Molecules
description
Transcript of Molecules
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Molecules
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Objectives
• Write the electron dot structure for an atom.• Explain how covalent bonds form molecules.
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Electron Dot Structures
Dots can be used to represent v/e.
Write electron dot structures for K, P, S, and Br.
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Covalent Bonds
How do non-metal atoms bond together?
Example: chlorine gas, Cl2 2e-
Both nuclei are attracted to the
molecular orbital.
Both nuclei repel each other.
This equilibrium establishesthe bond length.
+ +
covalent bond: an attractionbetween non-metal atomssharing v/e in a molecular orbital
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Molecules
molecule: group of neutral atoms held together with covalent bonds
molecular formula: indicates number and kinds of atoms in a molecule
H2O2 OO
HHhydrogenperoxide
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Objective
• Be able to draw a Lewis diagram when given a molecular formula.
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Lewis Structures
Draw Lewis structures for molecules containing the following elements: • nitrogen and fluorine• sulfur and chlorine• hydrogen and oxygen (water)
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Lewis StructuresDouble and triple covalent bonds can also form:• H2CO (double)• ClCN (triple)
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Objective
• Understand the concept of VSEPR theory.• Use VSEPR theory to determine the shape(s)
of a molecule.• Be able to identify common molecular shapes.
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VSEPR TheoryValence Shell Electron Pair Repulsion Theory: each pair of electrons (bonding pair or unshared pair) willrepel; molecule will adjust shape to maximize the anglesbetween each pair
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Molecular Shapes
CH4
methanetetrahedral(4 single)
NH3
ammoniapyramidal(3 single)
H2Sdihydrogen
sulfide
bent(2 single)
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Molecular Shapes
trignonal planar(2 single, 1 double)H2CO
formaldehdye
CO2
carbon dioxide
linear(2 double)
HCNhydrogencyanide
linear(1 single, 1 triple)
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Objectives
• Be able to determine the polarity of a covalent bond using a table of electronegativities.
• Be able to determine the polarity of a molecule based on the shape(s) present and the polarity of the covalent bonds within the molecule.
• Understand and apply the concept of molecular symmetry.
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Bond Polarity
Electrons are not always shared evenly between atoms.
H FAtom with higher electronegativityattracts the e- pair more!
-+ polar bond: unevenlyshared covalent bond
Br Br non-polar bond: evenlyshared covalent bond
2.2 4.0
3.0 3.0
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Bond Polarity
What is the polarity of a H – C bond?A bond between H – O?A bond between K – Cl?
0.5 – 1.9 = polar covalent (uneven sharing)
Electronegativity difference determines bond polarity:0.0 – 0.4 = non-polar covalent (even sharing)
2.0 – above = ionic bond (e- transfer)
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Molecular Polarity
Bond polarity and molecular shape must be considered when determining the polarity of a molecule.
dipole: a polar molecule with polarbonds in which partial charges canbe separated
+
-
-
+NH3 H2O
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Molecular Polarity
A molecule with all non-polar bondsis called a non-polar molecule.
Hydrocarbons (with just H and C) are always non-polar molecules.
Due to symmetry, a moleculewith polar bonds can be anon-polar molecule.
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Objectives
• Be able to determine the type of intermolecular bonding present in a molecular compound.
• Be able to predict the state of a molecular compound based on the type of intermolecular bonding present and the mass of the molecules.
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Intermolecular BondsCovalent bonds form within molecules (polar or non-polar).intermolecular bond: a bond between moleculesState of substance determined by strength
Non-polar molecules tend to be gaseous because they don’t attract each other.Examples: O2, N2, CO2
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London Dispersion Forces
London dispersion force: an intermolecular forcecaused by random distributions of electronsBrief partial charges cause attraction—more electronscauses stronger bonding!
Generally, in a non-polar molecule…gas: < 70 e-liquid:70 e- to 100 e-solid: > 100 e-
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Dipole Interactions
• dipole interaction: an attraction between dipoles that always results in a liquid or solid
• London force strength will determine state:liquid: < 100 e-solid: > 100 e-
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Hydrogen Bonds• hydrogen bond: a particularly strong dipole interaction that occurs between molecules containing –OH or –NH • always liquid or solid (depends on strength of London forces)
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Hydrogen Bonds
H-bonds occur in both H2O and DNA.
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Objectives
• Be able to draw a Lewis diagram for a polyatomic ion.
• Be able to draw a Lewis diagram for a molecule or ion containing coordinate covalent bonds.
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Molecular NamesCovalent Compound orMolecular Compound NomenclaturePrefixes1 mono- (mon-)2 di-3 tri-4 tetra- (tetr-)5 penta- (pent-)6 hexa- (hex-)7 hepta- (hept-)8 octa- (oct-)9 nona- (non-)10 deca- (dec-)
• use prefixes that match the number of atoms in the formula• end the second name with “-ide”• never start the first name with “mono-“• The o or a at the end of a prefix is often dropped when the word following the prefix begins with another vowel; however, it is okay to have an a before the i in iodide
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Diatomic Elements
• Many elements exist a paired atom molecules• H2, N2, O2, and halogens
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Macromoleculesmacromolecule: a huge moleculeC and Si produce macromolecules because eachelement can form four bonds per atom
diamond (C)
quartz (SiO2)
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Properties of Covalent Compounds
• Composed on non-metals only• Covalent bonding—exist as molecules.• Solid, liquid, or gas• Usually non-conductors of heat and electricity
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Polyatomic Ions and Coordinate Covalent Bonds (extra-credit)
Polyatomic ions are essentially charged molecules(usually with additional electrons).
Draw Lewis structures for NO2– and OH–
coordinate covalent bond: a bond that forms whenone atom donates both electrons to a bond
Examples: CO, SO3, CO32–