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Chapter 8 – Basic Concepts of Chemical Bonding

Chapter 8 – Basic Concepts of Chemical Bonding. Abegim Undieh. 8.1 – Chemical Bonds, Lewis Symbols, and the Octet Rule. When atoms or ions are strongly attached to one another, there is a chemical bond between them There are THREE types of chemical bonds: ionic, covalent, and metallic.

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Chapter 8 – Basic Concepts of Chemical Bonding

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  1. Chapter 8 – Basic Concepts of Chemical Bonding Abegim Undieh

  2. 8.1 – Chemical Bonds, Lewis Symbols, and the Octet Rule • When atoms or ions are strongly attached to one another, there is a chemical bond between them • There are THREE types of chemical bonds: ionic, covalent, and metallic

  3. Lewis Symbols • The electrons involved in chemical bonding are called valence electrons • Valence electrons are in the outer shell of an atom • Valence electrons are symbolized using Lewis Symbols • The Lewis Symbol of an element shows the chemical symbol for the element and a dot for each valence electron

  4. Picture of a Lewis Symbol • Lewis Dot Symbol for Se: http://www.cbu.edu/~mcondren/Se-Lewis-dot-structure.jpg

  5. The Octet Rule • All noble gases (besides He) have 8 valence electrons • Noble gases have the most stable electron arrangements • The Octet Rule: Atoms tend to gain, lose, or share electrons until they are surrounded by eight valence electrons

  6. 8.2 – Ionic Bonding • The bonding of a metal and a non-metal through the electrostatic attraction of opposite charges is called ionic bonding • In an ionic bond, the metal transfers electrons to the nonmetal • This occurs when there is a metal of low ionization energy and a nonmetal with a high electron affinity

  7. Formula showing a sample Ionic Bond Na + Cl  NaCl Na + Cl  Na+ + [Cl]- http://upload.wikimedia.org/wikipedia/commons/7/75/Ionic_bonding_animation.gif

  8. Ionic Bonding (cont.) • Ionic bonding is a very exothermic process • The measure of the stability of an ionic bond is called the lattice energy • Lattice energy – the energy required to completely separate a mole of a solid ionic compound into gaseous ions

  9. Ionic Bonding (cont.) • Formula for lattice energy: E = kQ1Q2 D Lattice energy increases as the charges on the ions increase and as their radii decrease Ionic radii do not vary much over a wide range, so the magnitude of lattice energy depends mainly on the ionic charges

  10. 8.3 – Sizes of Ions • Ionic size determines many properties of ionic solids, such as lattice energy, the way it packs in a solid, and the properties of the ions in solution • The size of an ion depends on • its nuclear charge • the number of electrons it possesses • the orbitals in which the valence electrons reside

  11. Sizes of Ions (cont) • In positive ions, electrons from the outermost orbital have been removed • This decreases the total electron-electron repulsion, allowing the atom to pull closer together • Therefore, cations are smaller than their parent atoms, and conversely.. • Anions are larger than their parent atoms • For ions of the same charge, size increases as you go down a group in the periodic table

  12. Sizes of Ions (cont) • an isoelectronic series is a series of ions that have the same number of electrons • Example: O2-, F-, Na+, Mg2+, Al3+ all have 10 electrons arranged in a 1s22s22p6 configuration like [Ne] • In an isoelectronic series, the radius of the ion decreases with increasing nuclear charge

  13. Ionic Radii http://www.chem.umass.edu/~botch/Chem111F04/Chapters/Ch8/IonicRadii.jpg

  14. 8.4 – Covalent Bonding • In covalent bonding, two nonmetals share electrons • The atoms are held together by the electrostatic attraction between the two nuclei

  15. Multiple Bonds • The sharing of a pair of electrons produces one covalent bond, called a single bond • In some cases, molecules form multiple bonds in order to achieve an octet

  16. Multiple Bonds • Two shared electrons produce a double bond • Three shared electrons produce a triple bond the triple bond in N2 as shown by a Lewis structure • As a rule, the distance between bonded atoms decreases as the number of shared electron pairs increases

  17. 8.5 – Bond Polarity and Electronegativity

  18. 8.6 – Drawing Lewis Structures

  19. 8.7 – Resonance Structures • In some molecules, the arrangement of atoms cannot fully be described by a single Lewis structure • This requires showing multiple equivalent Lewis structures, called resonance structures • The actual structure of the molecule is a blend of its resonance structures

  20. Resonance Structures (cont.) • Example: • In ozone, both of these resonance structures are equally as correct, and the molecule does not oscillate between the two: http://www.mikeblaber.org/oldwine/chm1045/notes/Bonding/Resonan/IMG00007.GIF Remember to put brackets ‘[ ]’ around each resonance structure that you draw, with the double arrow in-between

  21. Resonance in Benzene • Benzene, C6H6, has two equivalent Lewis structures, each showing three C-C single bonds and three C=C double bonds http://wpcontent.answers.com/wikipedia/commons/thumb/7/79/Benz3.svg/300px-Benz3.svg.png

  22. Resonance in Benzene • Benzene is often represented as such: • OR • Millions of organic compounds have structures based on Benzene

  23. 8.8 – Exceptions to the Octet Rule • Molecules with an odd number of electrons • Molecules in which an atom has less than an octet • Molecules in which an atom has more than an octet

  24. Odd Number of Electrons • In some cases, complete pairing of electrons is impossible and an octet around each atom cannot be achieved • Examples: ClO2, NO, NO2 ClO2 Lewis Diagram

  25. Less than an Octet • There are also cases in which an atom has less than 8 electrons • This most frequently occurs in compounds containing boron or beryllium • Example: http://www.up.ac.za/academic/chem/mol_geom/bf32.gif BF3 Lewis structure, showing that B only has 6 electrons surrounding it.

  26. More than an Octet • Many molecules/ions have more than an octet of electrons • This only occurs for elements in period 3 and beyond Diagram for PCl5, showing an expanded octet around P. http://www.wwnorton.com/college/chemistry/gilbert/concepts/chapter7/pcl5.gif

  27. More than an Octet • The larger the central atom, the larger the number of surrounding atoms it can hold • Expanded shells occur most often when the central atom is bonded to the smallest and most electronegative atoms, such as F, Cl, O

  28. 8.9 – Strengths of Covalent Bonds • Bond enthalpy – the enthalpy change for the breaking of a particular bond into a mole of gaseous substance • Bond enthalpy is always a positive quantity • The strengths of covalent bonds increase with the number of shared electron pairs

  29. Strengths of Covalent Bonds • The bond length between two bonded atoms is the distance between the two nuclei • The average bond length between two atoms decreases as the number of bonds between them increases

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