Presentation on theme: "Chemical Bonding Chapter 7. The Octet Rule Atoms tend to gain, lose, or share electrons in order to get a full set of valence electrons. “octet” – most."— Presentation transcript:
The Octet Rule Atoms tend to gain, lose, or share electrons in order to get a full set of valence electrons. “octet” – most atoms need 8 valence electrons for a full set Gaining or losing ions = ionic bonding Sharing = covalent bonding http://www.youtube.com/watch?v=_M9khs8 7xQ8&index=2&list=PL6ioyKxGFb1CnJ4tTw CcLxMCda7TXQDAV
Types of Ions Monatomic = “one-atom” –H +, Ca 2+, Br -, N 3- Polyatomic = “many-atoms” –NH 4 +, OH -, SO 4 2-,
Lewis Dot Structures Developed by American chemist Gilbert Lewis (1875-1946) Valence electrons represented by dots around the element symbol –No more than two dots per side Can be used to show rearrangement of electrons during chemical reactions
Binary Ionic Compounds Contain ions of only two elements Formula: Cation written first, then anion –Charges of ions written as superscripts, # of atoms in a compound written as subscripts Ratio written in lowest terms = empirical formula
Binary Ionic Compounds Draw the Lewis Dot Structures for sodium and chlorine Using an arrow, identify how the transfer of 1 electron can create 2 new ions
Sodium transfers an electron to chlorine. Sodium becomes a positive ion with a +1 charge. Chlorine becomes a negative ion with a -1 charge.
Binary Ionic Compounds Na + + Cl - NaCl The total (net) charge on the compound should be zero. You must determine how many of each ion will need to be in the compound to balance out the charges.
The Crisscross Method for Writing Compound Formulas Write the ion symbols (with their charges as superscripts) for the cation and anion Criss-cross the two charges, moving them diagonally from one ion’s superscript to the other ion’s subscript –Drop the sign!
Crisscross Method Practice magnesium ion and chloride ion Mg 2+ Cl -1 Mg Cl = MgCl 2
Compound Formula Practice magnesium ion + oxide ion Mg 2+ + O 2- Mg 2+ + O 2- MgO calcium ion and bromide ion Ca 2+ + Br - strontium ion and nitride ion Sr 2+ + N 3- Mg 2 O 2 CaBr 2 Sr 3 N 2
Naming Ionic Compounds Name the cation using its element name. Name the anion by dropping the ending of the element name and adding –ide. Ca 3 P 2 calcium phosphide If the anion is polyatomic, simply name it using the ion’s name Mg 3 (PO 4 ) 2 magnesium phosphate
Naming Ionic Compounds If the cation has more than one valence (it can have different charges), indicate the charge using roman numerals in parenthesis after the cation name. FeO = iron (II) oxide Fe 2 O 3 = iron (III) oxide
Covalent Bonds Formed by a shared pair of electrons between two atoms Make up molecules (which make up molecular substances) Between nonmetals
Formulas Empirical formula = lowest ratio of types of atoms in a compound Molecular formula = exact number of atoms of each element in a single molecule of a compound Structural formula = how atoms are bonded together
Formula Example: Glucose molecular formula C 6 H 12 O 6 empirical formula CH 2 O structural formula
Lewis Dot Structures For molecules: –Show pairs of electrons that are shared between atoms using 2 dots or 1 dash. –Leave electrons not involved in bonds as dots.
Lewis Dot Structures Draw the Lewis dot structures for: F 2 NH 3 H 2 O H 2 CO C 2 H 2
Exceptions to the Octet Rule Less than an octet –BF 3 More than an octet –SF 4 Odd number of electrons –NO
Properties of Covalent Bonds Polar covalent bonds –Unequal sharing b/c of electronegativity difference –More electronegative atom gets slightly negative charge (higher electron density) –Less electronegative atom gets slightly positive charge (lower electron density) Nonpolar covalent bonds –No electronegativity difference –Share electrons equally
Properties of Covalent Bonds Low melting points Soft, flexible Many won’t dissolve in water –Cannot conduct electricity even if they do dissolve (due to no charges being present)
Naming Covalent Compounds (Molecules) Prefixes must be added to tell the ratio of atoms in the compound. mono-1 di-2 tri-3 tetra-4 penta-5 hexa-6 hepta-7 octa-8 nona-9 deca-10
Naming Covalent Compounds (Molecules) Most electronegative element written last in formula and name. –Drop ending of this element’s name and add –ide. Si 2 Br 6 disilicon hexabromide Don’t include mono- prefix for 1 st element listed. CF 4 carbon tetrafluoride
Naming Covalent Compounds (Molecules) Shorten prefixes to make names easier to say. H2OH2O dihydrogen monoxide not dihydrogen monooxide Sometimes common names are used. O 2 = oxygen NH 3 = ammonia
Naming Hydrates Hydrates are ionic compounds that absorb water into their solid structures. –Anhydrous substances are water-free Naming: –Name the ionic compound –Using the prefixes that you have learned, identify the degree of hydration MgSO 4 7 H 2 O magnesium sulfate heptahydrate
Naming Acids Acids are molecular substances that dissolve in water to produce hydrogen ions (H + ). –Can separate into ions even though they are molecular compounds Hydrogen is the cation in acids.
Naming Acids If the anion ends in –ide –Begin the name with hydro- –Add the root name of the anion, but change the ending from –ide to –ic –Add the word acid HBr HCl H2SH2S Hydrochloric acid Hydrobromic acid Hydrosulfuric acid
Naming Acids If the anion ends in –ate –Do NOT begin with hydro- –Keep the root of the anion, but change the ending from –ate to –ic –Add the word acid HNO 3 H 3 PO 4 HC 2 H 3 O 2 nitric acid phosphoric acid acetic acid
Naming Acids If the anion ends in –ite –Do NOT begin with hydro- –Keep the root of the anion, but change the ending from –ite to –ous –Add the word acid HNO 2 H 2 SO 3 HClO 2 nitrous acid sulfurous acid chlorous acid