9/22/09

Post date: Sep 21, 2009 8:58:50 PM

Bell Ringer: Why do atoms form connections with each other?

    • Ionic bond analogy presentations

    • Properties of ionic compounds

      • Form networks, not molecules

        • Each ion is attracted equally to all those near it.

        • Smallest ratio of atoms is the formula unit for the compound

          • NaCl tells us we have one sodium for each chlorine

          • LiO2 tells us we have two oxygens for each lithium

      • Form solids at room temperature

      • Conductors?

        • Electricity is moving charges. In a network, each ion is held in place so charges cannot move.

        • When we dissolve or melt an ionic compound, the charges are free to move, making them conductive

    • Covalent Bonds

      • Atoms bond by sharing electrons

      • Occurs when atoms have nearly a complete outer electron level and similar atoms are near

        • Example: Chlorine has 7 valence electrons. If two Cl atoms come together and each share an electron, then each atom has access to 8 valence electrons! Hazzah!

      • This is where the structural formula method of showing compounds is handy!

        • We show the chemical symbol of an element and represent the valence electrons with dots. Chlorine is in the 7th column, so it has 7 valence electrons.

        • In order to fill up the outer level of electrons, Chlorine must share one of it's electrons with another Chlorine

          • The two chlorine atoms move close enough to share their 7th electrons

          • These shared electrons bond the two chlorine atoms together. We usually represent a bond (or a shared pair of electrons) with a straight line between the two atoms

        • Atoms can share more than one electron with other atoms. This happens when the atoms has only 5 or 6 valence electrons

          • Oxygen has 6 valence electrons

          • When two oxygen atoms come together, they each share 2 of their electrons to fill their outer electrons shells.

          • Again, we show each shared pair of electrons as a straight line

          • This type of bond, which has two lines between atoms (or 2 pairs of shared electrons) is called a double bond.

        • When two nitrogen atoms come together, they form a triple bond (they each share 3 electrons with the other) because nitrogen has 5 valence electrons

        • The more bonds there are between two atoms, the harder it is to break them apart (e.g. the bond between two nitrogen atoms is stronger than that between two oxygen atoms)

      • Atoms don't always share equally

        • When two different elements bond together, sometimes the electrons are more attracted to one nucleus than the other. This is called a polar covalent bond. (when electrons are equally attracted to each nucleus, we call it a nonpolar covalent bond)

        • Electrons are typically more attracted to elements that are farther to the right and higher up on the periodic table of elements.

  • Covalent bonds homework

      • Draw the structural diagrams (using straight lines for bonds) for the following compounds:

        • I2

        • S2

        • H20

        • NH3

        • P2

      • Refer to pages 186-187 for examples and help