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Chemical Bonds: Ionic and Covalent Compounds

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This chapter explores the concepts of ionic and covalent bonding in chemical compounds. It covers the octet rule, naming of compounds, Lewis dot structures, molecular geometry, and polarity.

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Chemical Bonds: Ionic and Covalent Compounds

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  1. Chapter 3 Chemical Bonds

  2. Note: order changed slightly • Will start with ionic bonding and ionic compounds, then move to covalent bonding and molecular compounds, where Lewis dot structures are used • Then will do molecular geometry and VSEPR theory and discuss polarity

  3. Key topics for this chapter • Octet rule and how it relates to quantum model of the atom • Differentiating between ionic and covalent bonds • Naming of ionic and molecular compounds • Use of Lewis dot structures and VSEPR theory to determine bonding and geometry of molecular compounds • Bond polarity • Molecule polarity

  4. Major themes • Octet rule: valence electrons make chemistry happen, core electrons play small role • Metals react with nonmetals to form ionic compounds • Nonmetals react with nonmetals to form molecular compounds • We learn the rules for writing names and formulas for ionic and molecular compounds • We learn about molecular shape and polarity

  5. Octet Rule

  6. Octet rule

  7. Electron Configurations of Cations and Anions Of Representative Elements Na [Ne]3s1 Na+ [Ne] Metal Atoms ___________ electrons so that cation has a noble-gas outer electron configuration. Ca [Ar]4s2 Ca2+ [Ar] Al [Ne]3s23p1 Al3+ [Ne] H 1s1 H- 1s2 or [He] Nonmetal Atoms ____________ electrons so that anion has a noble-gas outer electron configuration. F 1s22s22p5 F- 1s22s22p6 or [Ne] O 1s22s22p4 O2- 1s22s22p6 or [Ne] N 1s22s22p3 N3- 1s22s22p6 or [Ne] 8.2

  8. -1 -2 -3 +1 +2 +3 Cations and Anions Of Representative Elements 8.2

  9. Types of Chemical Bonds • According to the Lewis Model Ionic Bonding • An atom may lose or gain enough electrons to acquire the same electronic structure as its nearest noble gas neighbor (usually 8 valence e-) and become an ion • An ionic bond is the result of the force of attraction between a positive ion (cation) and a negative ion (anion) • Occurs between metals and nonmetals

  10. Ionic Bonds

  11. Formation of Ionic Bonds • Occur when electrons are transferred from a metal to a nonmetal

  12. Ionic Bond in NaCl

  13. Ionic Bond in CaCl2

  14. Formation of Ionic Bonds • Each atom attains a stable electronic structure • Number of protons/electrons in atoms no longer equal, charged particles result (ions) • Metals lose e-, form + ions (cations) • Nonmetals gain e-, form – ions (anions)

  15. A Closer Look at Salt

  16. Salt contains a large number of Na+ and Cl- ions

  17. Ions pack together tightly into crystalline solids • Strong attractive force between anions and cations holds crystal together

  18. Types of Chemical Bonds • According to the Lewis Model Covalent Bonding • An atom may share electrons with one or more other atoms to acquire a filled valence shell. • A covalent bond is the force of attraction between two atoms that share one or more pairs of electrons. • Occurs between nonmetals

  19. Covalent Bonds • A sharing of valence electrons between atoms • Usually occurs between nonmetallic elements • Nonmetals need to gain e- in order to attain a stable e- structure • Thus, when they combine, end up sharing valence electrons

  20. Covalent Bond Atom 1 Atom 2 • Attractive forces between nucleus of 1 atom and e- from other atom • Repulsive forces between nuclei, electron clouds • Attractive forces stronger, bond forms

  21. Covalent Bonds Bohr model of covalent bond • By Sharing e- • hydrogen obtains a completely filled 1st energy level • oxygen obtains a completely filled 2nd energy level (stable octet)

  22. Lewis, or electron, Dot Diagrams • Show the valence electrons • Consist of element symbol – represents nucleus and core electrons – and dots (valence e-)

  23. Elements and Covalent Bonds • Most nonmetallic elements do not exist in nature as individual atoms • e.g. Hydrogen exists as a diatomic molecule, H2 • By overlapping 1s orbitals, each H atom attains a stable e- structure

  24. Other Molecular Elements Diatomic Elements(Br2, I2, N2, Cl2, H2, O2, F2) • Hydrogen • All group 7A elements • Oxygen • Nitrogen BrINCIHOF!!! Memorize it

  25. General Rules for Bond Types Negative complex ions Nonmetal Nonmetal Covalent Bond Ionic Bond Metal

  26. What determines bond type? Generalization • metal + nonmetal = ionic bond • nonmetal + nonmetal = covalent bond Reality • Bonding type is on a continuum, from 100% ionic to 100% covalent 100% covalent 100% ionic

  27. Determining Predominant Bond type • Electronegativity is a measure of the tendency for atoms of an element to attract electrons in a chemical bond. • Originated with American chemist Linus Pauling (1901-1994), a 2x Nobel Prize winner from Stanford U - a relative scale

  28. Electronegativity (EN) Scale ranges from 0.7 to 4.0

  29. Electronegativity (EN) Trends in Periodic table

  30. Highly electronegative elements (N, O, F, Cl) are greedy for electrons • Predominant type of bonding (ionic/covalent) determined by differences in electronegativity (EN) between atoms in bond • EN > 1.9, predominantly ionic (metal, nonmetal) • EN 0.5 to 1.9, polar covalent • EN < 0.5, nonpolar covalent Nonmetal, nonmetal

  31. Determining Predominant Bond Type • Look at the electronegativity values of atoms involved in bond • Calculate electronegativity difference (EN) • Draw arrow in direction that e- cloud is pulled (more electronegative atom) • Also use lower case Greek symbol for delta, , along with + or - sign

  32. Example: Bond between H and Cl • Electronegativity values H= 2.1, Cl = 3.0 • EN = 3.0-2.1 = 0.9 = polar covalent bond • e- cloud pulled towards Cl atom = Polar Covalent Bond Cl H Cl H d+ d- e- rich electron poor region electron rich region 2.1 3.0 e- poor Direction e- cloud being pulled 9.5

  33. Result of differences in electro-negativity: • More electronegative element has greater share of e- cloud, it is electron rich • Less electronegative element is e- poor • Creation of bond dipoles: +and  -

  34. Example of Nonpolar Covalent Bonds • Diatomic elements e.g. H2 • Bonds between P and H = 2.1-2.1=0

  35. Bond Character Summary • Nonpolar covalent bond  electrons shared equally • Polar covalent bond  e- not shared equally, more electronegative atom has greater share of e- cloud • Ionic bond – e- transferred from one atom to other, creates a + and – ion

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