Chapter 3 Chemical Bonds Note order changed slightly
Chapter 3 Chemical Bonds
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
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
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
Octet Rule
Octet rule
Electron Configurations of Cations and Anions Of Representative Elements Na [Ne]3 s 1 Ca [Ar]4 s 2 Al [Ne]3 s 23 p 1 Na+ [Ne] Metal Atoms ______ electrons so that cation has a noble-gas outer electron configuration. Ca 2+ [Ar] Al 3+ [Ne] Nonmetal Atoms ______ electrons so that anion has a noble-gas outer electron configuration. H 1 s 1 H- 1 s 2 or [He] F 1 s 22 p 5 F- 1 s 22 p 6 or [Ne] O 1 s 22 p 4 O 2 - 1 s 22 p 6 or [Ne] N 1 s 22 p 3 N 3 - 1 s 22 p 6 or [Ne] 8. 2
-1 -2 -3 +3 +2 +1 Cations and Anions Of Representative Elements 8. 2
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
Ionic Bonds
Formation of Ionic Bonds • Occur when electrons are transferred from a metal to a nonmetal
Ionic Bond in Na. Cl
Ionic Bond in Ca. Cl 2
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)
A Closer Look at Salt
Salt contains a large number of Na+ and Cl- ions
Ions pack together tightly into crystalline solids • Strong attractive force between anions and cations holds crystal together
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
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
Covalent Bond Atom 1 Atom 2 • Attractive forces between nucleus of 1 atom and efrom other atom • Repulsive forces between nuclei, electron clouds • Attractive forces stronger, bond forms
Covalent Bonds Bohr model of covalent bond By Sharing e • hydrogen obtains a completely filled 1 st energy level • oxygen obtains a completely filled 2 nd energy level (stable octet)
Lewis, or electron, Dot Diagrams • Show the valence electrons • Consist of element symbol – represents nucleus and core electrons – and dots (valence e-)
Elements and Covalent Bonds • Most nonmetallic elements do not exist in nature as individual atoms • e. g. Hydrogen exists as a diatomic molecule, H 2 • By overlapping 1 s orbitals, each H atom attains a stable e- structure
Other Molecular Elements Diatomic Elements (Br 2, I 2, N 2, Cl 2, H 2, O 2, F 2) • Hydrogen • All group 7 A elements • Oxygen • Nitrogen Br. INCIHOF!!! Memorize it
General Rules for Bond Types Negative complex ions Nonmetal Covalent Bond Ionic Bond Metal Nonmetal
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% ionic
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 2 x Nobel Prize winner from Stanford U - a relative scale
Electronegativity (EN) Scale ranges from 0. 7 to 4. 0
Electronegativity (EN) Trends in Periodic table
• 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 Nonmetal, nonmetal • EN < 0. 5, nonpolar covalent
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
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 electron poor region 2. 1 H electron rich region 3. 0 Cl e- rich H Cl + e- poor Direction e- cloud being pulled 9. 5
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 -
Example of Nonpolar Covalent Bonds • Diatomic elements e. g. H 2 • Bonds between P and H = 2. 1 -2. 1=0
Bond Character Summary a) Nonpolar covalent bond electrons shared equally b) Polar covalent bond e- not shared equally, more electronegative atom has greater share of e- cloud c) Ionic bond – e- transferred from one atom to other, creates a + and – ion
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