Chemical Bonding

General information All bonds are the result of a (+) and (-) attraction. All bonds are the result of a (+) and (-) attraction. Only valence e- are directly involved in bonding Only valence e- are directly involved in bonding A pair of valence e- makes one bond. A pair of valence e- makes one bond. Transitional metals may have their valence e- come from the last two E.L. Transitional metals may have their valence e- come from the last two E.L.

Two types of bonds Ionic Bonds Ionic Bonds –Occur between metals and nonmetals –Occur between an element with a low IE and one with a high IE –Metal loses e- to the nonmetal –Attraction is between an anion and a cation, (so ions are present). Covalent bonds Covalent bonds –Occurs between nonmetals only –Both elements have high IE –Valence e- are shared –No ions are formed but dipoles may form.

Properties of ionic compounds The ions combine in the lowest ratio empirical formulas. Ionic charges are used to determine the empirical formula. Ex CaCl 2, K 2 S The ions combine in the lowest ratio empirical formulas. Ionic charges are used to determine the empirical formula. Ex CaCl 2, K 2 S Ionic compounds form crystals Ionic compounds form crystals Properties of ionic crystals Properties of ionic crystals –They have a distinct geometric arrangement to the ions –They are brittle and so shatter when hit.

Properties of Ionic crystals continued They have very high melting points. They have very high melting points. Many dissolve in water to form free moving ions which conduct electricity. Many dissolve in water to form free moving ions which conduct electricity. In the molten state, they are partial or semi conductors. The ions can only slide over each other very slowly. In the molten state, they are partial or semi conductors. The ions can only slide over each other very slowly. They are not conductive as solids. There is no movement in the ions. They are not conductive as solids. There is no movement in the ions.

Model of an NaCl crystal

Photos of ionic crystals

Photos ionic crystals

Properties of all covalent compounds Covalent compounds: Covalent compounds: –Rarely break apart to form ions in water. –Usually they do not conduct electricity under any circumstance.

Molecular geometry The shape of a molecule depends on: The shape of a molecule depends on: –The number of outer atoms. –The central atom’s number of valence e-. Specifically, are there any unbonded valence e?

Some examples of shapes

Pyramidal shape molecule

Covalent compounds can be divided into 2 categories Polar covalent Polar covalent –Molecules have a (+) and (-) end (dipole) to them. –A dipole is created when there is a difference of 0.5 or greater in electronegativity –A dipole is created when F,O,N, or Cl is in the molecule and they are not bonded to an identical element. Nonpolar covalent Nonpolar covalent –Molecules have no charge on them –Either there are no dipoles, or the dipoles cancel out.

Determining whether a covalent compound is polar or nonpolar. First: determine if there are any bonds with dipoles in the molecule. First: determine if there are any bonds with dipoles in the molecule. Second: determine the shape of the molecule. Second: determine the shape of the molecule. Third: determine if the dipoles cancel on the molecule. If they cancel, the molecule is nonpolar. If they add up to create a (+) or (-) end, then the molecule is polar. Third: determine if the dipoles cancel on the molecule. If they cancel, the molecule is nonpolar. If they add up to create a (+) or (-) end, then the molecule is polar.

First step: determining dipoles –A dipole is a bond with a (+) end and a (-) end. –Dipoles are created when F,O,N, and Cl, are bonded to any other element other than itself. –Exception: N bonded to Cl does not make a dipole.

Dipoles continued Determine which bonds have dipoles;Determine which bonds have dipoles; N-F, C-S, N-Br, O=O, N-Cl, H-S, N-H, H-O The bonds with dipoles are: N-F, N-Br, N-H, H-OThe bonds with dipoles are: N-F, N-Br, N-H, H-O The element which is more electronegative will have a partial (-) charge, the other element will have a partial (+) charge.The element which is more electronegative will have a partial (-) charge, the other element will have a partial (+) charge.

Second step. Determine the shape Use your chart on molecular geometry to determine the shape of the molecule. Use your chart on molecular geometry to determine the shape of the molecule. Note the number of outer atoms and if the central atom has an unbonded pair of valence electrons. Example: BH 3 and NH 3 BH 3 is planar, but NH 3 is pyramidal because the “N” has an unbonded pair of e- Note the number of outer atoms and if the central atom has an unbonded pair of valence electrons. Example: BH 3 and NH 3 BH 3 is planar, but NH 3 is pyramidal because the “N” has an unbonded pair of e-

Third and final step. Determining whether a molecule is polar or nonpolar. Applying the rules: Applying the rules: Example: NF 3 Example: NF 3 Time to use the whiteboard!!!!! Time to use the whiteboard!!!!! Leave space in your notes to write several examples. Leave space in your notes to write several examples.

Differences between the physical properties of polar and nonpolar molecules Nonpolar molecules Nonpolar molecules –Not soluble in water, makes a separate layer from water. –Volatile- easily evaporate –Low melting and boiling points. –Not very dense –At room temp.: small molecules are gases, larger ones are liquids or soft solids. Polar molecules Polar molecules –Soluble in water –Low volatility- do not evaporate easily –Relatively high melting and boiling points for their molar mass. –More dense than non polar –At room temp. very few are gases, most are liquids and solids. –Solids have crystal lattices

Examples of polar and nonpolar substances Polar substances Polar substances –Water –Vinegar –Blood –Alcohols –HCl –Acetone –Sugars –hair Nonpolar substances Nonpolar substances –Oils –Waxes –Most fuels, ex. Gasoline, methane –paint thinner –Sulfur. –Oil base paints