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What is a Molecule ?
• Molecule
– A collection of atoms bonded together
• Elemental molecules
– Atoms from same element
• Diatomic: H2, N2, F2 , O2, I2, Cl2 , Br2
• Polyatomic: P4, S8
What is a Molecule
• Molecules of Compounds
– Atoms of different elements
• Simple vs. Complicated
• Small vs. extremely large
Properties of Molecules
• Molecules have different properties than their elemental component.
Element Properties
O Odorless, colorless very reactive toxic gas
H Odorless colorless gas reactive
Properties of Molecules
Molecule Properties
O2Colorless odorless gas moderately reactive not flammable
H2Colorless odorless gas moderately reactive flammable
H2OColorless odorless liquid, low reactivity
H2O2Colorless odorless liquid high reactivity
Why do molecules form?
• To be like the noble gases
– The noble gases are perfect hence they do not react with anything
• They have 8 valence electrons in their outer most shell
Valence Electrons?
• The outermost electrons. Determines the chemical properties of an element
– The only electrons that bond
– The roman numerals at the top of each group A element
– Most elements prefer to have 8 electrons this is called the octet rule. Why?
The Octet Rule
Atoms tend to gain, lose, or share electrons until they have eight valence electrons.
Electron Dot diagrams
• A way of keeping track of valence electrons.
• How to write them– Write the symbol.– Put one dot for each
valence electron– Don’t pair up until you have
too.
X
The Electron Dot diagram for Nitrogen
Nitrogen has 5 valence electrons.
First we write the symbol.
NThen add 1 electron at a time to each side.
Until they are forced to pair up.
Electron Configurations for Cations
• Metals lose electrons to attain noble gas configuration.
• They make positive ions.
Electron Dots For Cations
• Metals will have few valence electrons• These will come off• Forming positive ions
Ca+2
Electron Configurations for Anions
• Nonmetals gain electrons to attain noble gas configuration.
• They make negative ions.
Electron Dots For Anions
• Nonmetals will have many valence .electrons.
• They will gain electrons to fill outer shell.
P P-3
Stable Electron Configurations
• All atoms react to achieve noble gas configuration.
• Noble gases 8 valence electrons .• Also called the octet rule.
Ar
How does H2 form?
++
• The nuclei repel
• But they are attracted to electrons
• They share the electrons
Covalent bonds• Nonmetals hold onto their valence
electrons.• They can’t give away electrons to bond.• Still want noble gas configuration.• Get it by sharing valence electrons with
each other.• By sharing both atoms get to count the
electrons toward noble gas configuration.
Covalent bonding
Fluorine has seven valence electrons A second atom also has seven By sharing electrons
F F
How to show how they formed• It’s like a jigsaw puzzle.
• I have to tell you what the final formula is.
• You put the pieces together to end up with the right formula.
• For example- show how water is formed with covalent bonds.
Water
H
O
Each hydrogen has 1 valence electron
Each hydrogen wants 1 more
The oxygen has 6 valence electrons
The oxygen wants 2 more
They share to make each other happy
Multiple Bonds
• Sometimes atoms share more than one pair of valence electrons.
• A double bond is when atoms share two pair (4) of electrons.
• A triple bond is when atoms share three pair (6) of electrons.
Carbon dioxide• CO2 - Carbon is central
atom ( I have to tell you)
• Carbon has 4 valence electrons
• Wants 4 more• Oxygen has 6 valence
electrons• Wants 2 more
O
C
Carbon dioxide The only solution is to share more Requires two double bonds Each atom gets to count all the atoms in
the bond
OCO
Carbon dioxide The only solution is to share more Requires two double bonds Each atom gets to count all the atoms in
the bond
OCO8 valence electrons
Carbon dioxide The only solution is to share more Requires two double bonds Each atom gets to count all the atoms in
the bond
OCO8 valence electrons
Carbon dioxide The only solution is to share more Requires two double bonds Each atom gets to count all the atoms in
the bond
OCO
8 valence electrons
– Hydrogen 2 valence e-
– Groups 1,2,3 get 2,4,6 valence e-
– Expanded octet more than 8 valence e-
(e.g. S, P, Xe)
– Radicals odd # of valence e-
• Exceptions:
A. Octet Rule
F B FFH O HN O
Very unstable!!
F FF S FF F
B. Drawing Lewis Diagrams• Find total # of valence e-.
• Arrange atoms - singular atom is usually in the middle.
• Form bonds between atoms (2 e-).
• Distribute remaining e- to give each atom an octet (recall exceptions).
• If there aren’t enough e- to go around, form double or triple bonds.
C. Polyatomic Ions
• To find total # of valence e-:– Add 1e- for each negative charge.– Subtract 1e- for each positive charge.
• Place brackets around the ion and label the charge.
D. Resonance Structures
• Molecules that can’t be correctly represented by a single Lewis diagram.
• Actual structure is an average of all the possibilities.
• Show possible structures separated by a double-headed arrow.
Atoms and ions
• Atoms are electrically neutral.
• Same number of protons and electrons.
• Ions are atoms, or groups of atoms, with a charge.
Atoms and Ions
• Different numbers of protons and electrons.
• Only electrons can move.
• Gain or lose electrons.
Anion
• A negative ion.• Has gained electrons.• Non metals can gain electrons.• Charge is written as a super script on
the right.
F-1 Has gained one electron
O-2 Has gained two electrons
Cations
Positive ions. Formed by losing electrons. More protons than electrons. Metals form cations.
K+1 Has lost one electron
Ca+2 Has lost two electrons
Two Types of Compounds
• Molecular compounds
– Made of molecules.
– Made by joining nonmetal atoms together into molecules.
Two Types of Compounds
• Ionic Compounds
– Made of cations and anions.– Metals and nonmetals.– The electrons lost by the cation are gained
by the anion.– The cation and anions surround each
other.– Smallest piece is a FORMULA UNIT.
Formula Unit• The smallest whole number ratio of
atoms in an ionic compound.
• Ions surround each other so you can’t say which is hooked to which.
Two Types of Compounds
Smallest piece
Melting Point
State
Types of elements
Formula Unit Molecule
Metal and Nonmetal
Nonmetals
solidSolid, liquid or gas
High >300ºC Low <300ºC
Ionic Molecular
Chemical Formulas
• Shows the kind and number of atoms in the smallest piece of a substance.
• Molecular formula- number and kinds of atoms in a molecule.
• CO2
Charges on ions
• For most of the Group A elements, the Periodic Table can tell what kind of ion they will form from their location.
• Elements in the same group have similar properties.
• Including the charge when they are ions.
Naming ions
• We will use the systematic way.
• Cation- if the charge is always the same (Group A) just write the name of the metal.
• Transition metals can have more than one type of charge.– Indicate the charge with roman numerals in
parenthesis.
Write Formulas for these• Potassium ion• Magnesium ion • Copper (II) ion• Chromium (VI) ion• Barium ion• Mercury (II) ion
Polyatomic ions
• Groups of atoms that stay together and have a charge.– Acetate C2H3O2
-1
– Nitrate NO3-1
– Nitrite NO2-1
– Hydroxide OH-1
– Permanganate MnO4-1
– Cyanide CN-1
Polyatomic ions
• Sulfate SO4-2
• Sulfite SO3-2
• Carbonate CO3-2
• Chromate CrO4-2
• Dichromate Cr2O7-
2
• Phosphate PO4-3
• Phosphite PO3-3
• Ammonium NH4+1
Naming Binary Ionic Compounds
• Binary Compounds - 2 elements.• Ionic - a cation and an anion.• To write the names just name the two
ions.• Easy with Representative elements.• Group A
• NaCl = Na+ Cl- = sodium chloride
• MgBr2 = Mg+2 Br- = magnesium bromide
Naming Binary Ionic Compounds
• The problem comes with the transition metals.
• Need to figure out their charges.• The compound must be neutral.• same number of + and – charges.• Use the anion to determine the charge
on the positive ion.
Naming Binary Ionic Compounds
• Write the name of CuO• Need the charge of Cu• O is -2• copper must be +2• Copper (II) oxide
• Name CoCl3
• Cl is -1 and there are three of them = -3• Co must be +3 Cobalt (III) chloride
Naming Binary Ionic Compounds
• Write the name of Cu2S.
• Since S is -2, the Cu2 must be +2, so each one is +1.
• copper (I) sulfide• Fe2O3 • Each O is -2 • Fe must be = + 3• iron (III) oxide
Ternary Ionic Compounds
• Will have polyatomic ions• At least three elements• name the ions
• NaNO3
• CaSO4
• CuSO3
• (NH4)2O
Writing Formulas
• The charges have to add up to zero.• Get charges on pieces.• Cations from name of table.• Anions from table or polyatomic.• Balance the charges.• Put polyatomics in parenthesis.
Writing Formulas
• Write the formula for calcium chloride.
• Calcium is Ca+2
• Chloride is Cl-1
• Ca+2 Cl-1
• CaCl2
Write the formulas for these• Lithium sulfide• tin (II) oxide• tin (IV) oxide• Magnesium fluoride• Copper (II) sulfate• Iron (III) phosphide• gallium nitrate• Iron (III) sulfide
Things to look for
• If cations have (), the number is their charge.
• If anions end in -ide they are probably off the periodic table (Monoatomic)
• If anion ends in -ate or -ite it is polyatomic
Molecular compounds
• made of just nonmetals
• smallest piece is a molecule
• can’t be held together because of opposite charges
• can’t use charges to figure out how many of each atom
Easier
• Ionic compounds use charges to determine how many of each
– Have to figure out charges
– Have to figure out numbers
• Molecular compounds name tells you the number of atoms
– Uses prefixes to tell you the number
Prefixes
• 9 nona-
• 10 deca-
• One exception is we don’t write mono- if there is only one of the first element.
Write formulas for these
• diphosphorus pentoxide• tetraiodide nonoxide• sulfur hexaflouride• nitrogen trioxide• Carbon tetrahydride• phosphorus trifluoride• aluminum chloride
Acids• Compounds that give off hydrogen ions
when dissolved in water
• Must have H in them (somewhere)
• will always be some H next to an anion
• The anion determines the name.
Naming acids• If the anion attached to hydrogen ends in -ide, put the
prefix hydro- and change -ide to -ic acid
• HCl - hydrogen ion and chloride ion
• hydrochloric acid
• H2S hydrogen ion and sulfide ion
• hydrosulfuric acid
Naming Acids
• If the anion has oxygen in it• it ends in -ate or -ite• change the suffix -ate to -ic acid
• HNO3 Hydrogen and nitrate ions
• Nitric acid• change the suffix -ite to -ous acid
• HNO2 Hydrogen and nitrite ions
• Nitrous acid
Writing Formulas
• Hydrogen will always be first
• name will tell you the anion
• make the charges cancel out.
• Starts with hydro- no oxygen, -ide
• no hydro, -ate comes from -ic, -ite comes from -ous