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Chemical Bonding. Chapter 8 AP Chemistry. Types of Chemical Bonds. Ionic – electrons are transferred from a metal to a nonmetal Covalent – electrons are shared between 2 nonmetals Metallic – between metal atoms. Lewis Symbols.
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Chemical Bonding Chapter 8 AP Chemistry
Types of Chemical Bonds • Ionic – electrons are transferred from a metal to a nonmetal • Covalent – electrons are shared between 2 nonmetals • Metallic – between metal atoms
Lewis Symbols • Since bonding involves the element’s valence electrons, Lewis symbols are used. • Lewis symbols consist of the element symbol plus a dot for each valence electron. • Example: [Ne]3s23p4 (sulfur)
Which is the correct Lewis Structure for Chlorine? Either of these two!
Octet Rule • Atoms tend to gain, lose or share electrons until they are surrounded by 8 valence electrons. • In doing so, they achieve the configuration of a noble gas and achieve stability.
Ionic Bonding • Metals will lose electrons to achieve a stable outer valence level, and nonmetals will take those electrons to also achieve a stable outer valence. • NaCl is composed of Na+ and Cl- ions, arranged in a 3-dimensional crystal.
Formation of NaCl • There is an electron transfer between Na and Cl. • The metal, Na, has a low ioniziation energy and the nonmetal, Cl has a high electron affinity.
Energetics of Ionic Bonds • The formation of NaCl is extremely exothermic (ΔH°f = -410.9kJ). Why? • Loss of e- is always endothermic – takes energy to remove an electron from an orbital. When an e- is gained, the process is exothermic.
Lattice Energy • Lattice energy is a measure of just how much stabilization results from arranging oppositely charged ions in an ionic compound. Lattice energy is the energy required to completely separate a mole of solid ionic compound into its gaseous ions.
Lattice Energy • The magnitude of lattice energy depends on the charges of the ions, their sizes, and their arrangement in the solid. • For a given arrangement of ions, the lattice energy increases as the charges on the ions increases and as their radii decrease. • In forming ions, transition metals lose the valence shell e- first, then as many d e- as needed to reach the charge of the ion.
Covalent Bonding • A chemical bond formed by sharing a pair of electrons is called a covalent bond. • The attractive forces of the nuclei and electrons must overcome the repulsion between electrons and nuclei.
Bond Polarity and Electronegativity • Polar Covalent Bond – one atom pulls harder for the shared electrons than the other atom, forming a dipole. • The difference in electronegativity between the atoms must be less than 1.7.
Bond Polarity • Non-polar Covalent Bonds – the atoms pull equally on the electrons. • The atoms must have the same electronegativity • The elements that have non-polar covalent bonds are the ones which are diatomic.
Dipoles F has a higher electronegativity, so it pulls harder on the (-) electrons creating an overall negative pole. C has a lower electronegativity, so the electrons are farther from it, and it has a partial positive charge.
Dipole Moment • The quantitative measure of the magnitude of a dipole is called the dipole moment, μ. • If a distance, r, separates two equal and opposite charges, Q+ and Q-, the magnitude is the product of Q and r. • μ = Qr
Dipole Moments • Reported in debyes (D), which equals 3.34 x 10-30 coulomb-meters (C-m) • Charge is measured in e (electronic charge), 1.60 x 10-19 C, and distance is in Å. • Suppose two charges 1+ and 1- are separated by 1.00Å. • μ = Qr = (1.60 x 10-19C)(1.00Å)(10-10 m/1Å) (1D/3.34 x 10-30) = 4.79 D
Drawing Lewis Structures • Sum the valence electrons from all atoms. • Write the symbols for the atoms to show which atoms are attached to which and connect them with a single bond. • Complete the octets around all of the atoms bonded to the central atom. • Place any leftover electrons on the central atom. • If there are not enough electrons to go around, try multiple bonds.
Resonance Structures • Multiple, but equally good representations for individual Lewis structures can be drawn for a molecule. • The resonance structures are “averaged”to give a more accurate description of the molecule.
Exceptions to the Octet Rule • The octet rule has limits when dealing with some of the transition metals and some covalent compounds. • Molecules and polyatomic ions with an odd number of electrons • Molecules and polyatomic ions in which an atom has fewer than an octet of valence electrons • Molecules and polyatomic ions in which an atom has more than an octet of valence electrons
Strengths of Covalent Bonds • The stability of a molecule is related to the strengths of the bonds it contains. • Bond enthalpy is the enthalpy change, ΔH, for breaking a particular bond in one mole of a gaseous substance. • ΔHrxn = Σ(bond enthalpies of bonds broken) – Σ(bond enthalpies of bonds formed)
Bond Enthalpies, continued • H—CH3 + Cl—Cl → Cl—CH3 + H—Cl • ΔHrxn = [D(C—H) + D(Cl—Cl)] – [D(C—Cl) + D(H—Cl)] • = (413 kJ + 242 kJ) – (328 kJ + 431 kJ) = -104 kJ • This reaction is exothermic because the bonds in the products are stronger than than the bonds in the reactants, and the ΔHrxn value is negative. • These are often averaged values and provide an estimate.