Chemical Bonding Notes PDF

Chapter 12 CB1 Chemical Bonding (Reading Section 12.1 – 12.5) Part 1 Chemical Bonding Part 2 Electronegativity Part 3 Bond polarity and Dipole Moment Part 4 Lewis Structures Part 5 Shapes of Molecules (VSEPR) In Chemistry 1, you have learned: Metals and non...

Chapter 12 CB1 Chemical Bonding (Reading Section 12.1 – 12.5) Part 1 Chemical Bonding Part 2 Electronegativity Part 3 Bond polarity and Dipole Moment Part 4 Lewis Structures Part 5 Shapes of Molecules (VSEPR) In Chemistry 1, you have learned: Metals and nonmetals form ionic compounds. e.g. NaCl Non-metals form covalent compounds (aka molecular compounds). e.g. CO2 We will look into the details why elements behave like this in this lesson. 2  Part 1 Chemical Bonding 3  Chemical Compounds Compound – chemically combination of two or more elements. Metal and non-metal elements form ionic compound. Non-metal and non-metal elements form molecular compound (aka covalent compound). 4  Chemical Bonds Chemical bonds are the forces that hold groups of atoms together to form compound or function as a unit. Atoms are bonded in two different ways: - by transferring electrons from one atom to another atom (ionic bond) - by sharing electrons among the participating atoms (covalent bond) 5  l e is a r t ic m i c p a l What - at o e m i c determ h su b o r c h many e ine how i c e f lectron Wh ponsibl ng?? involve (s) shou ld be res o n di d in bo nding? b a n t to e n t s w h y e l em g? ? W b o n d in form 6  Stable Compounds Stable bond is formed when the participating atoms achieve a noble gas electron configuration (i.e. contains the same no. of electrons as a noble gas atom such as He (2 electrons), Ne (10 electrons) and Ar (18 electrons)…etc. 7 Ionic Bond - Transferring Electrons Metals tend to form cations by losing one or more outermost electrons to achieve the previous noble gas configuration (i.e. same no. of electrons). Na - e- Na+ 11 e- - 1 e- 10 e- [same as Ne] - stable Nonmetals tend to form anions by gaining one or more electrons to achieve the next noble gas configuration (i.e. same no. of electrons). Cl + e- Cl- 8 Covalent Bond - Sharing Electrons Nonmetal atoms tend to share electrons to form covalent bonds. The shared electrons in the covalent bond belong to both atoms. Covalent – means sharing electrons. 9 Nearest Noble Gas Arrangement 10  Isoelectronic Series Definition - a series of ions/atoms containing the same number of electrons. Question: Is the Li+ ion the same as Helium 11  Concept check Identify which noble gas containing the same no. of electrons as the following particles: O2–, F–, Ne, Na+, Mg2+, and Al3+ 12 General Characteristics of Ionic Compounds Metal and non-metal form ionic compound Bond is ionic in nature Formulas are in simple ratios (i.e. reduce the subscript to simplest ratio) e.g. NaCl Crystalline structure at room temperature High melting points Good conductors of electricity when 13  General Characteristics of Molecular Compounds Formed by reaction of non-metals and non-metals Bond is covalent in nature Formulas are in true ratio (i.e. do not need to reduce the subscript to simplest ratio) e.g. C6H12O6 Substances may exist in any state of matter at room temperature e.g. Gas - CO2; liquid – H2O; solid - C6H12O6 14  Example of Ionic Compound Melting point: 801 oC Ionic bond in crystal lattice Good conductor of electricity NaCl with ionic bond 15 Example of Molecular Compound Melting point: - 78 °C Non-conductor of electricity CO2 with covalent bond (dry ice pellet) 16  Some Ionic and Molecular Compounds Examples Substan P.S. m.p. or b.p. Ionic or ce @ 25 molecul °C ar? Ammoni gas b.p. = −33.34 °C Molecula a r Dry ice gas b.p. = −78 °C Molecula r Ionic Table salt solid m.p. = 801 oC Ionic Glass solid m.p. = 1400 °C P.S. – Physical State; m.p. – melting point; b.p. – boiling point  Lesson Summary of Chemical Bonding For ionic compounds, metals tend to lose electrons (cation) while nonmetals tend to gain electrons (anion) to form ionic bond. For molecular compounds, nonmetal atoms tend to share electrons with other atoms to form covalent bond. Stable bond is formed when the participating atoms achieve a noble gas electron configuration. Octet rule is followed by representative elements. Duet rule is followed by hydrogen. Isoelectronic Series - a series of ions/atoms18  Part 2 Electronegati vity 19  Electronegativity (EN) EN is the ability of an atom to attract shared electrons to itself. It reflects the strength of the nuclear charge (positive charge from the proton) of the atom. Element with the most EN is fluorine (EN = 4.0). Elements with the least EN are cesium (atomic no. = 55) and francium (atomic no. = 87). Their EN value is 0.7. EN is a dimensionless quantity. For He, Ne and Ar is no data. Difference in EN between two atoms (∆EN)  Polar Covalent Bonds Unequal electron sharing results in a charge separation in the bond (partial positive and partial negative charge). The bond is polarized. It is called polar covalent bond. more unequal electron sharing = more polar of bond 21  The Trend of Electronegativity – Across a Period EN increases across a period from the left to right (excluding noble gases). In the 2nd period, F (EN = 4.0) > Li (EN = 1.0). Meaning: Fluorine attracts electrons better than Lithium. Explanation: The effective nuclear charge (due to protons) 22  The Trend of Electronegativity – Down a Group EN decreases from the top to bottom in a group. In Group 7A, F (EN = 4.0) > I (EN = 2.5). Meaning: Fluorine attracts electrons better than iodine. Explanation: - Atomic size increase in going down a group (i.e. atoms getting bigger as no. of electrons 23  EN of Non-metals In general, non-metals have higher EN. Therefore, they attract electrons better. So, they tend to gain electrons in bonding. 24  Concept Check If lithium and fluorine react, which has more attraction for an electron? Why? In a bond between fluorine and iodine, which has more attraction for an electron? Why? 25 Bond Character 26  An Example of Effect of Ionic Character on Physical Properties of Ionic Compounds 27  Bond Characters There are pure ionic bond (electrons completely transferred) e.g. bonding in NaCl There are pure covalent bond (aka nonpolar bond – equal sharing electrons). E.g. bonding in N2 molecule Other bonds will have some ionic and some covalent character. The difference in the electronegativity (∆EN) determines the bond characters in these cases. 28  Bond Characters (cont.) Different ionic bond can have different ionic character. Different polar covalent bond can have different polar character. This difference explains the differences in physical properties of substances. e.g. solubility of substances Terminology used for bond character comparison: Ionic bond – more or less ionic character Polar covalent bond – more or less polar 29  Difference in Bond Character – Ionic Bond Note the ∆EN for the following pairs: Na–Cl Mg–Cl Ag–Cl ENNa = 0.9 ENMg = 1.2 ENAg = 1.9 ENCl = 3.0 ENCl = 3.0 ENCl = 3.0 ∆EN = 3.0 – 0.9 ∆EN = 3.0 – 1.2 ∆EN = 3.0 – 1.9 = 2.1 = 1.8 = 1.1 30  Difference in Bond Character – Polar Covalent Bond Note the ∆EN for the following pairs: H–F H–O O–O ENH = 2.1 ENH = 2.1 ENO = 3.5 ENF = 4.0 ENO = 3.5 ENO = 3.5 ∆EN = 4.0 – 2.1 ∆EN = 3.5 – 2.1 ∆EN = 3.5 – 3.5 = 1.9 = 1.4 =0 H–F bond (∆EN = 1.9) has more polar 31  ∆EN in Covalent Bond ∆EN Bond Examples Charact er ENB = Non- H-H (∆EN = ENA polar 0) covalent H-P (∆EN = 0) (equal sharing) ENB ≠ Polar H-Cl (∆EN = ENA covalent 0.9) e.g. (unequal H-O (∆EN = H-O bond EN hasB > ENA polar more sharing) 1.4) than H- character  ∆EN in Ionic Bond ∆EN Bond Character Examples Ionic bond with Na+Cl- more ionic (∆EN = e.g. ENB >> character 2.1) ENA Ionic bond with Mg+S- less ionic (∆EN = e.g. ENB > character 1.3) EN Na+ACl- bond has more ionic character than + -  Notes about ∆EN (cont.) Bond may have partial ionic and partial covalent character. In covalent bond (nonmetal – nonmetal), ∆EN increases, more polar character. In ionic bond (metal - nonmetal), ∆EN increases, more ionic character. When ∆EN = zero, the electrons are shared equally. Pure covalent bond (nonpolar bond) is formed. Bond character affects the physical 34  Concept Check Which bond has greater ionic character? Na – F or Na - I 35  Concept Check Which bond has more polar character? O – Cl or O - Br 36  Lesson Summary of Electronegativity Electronegativity is a measure of the ability of an atom to attract shared electrons to itself. Electronegativity increases from left to right across a period and decreases down a group on periodic table. The difference in electronegativity of the atoms determine whether the bond has more polar or or ionic character. Equal sharing of electron form nonpolar covalent bond. Unequal sharing of electrons forms polar 37  Part 3 Bond Polarity and Dipole Moment (For covalent compound only) 38  Dipole Moment Property of a molecule whose charge distribution can be represented by a center of positive charge (δ +) and a center of negative charge (δ -). Unequal sharing of electrons. A measure of charge separation Dipole moment can be measured quantitatively, but we only need to know the concept.  Dipole Moment (cont.) Use an arrow to represent a dipole moment.  Point to the negative charge center with the crossed end of the arrow indicating the positive center of charge. Exercise – Drawing Dipole Moment Bond ∆EN Draw the Dipole Moment C-Cl P-H Si-I Br-Br 41  Dipole Moment (cont.) The overall molecule is still neutral The partial charges (partial positive and partial negative) on atoms within the molecule significant affect the molecular properties. e.g. Water is a covalent molecule with dipole moment and ionic compound can dissolve in it. 42  Concept Check 1 Why is sodium fluoride written as Na+ F - , but hydrogen monofluoride written as Hδ+ Fδ− ? Answer: Na+ F - indicates the bonding is ionic bond. Hδ+ Fδ− in hydrogen monofluoride indicates the partial charges in a polar covalent bond. 43  Concept Check 2 The C – O bond is described as more polar than the N – O bond. What does it mean? a) The electron sharing in C – O bond is more unequal than in N – O bond. b) The electron sharing in N – O bond is more unequal than in C – O bond. c) Cannot be determined since we do not know the ∆EN. 44 Dipole Moment in a Water Molecule 45  Significance of Dipole Moment affe affe Dipole moment ct  molecular polarity ct  affe ct Intermolecular force  Solubility and Boiling point Intermolecular force, solubility and boiling point – will be discussed in the next unit (Solutions). 46  Roadmap for Chemical Bonding 1.Metal – nonmetal  ionic bond (transferring) e.g. NaCl 2. Nonmetal – nonmetal  covalent bond (sharing) Polar covalent Bond Nonpolar covalent Bond Unequal sharing Equal sharing ∆EN > 0 ∆EN = 0 e.g. twin atoms More polar Less polar character character e.g. CO e.g. NO (∆EN = 1.0) (∆EN = 0.5) 47 Lesson Summary of Bond Polarity Dipole moment is the property of a molecule whose charge distribution can be represented by a center of positive charge (δ +) and a center of negative charge (δ -). Dipole moment is due to the unequal sharing of electrons. Dipole moment exists in a polar covalent bond. Dipole moment affect the molecular polarity which affect the boiling point 48