Valence ElectronsValence Electrons  Electrons in the outermost energy shell  Na has 1 valence electron  Mg has 2 valence electrons  Electrons involved in chemical reactions and form chemical bonds with other atoms.  Gained or lost by atoms in a reactions ***Except for transition elements, the number of valence electrons for an atom = group number in the periodic table !!!!

Octet RuleOctet Rule  Main group atoms gain/lose electrons in order to achieve a total of 8 electrons in their outermost electron shell.  Therefore, 8 valence electrons = satisfied atom.  Noble Gases = full octet, not reactive since they don’t need to gain/lose electrons.

Example 1: MgExample 1: Mg  What does it have to do to achieve 8 electrons?

Example 2: ClExample 2: Cl  What is it going to do to achieve 8 electrons?

Practice…. 1)N6) S 2)O7) Si 3)C8) B 4)H 5)Na

Lewis Dot StructuresLewis Dot Structures  Named for G.N. Lewis, who developed the octet rule.  Method of representing valence electrons for an atom.  Valence electrons are represented by dots around an element symbol  Electrons shown in pairs, any unpaired electrons are available to be gained/lost in a chemical reaction  Element symbol – nucleus and inner electrons around the atom  Used to describe chemical bonding.

Cations/Anions  What kind of atoms LOSE electrons and become positive?  What kind of atoms GAIN electrons and become negative?

Diatomic MoleculesDiatomic Molecules  Always exist in a chemical bond with another atom, even if the atom is of the same element  H 2, N 2, O 2, F 2, Cl 2, Br 2, I 2

Lewis Theory of Chemical BondingLewis Theory of Chemical Bonding 1)Valence electrons participate in chemical bonding. 2) Ionic Bonds form between metals and nonmetals. Valence electrons are transferred to nonmetals and ions are formed. 3) Covalent Bonds form between nonmetals. Bond involves the sharing of valence electrons 4)Most atoms aspire to noble gas configurations  Octet rule– 8 valence electrons  Exceptions: transition elements, H/Li/Be satisfied with 2

Electronegativity  Ability of an atom to draw electrons to it from another atom  Increase electronegativity, increase electron attraction for that atom  How does this characteristic relate to atomic radii?  Basic Trend: (why? )

What does this atomic characteristic have to do with chemical bonding?  Difference in electronegativity between 2 atoms determines type of covalent bond.  Small/no difference—nonpolar covalent  Slight difference—polar covalent  Huge difference—ionic bond

Example 1: Cl, Mg, SiExample 1: Cl, Mg, Si  Arrange in order of DECREASING electronegativity:

Example 2: As, N, SbExample 2: As, N, Sb  Arrange in order of INCREASING electronegativity.

Homework  Read over lab procedure  p #3, 4, 9, 10, 23, 39-40