Unit 7 (last one!!!!) Chapters 8,

Chemical Bonding and Molecular Geometry Lewis Symbols and the Octet Rule Ionic Bonding Covalent Bonding Molecular Geometry

Introduction Salt vs. Sugar solutions conductsolutions don’t electricityconduct electricity electrolytenon-electrolyte ionicmolecular Sugar and salt differ in the type of attractive forces between the atoms/ions in the compound. Chemical bond: strong attractive force that exists between atoms (or ions) in a compound

Chemical Bonds Three basic types of bonds –Ionic Electrostatic attraction between ions (cations- anions) –Covalent Sharing of electrons –Metallic Metal atoms bonded to several other atoms

Chemical Bonds Covalent Bonds: the attractive force between atoms in a molecule that results from sharing of one or more pairs of electrons –non-metals H 2 O : O H Cl 2 : Cl Cl H-O and Cl-Cl bonds result from sharing of electrons

Remember? Electron configuration for sodium: [Ne]3s 1 The 3s electron of a sodium atom is a valence electron. –electrons residing in the incomplete outer shell of an atom involved in chemical bonding and ion formation

Lewis Symbols Since valence electrons are involved in the formation of chemical bonds, it’s important to keep track of them. –electron-dot symbols or Lewis symbols simple way to depict valence electrons and track them during the formation of chemical bonds

To write Lewis Symbols You must be able to determine the number of valence electrons for the main group elements. For main group elements, the number of valence electrons for an element = group number of the element –N (group 5A) has 5 valence electrons –Br (group 7A) has 7 valence electrons

Lewis Symbols Lewis symbol has two components: –chemical symbol for the element –Plus a dot for each valence electron dots are placed on all 4 sides of the chemical symbol –all four sides of the symbol are equivalent up to 2 dots (electrons) per side (start with single dot per side of square, then double up)

Lewis Symbol for Oxygen Chemical symbol: O Group number: 6A # of valence electrons: 6 O

Lewis Symbols Draw the Lewis symbol for silicon. Chemical symbol: Si Group number: 4A # of valence electrons: 4 Si

Lewis Symbols Draw the Lewis symbol for argon. Chemical symbol: Ar Group number: 8A # of valence electrons: 8 Ar

Octet Rule The noble gases are particularly stable because their outer shell is full of electrons. –With the exception of He, all noble gases have 8 valence electrons. ns 2 np 6 Octet Rule: Atoms tend to gain, lose, or share electrons until they are surrounded by 8 valence electrons

Octet Rule The octet rule can be used to predict the charge of ions formed by main group elements as well as the structure of molecular compounds. BUT: There are many exceptions to the octet rule.

Covalent Bonding Molecular compounds are held together by covalent bonds that result from the sharing of electrons. Simplest example of a covalent bond is H Indicates 2 shared electrons = 1 bond

Covalent Bonding When 2 H atoms approach each other, electrostatic interactions occur between their respective electrons and their nuclei. –The two nuclei repel each other –The two electrons repel each other –The nuclei and the electrons attract each other.

Covalent Bonding The attractions between the nuclei and the electrons cause the electron density to concentrate between the two nuclei. The atoms in H 2 are held together by the electrostatic attraction of the two nuclei for the concentration of negative charge between them. The shared pair of electrons between the two nuclei acts as “glue”.

Covalent Bonding Lewis structures (also called electron-dot structures) can be used to represent the covalent bonds that are present in a molecule. –Symbol for each atom –Bond between atoms depicted using a solid line –Unshared electron pairs are shown around the appropriate atom

Lewis Structures Lewis structures are representations of molecules showing all valence electrons, bonding and nonbonding. NOTE: Octet of electrons around each Cl atom!

Covalent Bonding The formation of H 2 : H + H  H H or H H The Lewis structure for HF: H + F H F = H F

Covalent Bonding The bond between H and F in HF is called a single bond: –sharing of one pair of electrons In some molecules, atoms attain an octet of electrons by sharing more than one pair of electrons. –Double bond –Triple bond

Covalent Bonding Double bond: two electron pairs are shared between atoms –depicted using two lines to represent the two shared electron pairs O C O or O C O Carbon dioxide

Covalent Bonding Triple bond: three electron pairs are shared between atoms –depicted using three lines to represent the 3 pairs of shared electrons N + N N N or N N Nitrogen (N 2 )

Covalent Bonding In some molecular compounds, the bonding electrons are shared equally between the atoms in the molecule: H 2 F 2 N 2 Nonpolar covalent bond: bonding electrons are shared equally

Covalent Bonding In many molecular compounds, however, one atom attracts the bonding electrons more strongly than the other. ++ -- Fluorine attracts electrons more strongly than hydrogen. H – F ++ -- The fluorine end of the molecule has greater electron density than the hydrogen end. The H – F bond is a polar covalent bond.

Covalent Bonding Polar covalent bond: –a chemical bond in which the electrons are not shared equally one atom attracts the bonding electrons more strongly The polarity of a covalent bond can be determined using the difference in electronegativity between the two atoms.

Covalent Bonding Electronegativity: –The ability of an atom in a molecule to attract electrons to itself Range: 0.7 (Cs) (F) As electronegativity increases, the attraction that an atom has for electrons increases.

Covalent Bonding Trends to know: –Electronegativity increases: From left to right across a row From bottom to top of a column –The four most electronegative elements are: F (4.0) O (3.5) N (3.0) Cl (3.0)

Covalent Bonding Chemicals bonds exist along a continuum: The greater the difference in electronegativity between two atoms, the more polar their bond. F – F  en = 0 nonpolar covalent (en<0.5) H – F  en = 1.9 polar covalent (0.5<en<2.0) Li – F  en = 3.0 ionic (en>2.0) Ionic Bonds Polar Covalent Bonds Nonpolar Covalent Bonds

Testing Which of the following bonds is a polar covalent bond? Br-Br C-H O-H Al-Cl  EN= Polar covalent (0.5< en < 2.0)