Upload
chrystal-fox
View
214
Download
0
Embed Size (px)
DESCRIPTION
variable
Citation preview
Lesson 7Lesson 7
Chemical BondingChemical Bonding
Anything in black letters = write it in your notes (‘knowts’)
Review…Review…Elements in the same column have similar properties because…
they have the same number of valence electrons
electrons in the highest energy level
variable
Lewis Symbol – show valence electrons as dots around chemical symbol
C Na O Br
Section 1 – Ionic & Covalent Bonding
Lewis Symbols of Some Group A Elements
PeriodGroup
1A 2A 3A 4A 5A 6A 7A 8A
1
2
3
4
Octet Rule –
Atoms will gain or lose electrons to have 8 valence electrons.
metals lose e-; to form ca+ions
nonmetals tend to gain e-; to form anions
Normal Ion Charges
Ionic bonds are formed by electron transfer.
(between metal & nonmetal)
Covalent bonds are formed by electron sharing.
(between 2 nonmetals)
The more electronegative element acquires most of the e- charge
Ionic compounds consist of cations and anions arranged in repeating patterns;
NOT as molecules
A formula unit is the lowest ratio of ions in an ionic compound.
O2H2
double bondsingle bond
Each atom shares e- with another to complete the octet.
Covalent Bond
IONIC COVALENT
transfer of e- (ions!) sharing of e- (COvalent)
formula units molecules
metal & nonmetal 2 nonmetals
high melting points low melting points
(most are gases or liquids at room temp)
conduct e- when molten or dissolved (aqueous)
(solid crystals at room temp)
COMPOUNDS
Collection of water molecules
Molecule of water
Chemical formula H2O
Array of sodium ions and
chloride ions
Formula unit ofsodium chloride
Chemical formulaNaCl
FORMULA UNITS vs. MOLECULES
Covalent bonding involves a sharing of electrons between atoms but this sharing may not be equal…
Section 2 – Polar Covalent Bonding
First, a review of electronegativity….
Noble gases do not have e-neg values
Electronegativity Review…
B<H<C
A nonpolar covalent bond occurs between 2 identical atoms.
Since each atom in a nonpolar bond has the same electronegativity, the electrons are shared equally.
7 nonpolar (diatomic) molecules (H2, N2, O2, F2, Cl2, Br2, I2)
Polar Covalent Bond (polar bond) – covalent bond in which the electrons are shared unequally.
The more electronegative atom attracts electrons more and has a slightly negative charge (δ-)
δ-δ+
partial positive charge
partial negative charge
partial positive charge
partial negative charge
H Clδ-δ+
H Cl
A bond or molecule with +/- charged ends is also called a dipole. “die-pull”
The polar nature of a bond can also be shown by an arrow pointing to the more electronegative atom.
H—ClIdentify the bonds between these elements as ionic, polar covalent or nonpolar covalent.
a)H – Br b) K – Cl
c)C – O d) Li – O
e)Cl – F f) Br – Br
g)H – O h) H – Br
Place a δ- symbol above the more electronegative atom in the bond.
If the polar bonds in a molecule cancel out, the molecule is nonpolar.
When the polar bonds do NOT cancel out, the molecule is polar.
We will not cover molecular geometries and shapes except for that of water…
unshared pairs of e-
H2O
(lone or nonbonding pair)
A molecule of water has a bent shape due to the space needed by the lone pairs of e-
So far, we have talked about bonding between atoms (ionic, covalent)
Now, we will talk about bonding between molecules
The bonds between separate molecules are much weaker than ionic or covalent bonds, but without these forces there would be no liquids or solids.
Section 3 – Bonding Between Molecules
Forces between separate molecules are called Intermolecular Forces (or Van der Waals Forces)
2 types of intermolecular forces that we will discuss,
Hydrogen Bonding
Dispersion Forces (London Forces)
Hydrogen Bond – intermolecular force between H in one molecule and an electronegative atom (N, O or F) in another nearby molecule.
Hydrogen Bonds in H2O
Opposites partial poles attract
hydrogen bond
Dispersion Forces – temporary attractive forces between molecules due to electron dispersion (motion)
F2 melts at 53 K, Boils at 85 K
Without dispersion forces, nonpolar molecules could never be liquids or solids.
Molecule Melt Pt Boil Pt
F2 53 K 85 K
Cl2 171.6 K 239.1 K
Br2 265.8 K 332.0 K
I2 386.8 K 457.4 K
Dispersion Force
Cohesion – attraction to same substance.
Adhesion – attraction to different substance
butane (C4H10) 58 amu
acetone (C3H6O) 58 amu
b.p. = -0.5°C gas @ room temp
b.p. = 56.1°C liquid @ room temp
A tale of two molecules…
1. Why does acetone have a higher boiling point?
2. What would cause butane molecules to stick to each other to become a liquid
Why does acetone have a higher boiling point?
the opposite partial charges on each molecule hold the molecules together.
δ+
δ-
δ+
etc…
etc…etc…
etc… butane is a nonpolar molecule and does not have this type of intermolecular attraction
2. What would cause butane molecules to stick to each other to become a liquid?
butane, like all molecules, has electrons that are randomly moving. This produces temporary poles within the molecule.
The valence electrons in metals are loosely held and are free to move.
The properties of metals can be explained by the ‘sea of electrons’ model.
Section 4 – Metallic Bonding
Alloy – mixture of metals.Steel –
Stainless Steel –
Bronze –
Solder –
Brass –
Sterling Silver –
Amalgam –
Nichrome –
Alloy – mixture of metals.Steel – Fe & C
Stainless Steel – steel w/ Cr, Ni, or Mn
Bronze – Cu & Sn
Solder – Sn & Pb
Brass – Cu & Zn
Sterling Silver – Ag & usually Cu
Amalgam – Hg w/ other metals (Ag, Sn, Cu)
Nichrome – Ni & Cr