chemical bonding chapter 7. the octet rule atoms tend to gain, lose, or share electrons in order to...
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=_M9khs87xQ8&index=2&list=PL6ioyKxGFb1CnJ4tTwCcLxMCda7TXQDAV
Properties of Ionic Compounds
• High melting points
• Brittle
• Usually salts
• Many dissolve in water– Can conduct electricity because ions separate
and are charged in the solution
Ionic Bonds
• Positively charged ion attracted to negatively charged ion– Positive ions = cations– Negative ions = anions
• Metal + nonmetal– Metals form cations– Nonmetals form anions
Types of Ions
• Monatomic = “one-atom”– H+, Ca2+, Br-, N3-
• Polyatomic = “many-atoms”– NH4
+, OH-, SO42-,
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!
Compound Formula Practice
magnesium ion + oxide ion Mg2+ + O2-
Mg2+ + O2- MgO
calcium ion and bromide ion Ca2+ + Br-
strontium ion and nitride ion Sr2+ + N3-
Mg2O2
CaBr2
Sr3N2
Naming Ionic Compounds
• Name the cation using its element name.• Name the anion by dropping the ending of
the element name and adding –ide.
Ca3P2
calcium phosphide• If the anion is polyatomic, simply name it
using the ion’s name
Mg3(PO4)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
Fe2O3 = 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
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.
Exceptions to the Octet Rule
• Less than an octet– BF3
• More than an octet– SF4
• 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.
Si2Br6
disilicon hexabromide
• Don’t include mono- prefix for 1st element listed.
CF4
carbon tetrafluoride
Naming Covalent Compounds (Molecules)
• Shorten prefixes to make names easier to say.
H2O
dihydrogen monoxide
not dihydrogen monooxide
• Sometimes common names are used.O2 = oxygen
NH3 = 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
MgSO4 7 H2Omagnesium 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
H2S
Hydrochloric acidHydrobromic 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
HNO3
H3PO4
HC2H3O2
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
HNO2
H2SO3
HClO2
nitrous acid
sulfurous acid
chlorous acid
_______ate_______ide ________ite
hydro____icacid
______icacid
______ousacid
anion
acid
add hydrogen ions (H+)