Chemical Bonds. Chemistry 100. Bettelheim, Brown, Campbell & Farrell. Introduction to General, Organic and Biochemistry Chapter 3

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1 hemistry 100 Bettelheim, Brown, ampbell & arrell Ninth Edition Introduction to General, rganic and Biochemistry hapter 3 hemical Bonds Electronic Structure & hemical Valence An atom that loses one or more electrons becomes a positively charged ion called a cation. Na Na+ + 1 e- 1s 2, 2s 2, 2p 6, 3s 1 1s 2, 2s 2, 2p 6 = [Ne]+ Mg Mg e- 1s 2, 2s 2, 2p 6, 3s 2 1s 2, 2s 2, 2p 6 = [Ne]2+ An atom that gains one or more electrons becomes a negatively charged ion called an anion. + 1 e- - 1s 2, 2s 2, 2p 5 1s 2, 2s 2, 2p 6 = [Ne]- The ctet Rule Some Limitations The octet rule gives us a good way to understand why Group 1A-7A elements form the ions they do; but it is not perfect. Ions of period 1 and 2 elements with charges greater than +2 are unstable. or example, boron does not lose its three valence electrons to become B 3+, nor does carbon lose its four valence electrons to become 4+. Below period 2, +3 ions are stable, e.g. Al 3+, Sc 3+, e 3+, etc. form. Ions of period 1 and 2 elements with charges greater than -3 are also unstable. or example, carbon does not gain four valence electrons to become 4-, and -3 ions only form in period 2 (N 3- ). The octet rule does not apply to Group 1B-7B (the transition elements), most of which form ions with two or more different positive charges. 1

2 orming hemical Bonds The Lewis Model Ionic Bond - An atom may lose or gain enough electrons to acquire a filled valence shell and become an ion. When two ions of opposite charges are brought together an ionic bond results from the force of attraction between the cation and anion. ovalent Bond - An atom may share electrons with one or more other atoms so as to acquire a filled valence shell. A covalent bond is the result of the force of attraction between two atoms that share one or more pairs of electrons. Electronegativity & hemical Bonds In forming an ionic compound the less electronegative element loses one or more electrons to the more electronegative element. The resulting ations and Anions form the ionic bonds. Li + Li + - 1s 2, 2s 1 1s 2, 2s 2, 2p 5 [e]+ [Ne]- In forming a covalent compound the less electronegative element shares one or more electrons with the more electronegative element. The resulting molecule is polar because the more electronegative element gets more than an equal share. + Naming Ionic ompounds In any ionic compound the total number of positive charges must equal the total number of negative charges, the formula must be neutral! lithium ion and fluoride ion form Li barium ion and bromide ion form BaBr 2 aluminum ion and oxide ion form Al 2 3 potassium ion and bicarbonate ion form K 3 sodium ion and phosphate ion form Na 3 P 4 The name does not indicate the number of each ion if the element forms only one kind of ion, e.g. aluminum oxide. 2

3 Naming Ionic ompounds If a metal atom forms more than one cation we must indicate which one in the name: Iron (III) chloride is el 3 Iron(II) chloride is el 2 Manganese(IV) oxide is Mn 2 Manganese(III) oxide is Mn 2 3 Manganese(II) oxide is Mn Manganese(VII) oxide is Mn 2 7 In every case the chemist must know the charges of ions which are constant. Naming Ionic ompounds or ionic compounds that contain polyatomic ions name the positive ion first followed by the name of the negative ion: N 4 N 3 is ammonium nitrate a 3 is calcium carbonate Na 2 P 4 is sodium dihydrogen phosphate Na is sodium hydroxide u 3 is copper(ii) carbonate e 2 ( 3 ) 3 is iron(iii) carbonate es 4 is iron(ii) sulfate Lewis Dot Electron onfigurations Lewis Dot onfigurations as shown below are more useful in putting together covalently bonded compounds. 1A 2A 3A 4A 5A 6A 7A 8A Li e Be B N Ne Na Mg Al Si P S l Ar 3

4 Drawing Lewis Dot Structures To create a Lewis dot structure, choose the element nearest the center of the periodic chart: in 2 2. Pair one electron in the other atoms to the carbon to create the four single bonds. ydrogen and fluorine form only one bond each, they must always be on the outer perimeter of the molecule. Drawing Lewis Dot Structures Draw the Structure of methanol: 3 arbon is nearest the center, xygen has two unpaired electrons, hydrogen has only one. onnect arbon and xygen, then add hydrogens. Always connect the higher valence elements before adding monovalent elements. Drawing Lewis Dot Structures Draw the structure of formaldehyde: 2 onnect the arbon and xygen, since there are only two hydrogens, connect a second pair of & electrons. a double bond When two atoms share two pairs of electrons a double bond is formed. Always connect the higher valent elements so that the remaining unpaired electrons match the number of monovalent elements. 4

5 Drawing Lewis Dot Structures Draw the structure of hydrogen cyanide: N onnect the arbon and Nitrogen so the remaining unpaired electrons match the number of hydrogens. N N a triple bond Draw the structure of acetylene: 2 2 onnect the two arbons so the remaining unpaired electrons match the number of hydrogens. a triple bond Drawing Lewis Dot Structures Elements beyond the second period can violate the octet rule and expand their octet. Write the structure of Phosphorus(V) luoride: P 5 P P We split the pair on phosphorus and bond fluorine atoms to each unpaired electron. The result is a compound with 10 electron pairs around phosphorus. Drawing Lewis Dot Structures Draw the structure of sulfuric acid: 2 S 4 onnect the oxygen atoms to sulfur by using two models of oxygen from the table of electron configurations. S S Note that sulfur has twelve electrons in its valence shell if the two red oxygens are viewed as double bonds. These sulfur-oxygen bonds are called dative bonds. 5

6 Naming Molecular ompounds A molecular compound is a compound in which all bonds are covalent. When naming binary molecular compounds the less electronegative element is named first (it is generally written first in the formula). The prefixes mono-, di-, tri-, tetr-, etc. are used to show the number of atoms of each element; mono- is commonly omitted when it refers to the first atom, and is rarely used with the second atom: is carbon monoxide; N is nitrogen oxide S 2 is sulfur difluoride, N 2 is dinitrogen oxide N 2 4 is dinitrogen tetroxide, N 2 is nitrogen dioxide The Geometric Structures of Molecules Valence-Shell Electron-Pair (VSEPR) Model Valence electrons of an atom may be involved in forming single, double, or triple bonds, or they may be unshared. Each involvement creates a negatively charged region of electron density around the nucleus. Because like charges repel each other, the various regions of electron density around an atom spread so that each is as far away from the others as possible while remaining connected to the atomic core. The shape of a molecule reflects this distribution of electron density about each atom. The Geometric Structures of Molecules Predicting the shape of a molecule of 2 2 : ount the regions of electron density about the element(s) nearest the center of the Periodic hart. Then apply the following criteria: 2 = linear, angles 180E 3 = trigonal, angles 120E 4 = tetrahedral, angles 109.5E 5 = trigonal bipyramid, angles 120E and 90E 6 = octahedral, angles 90E 6

7 The Geometric Structures of Molecules Predicting the shape of a molecule of 2 2 : ount the regions of electron density about the element nearest the center of the Periodic hart. 4 = tetrahedral, angles 109.5E 4 The Geometric Structures of Molecules Predicting the shape of a molecule of Predicting the shape of a molecule of 2 4 The Geometric Structures of Molecules Predicting the shape of a molecule of formaldehyde: 2 3 a double bond Predicting the shape of a molecule of acetylene: a triple bond 7

8 The Geometric Structures of Molecules Predicting the shape of a molecule of P 5 P Predicting the shape of a molecule of S 6 S 6 5 P S Geometry and Polarity of Molecules Molecules are polar if their bonds are polar and the geometry of molecules doesn t cancel out the effect. Polarity in covalent bonds is caused when the ΔEN is between 0.5 and 1.9 Pauling units. This polarity creates a dipole in which one end is negative the other is positive. This is depicted by: δ+ δ- When the effect of all the arrows along bonds are added vectorally the result is a dipole moment. This means the molecule is polar. In some molecules the bond dipoles cancel and the molecule is non-polar. Geometry and Polarity of Molecules Water is polar: arbon dioxide is not polar: zero dipole moment 2 2 is polar: But 4 is not polar! 8