrounded by eight valence electrons. Rules of the game- –i) O.R. works mostly for second period elements. Many exceptions especially with 3rd period elements (d-orbitals) –ii) H prefers 2 e- (electron deficient) –iii) :C: N: :O: :F: 4u.p 3u.p 2u.p. 1u.p. up = unpaired e- 4 bonds 3 bonds 2 bonds 1 bond – O=C=O NN O=O F-F –iv) H & F are terminal in the structural formula (Never central)  When compounds are formed they tend to follow the Octet Rule. –Octet Rule: An atom will gain or loose e-(s) until it is surrounded by eight valence electrons. (Seek a full octet) Rules of the game- –i) H prefers 2 e- (electron deficient) –i) :C: N: :O: :F: 4 u.p 3u.p 2u.p. 1u.p. up = unpaired e- 4 bonds 3 bonds 2 bonds 1 bond – O=C=O NN O=O F-F –iii) H & F are terminal in the structural formula (Never central)  Atomic Connectivity (Bond Capabilities) The atomic arrangement for a molecule is usually given. CH2ClF HNO3 CH3COOH H2Se H2SO4 O3 In general when there is a single central atom in the molecule, CH2ClF, SeCl2, O3 (CO2, NH3, PO43-), the central atom is the first atom in the chemical formula. Except when the first atom in the chemical formula is Hydrogen (H) or fluorine (F). In which case the central atom is the second atom in the chemical formula. Find the central atom for the following: 1) H2O a) H b) O 2) PCl3 a) P b) Cl 3) SO3 a) S b) O 4) CO32- a) C b) O 5) BeH2 a) Be b) H 6) IO3- a) I b) O How to Write a Lewis Dot Diagram: Try Lithium: Li 1.How many valence electrons? 2.Write the symbol. 3.Start on the right. 4.Go clockwise. 5.Fill in up to two dots on a side.  How to… Try Chlorine: Cl 1.How many valence electrons? 2.Write the symbol. 3.Start on the right. 4.Go clockwise. 5.Fill in up to two dots on a side. Copyright © Houghton Mifflin Company.All rights reserved. Presentation of Lecture Outlines, 9–32  Writing Lewis Dot Formulas The Lewis electron-dot formula of a covalent compound is a simple two- dimensional representation of the positions of electrons in a molecule. – Bonding electron pairs are indicated by either two dots or a dash. – In addition, these formulas show the positions of lone pairs of electrons. Copyright © Houghton Mifflin Company.All rights reserved. Presentation of Lecture Outlines, 9–33  Writing Lewis Dot Formulas The following rules allow you to write electron-dot formulas even when the central atom does not follow the octet rule. – To illustrate, we will draw the structure of PCl3, phosphorus trichloride. PCl 3 Copyright © Houghton Mifflin Company.All rights reserved. Presentation of Lecture Outlines, 9–34  Writing Lewis Dot Formulas Step 1: Total all valence electrons in the molecular formula. That is, total the group numbers of all the atoms in the formula. PCl 3 26 e- total 5 e- (7 e-) x 3 – For a polyatomic anion, add the number of negative charges to this total. – For a polyatomic cation, subtract the number of positive charges from this total. Copyright © Houghton Mifflin Company.All rights reserved. Presentation of Lecture Outlines, 9–35  Writing Lewis Dot Formulas Step 2: Arrange the atoms radially, with the least electronegative atom in the center. Place one pair of electrons between the central atom and each peripheral atom. Cl Cl P Cl Copyright © Houghton Mifflin Company.All rights reserved. Presentation of Lecture Outlines, 9–36  Writing Lewis Dot Formulas Step 3: Distribute the remaining electrons to the peripheral atoms to satisfy the octet rule. :Cl: :Cl : : : P :Cl : : Copyright © Houghton Mifflin Company.All rights reserved. Presentation of Lecture Outlines, 9–37  Writing Lewis Dot Formulas Step 4: Distribute any remaining electrons to the central atom. If there are fewer than eight electrons on the central atom, a multiple bond may be necessary. :Cl: :Cl : : : : P :Cl : : Copyright © Houghton Mifflin Company.All rights reserved. Presentation of Lecture Outlines, 9–38 Copyright © Houghton Mifflin Company.All rights reserved. Presentation of Lecture Outlines, 9–39 Copyright © Houghton Mifflin Company.All rights reserved. Presentation of Lecture Outlines, 9–40  Lewis Dot Structure of CO2 by Bonds Table A. Calculate Octet electrons and Total Valence electrons to determine number of bonds B. Calculate the number of bonds in compound structure. = (24- 16) = 8 = 4 bonds 2 2 C. Calculate the remaining electrons to add to structure to complete Lewis dot structure. 16 - 8 = 8 e-Remaining O C O O C O O C O #3. Place the remaining 8 electrons in #1 Write atom #2. Draw the four bonds the structure to complete the Lewis connectivity for CO2. in the structure. Structure  Lewis Dot Structure of ClO4- by Bonds Table A. Calculate B. Number of Bonds. # bonds = (40- 32) = 8 = 4 bonds 2 2 C. Remaining electrons. 32 - 8 = 24 e-Remaining Writing Lewis Structure: #3. Place the remaining 24 #2. Draw the four electrons in the structure such that #1. Write atom bonds in the each atom has an octet to complete connectivity for ClO4-. structure. the Lewis Structure O O O Cl O O Cl O O O  Lewis Structures: Recitation a. Draw the Electron dot structure. b. Determine the remaining electron. c. Determine the number of bonds. a) CH2ClF b) SO2 c)SO4 d) H3PO4  Exercises a. Draw the Electron dot structure. b. Determine the remaining electron. c. Determine the number of bonds. 1. H2S 8. CO3-2 2. SO3 9. CH2F2 3. CH2Br2 10.phosphite ion, PO3-3 4. HCN 11. SCl2 5. NH4+ 12. COCl2 6. NO3- 7. PO4-3  Lewis Structures Multiple Bonds One lone pair of e− can be converted into one bonding pair of e− for each 2 e− needed to complete an octet on a Lewis Structure. A double bond contains four electrons in two 2 e− bonds. O O A triple bond contains six electrons in three 2 e− bonds. N N 45  Lewis Structures Multiple Bonds Example Draw the Lewis Structure for C2H4. Step Arrange the atoms. H C C H H H Step Count the valence e−. 2 C x 4 e− = 8 e− 4 H x 1 e− = 4 e− 12 e− total 46  Lewis Structures Multiple Bonds Step Add the bonds and lone pairs. 5 bonds x 2 e− = 10 e− H C C H + 1 lone pair x 2 e− = 2 e− H H 12 e− C still does not All valence e− have have an octet. been used. 47  Lewis Structures Multiple Bonds Step Change one lone pair into one bonding pair of e–, forming a double bond. H C C H H C C H H H H H Answer Each C now has an octet. 48  Delocalized Bonding: Resonance According to theory, one pair of bonding electrons is spread over the region of all three atoms. O O O – This is called delocalized bonding, in which a bonding pair of electrons is spread over a number of atoms. Copyright © Houghton Mifflin Company.All rights reserved. Presentation of Lecture Outlines, 9–49  Delocalized Bonding: Resonance According to the resonance description, you describe the electron structure of molecules with delocalized bonding by drawing all of the possible electron-dot formulas. : O : O and :O : : O: : : O : : : : O – These are called the resonance formulas of the molecule. Copyright © Houghton Mifflin Company.All rights reserved. Presentation of Lecture Outlines, 9–50  Exceptions to the Octet Rule Although many molecules obey the octet rule, there are exceptions where the central atom has more than eight electrons. – Generally, if a nonmetal is in the third period or greater it can accommodate as many as twelve electrons, if it is the central atom. – These elements have unfilled “d” subshells that can be used for bonding. Copyright © Houghton Mifflin Company.All rights reserved. Presentation of Lecture Outlines, 9–51  Exceptions to the Octet Rule Most of the common elements generally follow the octet rule. H is a notable exception, because it needs only 2 e− in bonding. Elements in group 3A do not have enough valence e− to form an octet in a neutral molecule. F F B F only 6 e− on B 52  Exceptions to the Octet Rule Elements in the third row have empty d orbitals available to accept electrons. Thus, elements such as P and S may have more than 8 e− around them. O O HO P OH HO S OH OH O 10 e− on P 12 e− on S 53  Exceptions to the Octet Rule For example, the bonding in phosphorus pentafluoride, PF5, shows ten electrons surrounding the phosphorus. : :F: : F: : : : :F P F: : :F: : : Copyright © Houghton Mifflin Company.All rights reserved. Presentation of Lecture Outlines, 9–54  Exceptions to the Octet Rule In xenon tetrafluoride, XeF4, the xenon atom must accommodate two extra lone pairs. : : :F : F: : : :F Xe : : F: : : : Copyright © Houghton Mifflin Company.All rights reserved. Presentation of Lecture Outlines, 9–55  Formal Charge and Lewis Structures In certain instances, more than one feasible Lewis structure can be illustrated for a molecule. For example, H C N: or H N C: – The concept of “formal charge” can help discern which structure is the most likely. Copyright © Houghton Mifflin Company.All rights reserved. Presentation of Lecture Outlines, 9–56  Formal Charge and Lewis Structures The formal charge of an atom is determined by subtracting the number of electrons in its “domain” from its group number. group 1 e- 4 e- 5 e- 1 e- 4 e- 5 e- number H C N: or H N C: “domain” I IV V I V IV electrons – The number of electrons in an atom’s “domain” is determined by counting one electron for each bond and two electrons for each lone pair. Copyright © Houghton Mifflin Company.All rights reserved. Presentation of Lecture Outlines, 9–57  Formal Charge and Lewis Structures The most likely structure is the one with the least number of atoms carrying formal charge. If they have the same number of atoms carrying formal charge, choose the structure with the negative formal charge on the more electronegative atom. formal H C or N: H N C: charge 0 0 0 0 +1 -1 – In this case, the structure on the left is most likely correct. Copyright © Houghton Mifflin Company.All rights reserved. Presentation of Lecture Outlines, 9–58  -END- Copyright © Houghton Mifflin Company.All rights reserved. Presentation of Lecture Outlines, 9–59  THANK YOU! Copyright © Houghton Mifflin Company.All rights reserved. Presentation of Lecture Outlines, 9–60  UP NEXT…. Section 2. Ionic, Covalent and Metallic Bonding 61