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Bonding – General Concepts

Bonding – General Concepts. Why Do Chemical Reactions Occur?. Bond Length Diagram. Problems and questions — How is a molecule or polyatomic ion held together? Why are atoms distributed at strange angles? Why are molecules not flat? Can we predict the structure?

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Bonding – General Concepts

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  1. Bonding – General Concepts

  2. Why Do Chemical Reactions Occur?

  3. Bond Length Diagram

  4. Problems and questions — How is a molecule or polyatomic ion held together? Why are atoms distributed at strange angles? Why are molecules not flat? Can we predict the structure? How is structure related to chemical and physical properties? Chemical Bonding

  5. There are 3 forms of bonding: _________—complete transfer of 1 or more electrons from one atom to another (one loses, the other gains) forming oppositely charged ions that attract one another _________—some valence electrons shared between atoms _________ – holds atoms of a metal together Review of Chemical Bonds Most bonds are somewhere in between ionic and covalent.

  6. The type of bond can usually be calculated by finding the difference in electronegativity of the two atoms that are going together.

  7. Electronegativity Difference • If the difference in electronegativities is between: • 1.7 to 4.0: Ionic • 0.3 to 1.7: Polar Covalent • 0.0 to 0.3: Non-Polar Covalent • Example: NaCl • Na = 0.8, Cl = 3.0 • Difference is 2.2, so • this is an ionic bond!

  8. Ionic Bonds Positive cations and the negative anions are attracted to one another (remember the Paula Abdul Principle of Chemistry: Opposites Attract!) Therefore, ionic compounds are usually between metals and nonmetals (opposite ends of the periodic table).

  9. Ionic Bonds • Electrons are transferred • Electronegativity differences are generally greater than 1.7 • The formation of ionic bonds is always exothermic!

  10. Determination of Ionic Character Electronegativity difference is not the final determination of ionic character Compounds are ionic if they conduct electricity in their molten state

  11. Coulomb’s Law “The energy of interaction between a pair of ions is proportional to the product of their charges, divided by the distance between their centers”

  12. Which bond will be the strongest? • Na & F • Or • Na & Cl • Mg & F • Or • Na & F

  13. Table of Ion Sizes

  14. Sodium Chloride Crystal Lattice Ionic compounds form solids at ordinary temperatures. Ionic compounds organize in a characteristic crystal lattice of alternating positive and negative ions.

  15. Estimate Hf for Sodium Chloride Na(s) + ½ Cl2(g)  NaCl(s) Na(s)  Na(g) + 109 kJ Na(g)  Na+(g) + e- + 495 kJ ½ Cl2(g)  Cl(g) + ½(239 kJ) Cl(g) + e- Cl-(g) - 349 kJ Na+(g) + Cl-(g)  NaCl(s) -786 kJ Na(s) + ½ Cl2(g)  NaCl(s) -412 kJ/mol

  16. Electron distribution is depicted withLewis (electron dot) structures This is how you decide how many atoms will bond covalently! (In ionic bonds, it was decided with charges) G. N. Lewis 1875 - 1946 Electron Distribution in Molecules

  17. Covalent Bonds Polar-Covalent bonds • Electrons are unequally shared • Electronegativity difference between .3 and 1.7 Nonpolar-Covalent bonds • Electrons are equally shared • Electronegativity difference of 0 to 0.3

  18. HCl is POLAR because it has a positive end and a negative end. (difference in electronegativity) Bond Polarity Cl has a greater share in bonding electrons than does H. Cl has slight negative charge (-d) and H has slight positive charge (+ d)

  19. This is why oil and water will not mix! Oil is nonpolar, and water is polar. The two will repel each other, and so you can not dissolve one in the other Bond Polarity

  20. “Like Dissolves Like” Polar dissolves Polar Nonpolar dissolves Nonpolar Bond Polarity

  21. Bond Length and Energy Bonds between elements become shorter and stronger as multiplicity increases.

  22. What is the relationship between bond length and bond energy?

  23. Bond Energy and Enthalpy Energy required Energy released D = Bond energy per mole of bonds Breaking bonds always requires energy Breaking = endothermic Forming bonds always releases energy Forming = exothermic

  24. The Octet Rule Combinations of elements tend to form so that each atom, by gaining, losing, or sharing electrons, has an octet of electrons in its highest occupied energy level. Diatomic Fluorine

  25. Formation of Water by the Octet Rule

  26. 2nd row elements C, N, O, F observe the octet rule (HONC rule as well). 2nd row elements B and Be often have fewer than 8 electrons around themselves - they are very reactive. 3rd row and heavier elements CAN exceed the octet rule using empty valence d orbitals. When writing Lewis structures, satisfy octets first, then place electrons around elements having available d orbitals. Comments About the Octet Rule

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