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Children s club lecture 20-04-2010

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Children s club lecture 20-04-2010

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    2. CHEMICAL BONDING To play the movies and simulations included, view the presentation in Slide Show Mode. To play the movies and simulations included, view the presentation in Slide Show Mode.

    3. Chemical Bonding 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?

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    6. Review of Chemical Bonds 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

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    8. Characteristics of Covalent Compounds Covalent compounds are formed by the mutual sharing of electrons. There is no transfer of electrons from one atom to another and therefore no charges are created on the atom. No ions are formed. These compounds exist as neutral molecules and not as ions. Although some of the covalent molecules exist as solids, they do not conduct electricity in fused or molten or dissolved state. ii) They possess low melting and boiling points. This is because of the weak intermolecular forces existing between the covalent molecules. Since, no strong coulombic forces are seen; some of covalent molecules are volatile in nature. Mostly covalent compounds possess low melting and boiling points. iii) Covalent bonds are rigid and directional therefore different shapes of covalent molecules are seen. iv) Most of the covalent molecules are non polar and are soluble in nonpolar (low dielectric constant) solvents like benzene, ether etc and insoluble in polar solvents like water. Carbon tetrachloride (CCl4) is a covalent nonpolar molecule and is soluble in benzene. 8

    9. Co-Ordinate Covalent Bond Covalent type bond in which both the electrons in the shared pair come from one atom is called a coordinate covalent bond. Co-ordinate covalent bond is usually represented by an arrow () pointing from donor to the acceptor atom. Co-ordinate Covalent bond is also called as dative bond, donor acceptor bond, semi- polar bond or co-ionic bond. The electrostatic force of attraction which holds the oppositely charged ions together is known as ionic bond or electrovalent bond. Ionic compounds will be formed more easily between the elements with comparatively low ionization enthalpy and elements with comparatively high negative value of electron gain enthalpy. 9

    10. General Properties of Ionic Compounds Ionic compounds usually exist in the form of crystalline solids. They have high melting and boiling points and this ionic compounds are generally soluble in water and other polar solvents having high dielectric constants. Ionic compounds are good conductors of electricity in the solutions or in their molten states. The chemical reactions of ionic compounds are characteristic of the constituent ions and are known as ionic reactions. In ionic - compounds, each ion is surrounded by oppositely charged ions uniformly distributed all around the ion and therefore, electrical field is nondirectional. 10

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

    12. 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

    13. Ionic Bonds All those ionic compounds were made from ionic bonds. Weve been through this in great detail already. Positive cations and the negative anions are attracted to one another!)

    14. Electron Distribution in Molecules Electron distribution is depicted with Lewis (electron dot) structures This is how we can decide how many atoms will bond covalently! (In ionic bonds, it was decided with charges)

    15. Bond and Lone Pairs Valence electrons are distributed as shared or BOND PAIRS and unshared or LONE PAIRS.

    16. Bond Formation A bond can result from an overlap of atomic orbitals on neighboring atoms.

    17. Review of Valence Electrons Remember from the electron chapter that valence electrons are the electrons in the OUTERMOST energy level thats why we did all those electron configurations! B is 1s2 2s2 2p1; so the outer energy level is 2, and there are 2+1 = 3 electrons in level 2. These are the valence electrons! Br is [Ar] 4s2 3d10 4p5 How many valence electrons are present?

    18. Review of Valence Electrons Number of valence electrons of a main (A) group atom = Group number

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    20. Valence Shell Electron Pair Repulsion (VSEPR) theory

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    23. Steps for Building a Dot Structure Ammonia, NH3 1. Decide on the central atom; never H. Why? If there is a choice, the central atom is atom of lowest affinity for electrons. (Most of the time, this is the least electronegative Therefore, N is central on this one 2. Add up the number of valence electrons that can be used. H = 1 and N = 5 Total = (3 x 1) + 5 = 8 electrons / 4 pairs

    24. 3. Form a single bond between the central atom and each surrounding atom (each bond takes 2 electrons!)

    25. Carbon Dioxide, CO2 1. Central atom = 2. Valence electrons = 3. Form bonds.

    26. Carbon Dioxide, CO2

    27. Double and even triple bonds are commonly observed for C, N, P, O, and S

    28. Violations of the Octet Rule (Honors only) Usually occurs with B and elements of higher periods. Common exceptions are: Be, B, P, S, and Xe.

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    30. MOLECULAR GEOMETRY

    31. VSEPR Valence Shell Electron Pair Repulsion theory. Most important factor in determining geometry is relative repulsion between electron pairs.

    32. Some Common Geometries

    33. Structure Determination by VSEPR Water, H2O

    34. Structure Determination by VSEPR Ammonia, NH3 The electron pair geometry is tetrahedral.

    35. Bond Polarity HCl is POLAR because it has a positive end and a negative end. (difference in electronegativity)

    36. 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

    37. Bond Polarity Like Dissolves Like Polar dissolves Polar Nonpolar dissolves Nonpolar

    38. Hybridization of Atomic Orbitals Put on your 3-D glasses!

    39. Hybridization Uses modifications of molecular models to account for observed structures of molecules or ions Is a mixing of the native atomic orbitals to form special hybrid orbitals for bonding The special orbitals will then strive to be as far away from each other in space as they can be

    40. Types of Hybridization sp3 sp2 sp dsp3 d2sp3

    41. Types of Bonds Sigma (s) bonds End-to-end bonding There is an overlap One in every type of bond Pi (p) bonds Side-to-side bonding There is no overlap 1 in a double bond 2 in a triple bond

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    92. Thank You for your kind attention ! Questions?

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