Section 9 Chemical Bonding Lewis Structures Material was

Section 9 Chemical Bonding – Lewis Structures Material was developed by combining Janusa’s material with the lecture outline provided with Ebbing, D. D. ; Gammon, S. D. General Chemistry, 8 th ed. , Houghton Mifflin, New York, NY, 2005. Majority of figures/tables are from the Ebbing lecture outline.

Lewis Electron-Dot Symbols • A Lewis electron-dot symbol is a symbol in which the electrons in the valence shell of an atom or ion are represented by dots placed around the letter symbol of the element. . P. : S. : Cl. : Ar: Na. . Mg. . Al. . Si. . . Group II Group IV Group VIII : : : : . Group III : : Group I – Note that the group number indicates the number of valence electrons. 2 Material was developed by combining Janusa’s material with the lecture outline provided with Ebbing, D. D. ; Gammon, S. D. General Chemistry, 8 th ed. , Houghton Mifflin, New York, NY, 2005. Majority of figures/tables are from the Ebbing lecture outline.

Covalent Bonds • The tendency of atoms in a molecule to have eight electrons in their outer shell (two for hydrogen) is called the octet rule. 3 Material was developed by combining Janusa’s material with the lecture outline provided with Ebbing, D. D. ; Gammon, S. D. General Chemistry, 8 th ed. , Houghton Mifflin, New York, NY, 2005. Majority of figures/tables are from the Ebbing lecture outline.

Lewis Electron-Dot Formulas • A Lewis electron-dot formula is an illustration used to represent the transfer of electrons during the formation of an ionic bond. – As an example, let’s look at the transfer of electrons from magnesium to fluorine to form magnesium fluoride. 4 Material was developed by combining Janusa’s material with the lecture outline provided with Ebbing, D. D. ; Gammon, S. D. General Chemistry, 8 th ed. , Houghton Mifflin, New York, NY, 2005. Majority of figures/tables are from the Ebbing lecture outline.

Lewis Electron-Dot Formulas • The magnesium has two electrons to give, whereas the fluorines have only one “vacancy” each. 2+ Mg [: F: ] : : [: F: ] - . F: : : : F. . Mg. - – Consequently, magnesium can accommodate two fluorine atoms. HW 64 Material was developed by combining Janusa’s material with the lecture outline provided with Ebbing, D. D. ; Gammon, S. D. General Chemistry, 8 th ed. , Houghton Mifflin, New York, NY, 2005. Majority of figures/tables are from the Ebbing lecture outline. 5

Electron Configurations of Ions • As metals lose electrons to form cations and establish a “noble gas” configuration, the electrons are lost from the valence shell first. – For example, magnesium generally loses two electrons from its 3 s subshell to look like neon. [Ne]3 s 2 [Ne] 6 Material was developed by combining Janusa’s material with the lecture outline provided with Ebbing, D. D. ; Gammon, S. D. General Chemistry, 8 th ed. , Houghton Mifflin, New York, NY, 2005. Majority of figures/tables are from the Ebbing lecture outline.

Electron Configurations of Ions • Transition metals also lose electrons from the valence shell first, which is not the last subshell to fill according to the aufbau sequence. – For example, zinc generally loses two electrons from its 4 s subshell to adopt a “pseudo”-noble gas configuration. [Ar]4 s 23 d 10 [Ar]3 d 10 Material was developed by combining Janusa’s material with the lecture outline provided with Ebbing, D. D. ; Gammon, S. D. General Chemistry, 8 th ed. , Houghton Mifflin, New York, NY, 2005. Majority of figures/tables are from the Ebbing lecture outline. 7

Covalent Bonds • When two nonmetals bond, they often share electrons since they have similar attractions for them. This sharing of valence electrons is called the covalent bond. – These atoms will share sufficient numbers of electrons in order to achieve a noble gas electron configuration (that is, eight valence electrons). 8 Material was developed by combining Janusa’s material with the lecture outline provided with Ebbing, D. D. ; Gammon, S. D. General Chemistry, 8 th ed. , Houghton Mifflin, New York, NY, 2005. Majority of figures/tables are from the Ebbing lecture outline.

Lewis Structures • You can represent the formation of the covalent bond in H 2 as follows: H . +. H : H H – This uses the Lewis dot symbols for the hydrogen atom and represents the covalent bond by a pair of dots. 9 Material was developed by combining Janusa’s material with the lecture outline provided with Ebbing, D. D. ; Gammon, S. D. General Chemistry, 8 th ed. , Houghton Mifflin, New York, NY, 2005. Majority of figures/tables are from the Ebbing lecture outline.

Lewis Structures • The shared electrons in H 2 spend part of the time in the region around each atom. : H H – In this sense, each atom in H 2 has a helium configuration. 10 Material was developed by combining Janusa’s material with the lecture outline provided with Ebbing, D. D. ; Gammon, S. D. General Chemistry, 8 th ed. , Houghton Mifflin, New York, NY, 2005. Majority of figures/tables are from the Ebbing lecture outline.

Lewis Structures • The formation of a bond between H and Cl to give an HCl molecule can be represented in a similar way. : : . +. Cl: : : H H Cl – Thus, hydrogen has two valence electrons about it (as in He) and Cl has eight valence electrons about it (as in Ar). 11 Material was developed by combining Janusa’s material with the lecture outline provided with Ebbing, D. D. ; Gammon, S. D. General Chemistry, 8 th ed. , Houghton Mifflin, New York, NY, 2005. Majority of figures/tables are from the Ebbing lecture outline.

Lewis Structures • Formulas such as these are referred to as Lewis electron-dot formulas or bonding pair Lewis structures. : : H Cl lone pair – An electron pair is either a bonding pair (shared between two atoms) or a lone pair (an electron pair that is not shared). 12 Material was developed by combining Janusa’s material with the lecture outline provided with Ebbing, D. D. ; Gammon, S. D. General Chemistry, 8 th ed. , Houghton Mifflin, New York, NY, 2005. Majority of figures/tables are from the Ebbing lecture outline.

Coordinate Covalent Bonds • When bonds form between atoms that both donate an electron, you have covalent bond: A . +. B : A B – It is, however, possible that both electrons are donated by one of the atoms. This is called a coordinate covalent bond. A + : B : A B Material was developed by combining Janusa’s material with the lecture outline provided with Ebbing, D. D. ; Gammon, S. D. General Chemistry, 8 th ed. , Houghton Mifflin, New York, NY, 2005. Majority of figures/tables are from the Ebbing lecture outline. 13

Multiple Bonds • In the molecules described so far, each of the bonds has been a single bond, that is, a covalent bond in which a single pair of electrons is shared. – It is possible to share more than one pair. A double bond involves the sharing of two pairs between atoms. : : H : : C : C H : H H or Material was developed by combining Janusa’s material with the lecture outline provided with Ebbing, D. D. ; Gammon, S. D. General Chemistry, 8 th ed. , Houghton Mifflin, New York, NY, 2005. Majority of figures/tables are from the Ebbing lecture outline. 14

Multiple Bonds • Triple bonds are covalent bonds in which three pairs of electrons are shared between atoms. C : : : : H H or 15 Material was developed by combining Janusa’s material with the lecture outline provided with Ebbing, D. D. ; Gammon, S. D. General Chemistry, 8 th ed. , Houghton Mifflin, New York, NY, 2005. Majority of figures/tables are from the Ebbing lecture outline.

Bond Length and Bond Order • The bond order, determined by the Lewis structure, is the number of pairs of electrons in a bond. – Bond length depends on bond order. – As the bond order increases, the bond gets shorter and stronger. C C C Bond length Bond energy Bond Order 154 pm C 346 k. J/mol 1 134 pm C 602 k. J/mol 2 120 pm C 835 k. J/mol 3 16 Material was developed by combining Janusa’s material with the lecture outline provided with Ebbing, D. D. ; Gammon, S. D. General Chemistry, 8 th ed. , Houghton Mifflin, New York, NY, 2005. Majority of figures/tables are from the Ebbing lecture outline.

Multiple Bonds • Why does multiple bond occur? Ex. N 2 17 Material was developed by combining Janusa’s material with the lecture outline provided with Ebbing, D. D. ; Gammon, S. D. General Chemistry, 8 th ed. , Houghton Mifflin, New York, NY, 2005. Majority of figures/tables are from the Ebbing lecture outline.

Polar Covalent Bonds • A polar covalent bond is one in which the bonding electrons spend more time near one of the two atoms involved. – When the atoms are alike, as in the H-H bond of H 2 , the bonding electrons are shared equally (a nonpolar covalent bond). – When the two atoms are of different elements, the bonding electrons don’t have to shared equally, resulting in a “polar” bond (i. e HCl). 18 Material was developed by combining Janusa’s material with the lecture outline provided with Ebbing, D. D. ; Gammon, S. D. General Chemistry, 8 th ed. , Houghton Mifflin, New York, NY, 2005. Majority of figures/tables are from the Ebbing lecture outline.

Polar Covalent Bonds • For example, the bond between carbon and oxygen in CO 2 is considered polar because the shared electrons spend more time orbiting the oxygen atoms. : : d- d+ d- – The result is a partial negative charge on the oxygens (denoted d-) and a partial positive charge on the carbon (denoted d+) 19 Material was developed by combining Janusa’s material with the lecture outline provided with Ebbing, D. D. ; Gammon, S. D. General Chemistry, 8 th ed. , Houghton Mifflin, New York, NY, 2005. Majority of figures/tables are from the Ebbing lecture outline.

Dipole Moment and Molecular Geometry • The dipole moment is a measure of the degree of charge separation in a molecule. – We can view the polarity of individual bonds within a molecule as vector quantities. – Thus, molecules that are perfectly symmetric have a zero dipole moment. These molecules are considered nonpolar. d+ d- d 20 Material was developed by combining Janusa’s material with the lecture outline provided with Ebbing, D. D. ; Gammon, S. D. General Chemistry, 8 th ed. , Houghton Mifflin, New York, NY, 2005. Majority of figures/tables are from the Ebbing lecture outline.

Dipole Moment and Molecular Geometry • However, molecules that exhibit any asymmetry in the arrangement of electron pairs would have a nonzero dipole moment. These molecules are considered polar. d- : d- H N H H d+ Material was developed by combining Janusa’s material with the lecture outline provided with Ebbing, D. D. ; Gammon, S. D. General Chemistry, 8 th ed. , Houghton Mifflin, New York, NY, 2005. Majority of figures/tables are from the Ebbing lecture outline. d+ 21

Polar Covalent Bonds • Electronegativity is a measure of the ability of an atom in a molecule to draw bonding electrons to itself. – In general, electronegativity increases from the lower-left corner to the upper-right corner of the periodic table. – The current electronegativity scale, developed by Linus Pauling, assigns a value of 4. 0 to fluorine and a value of 0. 7 to cesium. 22 Material was developed by combining Janusa’s material with the lecture outline provided with Ebbing, D. D. ; Gammon, S. D. General Chemistry, 8 th ed. , Houghton Mifflin, New York, NY, 2005. Majority of figures/tables are from the Ebbing lecture outline.

Electronegativities Generally, m-nm ionic DEN 1. 8 nm-nm covalent DEN < 1. 8 23 Material was developed by combining Janusa’s material with the lecture outline provided with Ebbing, D. D. ; Gammon, S. D. General Chemistry, 8 th ed. , Houghton Mifflin, New York, NY, 2005. Majority of figures/tables are from the Ebbing lecture outline.

Polar Covalent Bonds • The absolute value of the difference in electronegativity of two bonded atoms gives a rough measure of the polarity of the bond. – When this difference is small (less than 0. 5), the bond is nonpolar (i. e H-C). – When this difference is large (greater than 0. 5 but less than 1. 8), the bond is considered polar (i. e. H-Cl). – If the difference exceeds approximately 1. 8, sharing of electrons is no longer possible and the bond becomes ionic. – Generally, N, O, F, Cl, Br and another nonmetal – polar bond; other nonmetal-nonmetal combinations – nonpolar. 24 HW 65 Material was developed by combining Janusa’s material with the lecture outline provided with Ebbing, D. D. ; Gammon, S. D. General Chemistry, 8 th ed. , Houghton Mifflin, New York, NY, 2005. Majority of figures/tables are from the Ebbing lecture outline.

Writing Lewis Dot Formulas • The Lewis electron-dot formula of a covalent compound is a simple twodimensional 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 with dots. 25 Material was developed by combining Janusa’s material with the lecture outline provided with Ebbing, D. D. ; Gammon, S. D. General Chemistry, 8 th ed. , Houghton Mifflin, New York, NY, 2005. Majority of figures/tables are from the Ebbing lecture outline.

By following the rules below, the Lewis Dot Structure can be written for covalent compounds and polyatomic ions. 1. Count the total number of valence electrons; adjust for charge if an ion - add one electron for each negative charge, subtract one electron for each positive charge. 2. Draw skeleton of structure - typically least EN atom is the central atom (most obvious one most of the time, except H is never it – H always on outside). 3. Complete the octet around each atom surrounding the central atom (except H only needs 2 electrons, Be only needs 4 electrons, B and Al typically have 6 electrons for full “octet”). 26 Material was developed by combining Janusa’s material with the lecture outline provided with Ebbing, D. D. ; Gammon, S. D. General Chemistry, 8 th ed. , Houghton Mifflin, New York, NY, 2005. Majority of figures/tables are from the Ebbing lecture outline.

4. Count the number of electrons placed in step 3. 5. Compare the number of electrons placed to that available (subtract placed in #4 from available in #1 [step 1 available – step 4 placed]. a. ) equal – verify that the central atom has 8 electron around it (watch exceptions); i. ) If it does, then you’re finished; except could check formal charge (learn later) ii. ) If it doesn’t, this suggests that a multiple bond is present (2 e- fewer than an octet suggests a double bond; four fewer suggests a triple bond or two double bonds). To obtain a multiple bond, move one or two electron pairs (depending on whether the bond is to be double or triple) from a surrounding atom to the bond connecting the central atom. Typically multiple bonds are done with C, N, O, and S. b. ) positive number – verify that the central atom has 8 electron around it (watch exceptions); i. ) If it doesn’t, then add electrons to the central atom to obtain an octet and account for these in your comparison. ii. ) If you have an octet around the central atom and there are still electrons left over, place the extra electrons on central atom. Typically this occurs when d orbitals are available (n=3 or higher) which allows for more than an octet. Typically C – 4 bonds 27 H – 1 bond Material was developed by combining Janusa’s material with the lecture outline provided with Ebbing, D. D. ; Gammon, S. D. General Halogen – 1 bond, 3 lone pairs Chemistry, 8 th ed. , Houghton Mifflin, New York, NY, 2005. Majority of figures/tables are from the Ebbing lecture outline.

Writing Lewis Dot Formulas • HCl • H 2 O • NH 3 28 Material was developed by combining Janusa’s material with the lecture outline provided with Ebbing, D. D. ; Gammon, S. D. General Chemistry, 8 th ed. , Houghton Mifflin, New York, NY, 2005. Majority of figures/tables are from the Ebbing lecture outline.

Writing Lewis Dot Formulas • BF 3 • As. F 5 • COCl 2 29 Material was developed by combining Janusa’s material with the lecture outline provided with Ebbing, D. D. ; Gammon, S. D. General Chemistry, 8 th ed. , Houghton Mifflin, New York, NY, 2005. Majority of figures/tables are from the Ebbing lecture outline.

Writing Lewis Dot Formulas • CH 4 O • NO 3 - 30 Material was developed by combining Janusa’s material with the lecture outline provided with Ebbing, D. D. ; Gammon, S. D. General Chemistry, 8 th ed. , Houghton Mifflin, New York, NY, 2005. Majority of figures/tables are from the Ebbing lecture outline.

Delocalized Bonding: Resonance – Experiments show, however, that all bonds are identical. – This is called delocalized bonding, in which a bonding pair of electrons is spread over a number of atoms. – According to the resonance description, you describe the electron structure of molecules with delocalized bonding by drawing all of the possible electron-dot formulas. These are called the resonance formulas of the molecule. 31 Material was developed by combining Janusa’s material with the lecture outline provided with Ebbing, D. D. ; Gammon, S. D. General Chemistry, 8 th ed. , Houghton Mifflin, New York, NY, 2005. Majority of figures/tables are from the Ebbing lecture outline.

Writing Lewis Dot Formulas • NH 4+ • Xe. F 4 • HCN Material was developed by combining Janusa’s material with the lecture outline provided with Ebbing, D. D. ; Gammon, S. D. General Chemistry, 8 th ed. , Houghton Mifflin, New York, NY, 2005. Majority of figures/tables are from the Ebbing lecture outline. HW 66 32

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. 33 Material was developed by combining Janusa’s material with the lecture outline provided with Ebbing, D. D. ; Gammon, S. D. General Chemistry, 8 th ed. , Houghton Mifflin, New York, NY, 2005. Majority of figures/tables are from the Ebbing lecture outline.

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. • F. C. = group # (valence e-) - # bonds - # unbonded e– 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. 34 Material was developed by combining Janusa’s material with the lecture outline provided with Ebbing, D. D. ; Gammon, S. D. General Chemistry, 8 th ed. , Houghton Mifflin, New York, NY, 2005. Majority of figures/tables are from the Ebbing lecture outline.

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 charge or H C 0 0 N: 0 H 0 N C: +1 -1 – In this case, the structure on the left is most likely correct. Material was developed by combining Janusa’s material with the lecture outline provided with Ebbing, D. D. ; Gammon, S. D. General Chemistry, 8 th ed. , Houghton Mifflin, New York, NY, 2005. Majority of figures/tables are from the Ebbing lecture outline. 35

Bond Length and Bond Order • Bond length (or bond distance) is the distance between the nuclei in a bond. – Knowing the bond length in a molecule can sometimes give clues as to the type of bonding present. – Covalent radii are values assigned to atoms such that the sum of the radii of atoms “A” and “B” approximate the A-B bond length. 36 Material was developed by combining Janusa’s material with the lecture outline provided with Ebbing, D. D. ; Gammon, S. D. General Chemistry, 8 th ed. , Houghton Mifflin, New York, NY, 2005. Majority of figures/tables are from the Ebbing lecture outline.

Bond Length and Bond Order – For example, to predict the bond length of CCl, you add the covalent radii of the two atoms. C Cl Bond length Material was developed by combining Janusa’s material with the lecture outline provided with Ebbing, D. D. ; Gammon, S. D. General Chemistry, 8 th ed. , Houghton Mifflin, New York, NY, 2005. Majority of figures/tables are from the Ebbing lecture outline. 37

Bond Energy • We define the A-B bond energy (denoted BE) as the average enthalpy change for the breaking of an A-B bond in a molecule in its gas phase. – The enthalpy, DH, of a reaction is approximately equal to the sum of the bond energies of the reactants minus the sum of the bond energies of the products. broken formed 38 Material was developed by combining Janusa’s material with the lecture outline provided with Ebbing, D. D. ; Gammon, S. D. General Chemistry, 8 th ed. , Houghton Mifflin, New York, NY, 2005. Majority of figures/tables are from the Ebbing lecture outline.

Bond Energy • To illustrate, let’s estimate the DH for the following reaction. – In this reaction, one C-H bond and one Cl-Cl bond must be broken. – In turn, one C-Cl bond and one H-Cl bond are formed. Material was developed by combining Janusa’s material with the lecture outline provided with Ebbing, D. D. ; Gammon, S. D. General Chemistry, 8 th ed. , Houghton Mifflin, New York, NY, 2005. Majority of figures/tables are from the Ebbing lecture outline. 39

Bond Energy • Referring to tables for the bond energies, a little simple arithmetic yields DH. C-H 411 k. J/mol C-Cl 327 k. J/mol Cl-Cl 240 k. J/mol H-Cl 428 k. J/mol Material was developed by combining Janusa’s material with the lecture outline provided with Ebbing, D. D. ; Gammon, S. D. General Chemistry, 8 th ed. , Houghton Mifflin, New York, NY, 2005. Majority of figures/tables are from the Ebbing lecture outline. HW 67 40