Descriptive Chemistry
Cosmic abundance of the Elements
Element
H
He
Atomic No.
1
2
Rel. Abundance
3.5 x 108
3.5 x 107
C
N
O
F
6
7
8
9
80,000
160,000
210,000
90
Ne
Na
Mg
10
11
12
10,000
462
8,870
Al
Si
13
14
882
10,000
Cr
Mn
Fe
24
25
26
95
77
18,300
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Occurrence of the Elements on Earth
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Abundance of elements
Terrestrial abundance; On Earth!
Element Atomic No.
H
1
He
2
C
6
N
7
O
8
F
9
PPM
8,700
0.003
800
300
495,000
270
Rank
9
Na
Mg
Al
Si
11
12
13
14
26,000
19,000
75,000
257,000
6
8
3
2
K
Ca
Ti
Fe
19
20
22
26
24,000
34,000
5,800
47,000
7
5
10
4
1
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Abundance of elements
Terrestrial abundance
O
Si 27.72%
46.6%
10
9
8
% by weight
7
6
5
4
3
2
1
0
H
B
C
N
F
Na Mg Al
P
O
S
Cl
K
Ca
Ti
V
Cr Mn Fe Co Cu Zn
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Si
Se Rb Mo Sn
I
4
Isolation of the Elements From their Natural state
Sources of Gas-Phase Nonmetals:
1) HEAVY Noble Gases, O2, N2
2) Helium
3) Halogens
a)
b)
4) Hydrogen
a) Steam Reforming
CH4(g) + H2O(g) ! CO(g) + 3 H2(g)
CO(g) + H2O(g) ! CO2(g) + H2(g)
b)
c)
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Isolation of the Elements From their Natural state
Sources of Other Nonmetals:
1) Carbon
2) Sulfur
3) P
4) Se, Te
5)! Boron
6)! Metalloids
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Periodic Trends Summary
n
Zeff
1) Atomic Size
2) ionization energy
3) electron affinity
both + and - ; halogens most negative
4) metallic character
- metals lose electrons
- nonmetals gain electrons
increasing metallic character
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Valence in Period II
N Max. of 4 bonds, 1 lone pair; usually has a valence of 3
HNO3
(4 bonds to N)
NH3, NCl3, CH3NH2, HNO2
O Max. of 4 bonds, 2 lone pairs; usually has a valence of 2
H2O, OF2, H2C=O
C Max. of 4 bonds, 0 lone pairs; always has a valence of 4
can gain 4 or lose 4 electrons to make an octet
So carbon always makes 4 bonds
CH4
(4 single bonds)
O=C=O
(2 double bonds)
H-C!C-H (1 single + 1 triple bond)
H 2N
H 2N
C=O
(2 single + 1 double bond)
(urea)
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Bonding in Period II vs. III
Valence: Heavy Group V elements can gain 3 electrons or lose 5 to make an
octet; " expect either 5 covalent bonds (PF5) or 3 covalent bonds (PF3)
Multiple Bonds: elements past the second row are too big to allow good
sidewise overlap of p-orbitals
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Difference in Bonding: Period 2 vs. 3
O2 is molecular (O=O, has a double bond)
But S forms rings with single bonds (e.g., S8)
N2 has a triple bond (:N!N:, very stable)
But phosphorus is found in several forms (white, red, black),
all of which have only single bonds.
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Chemistry of Hydrogen
Forms binary hydrides (hydrogen and one other element)
ionic, metallic or molecular hydrides
Molecular hydrides: H bonded covalently to a non-metal
Examples: HCl, HBr, CH4, H2Se, SiH4, …
acidic or show no acid-base properties
acid strength increases from left to right
PH3 < H2S < HCl
acid strength increases going down family
H2O < H2S < H2Se < H2Te
NH3 and other amines are basic (lone pair)
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Ionic Hydrides: Hydrogen plus a group 1 or 2 metal
Examples: LiH, LiAlH4, CaH2
these metals are much less electronegative than hydrogen;
H gains electrons, produces a
hydride anion H **high melting ionic solids, very basic, strong reducing agents
CaH2(s) + H2O (l) # H2(g) + Ca(OH)2 (aq)
Metallic hydrides: H plus a transition metal
many retain their metallic characteristics
If ratio of M:H is not fixed;
hydrogen atoms are absorbed
into the interstices of the metal
lattice
If M-H ratio is fixed;
organometallic metal
hydrides or metaldihydrogen complexes
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Fe(H2)(H)2 (PR3) 3
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Group III; B, Al, Ga, In, Tl
Properties of Boron
"!Only group 3 element that is a nonmetal
"!Network covalent solid (mp=2300ºC)
Sources of Boron:
Rare, found in one mineral
Borax: Na2B4O7.(OH2)10
Chemistry of Boron:
"!Oxide B2O3 is used to make Pyrex glass
(anhydride of boric acid H3BO3)
"!Boranes (B plus H) are
electron deficient (6 v.e.)
"!Borohydrides (borane anions); source of H! ions
"!Boron Nitride, BN
Boron is one element to the
left of carbon.
Nitrogen is one element to the
right of carbon.
BN hardness = 9.8
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The Inert Pair Effect:
As you get closer to the bottom of the group, there is an increasing
tendency for the s2 electrons not to be used in bonding.
Trends in Group III; B, Ga, In, Tl
•! Inert Pair Effect
•! On descending the group +1 oxidation state becomes more stable than +3
•! Oxides and hydroxides become more basic
Going down the group metallic character increases:
B(OH)3 is a weak acid
Al(OH)3 and Ga(OH)3 are amphoteric
In(OH)3 is basic
•! Bonding: covalent # ionic going down the group
B is most electronegative (ONLY covalent bonds)
Al, Ga, and In form both covalent and ionic bonds.
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Group IV: C, Si, Ge, Sn, Pb
Chemistry of carbon
#!Organic Chemistry (C bonded to H)
#!Inorganic Chemistry (C not bonded to H)
$! Oxides
CO, CO2
$! Carbides
1. ionic (e.g., CaC, C with active metals )
contain C4- or C22 - (-C!C-)
C4- : Be2C, Al4C3
reacts with water to form CH4
C22 - : CaC2
reacts with water to give HC!CH
2. covalent (e.g., SiC, covalent bonds with metalloids and nonmetals)
SiC: does not react with water; very hard
3. interstitial (e.g., steel, C incorporated into interstitial spaces of transition
metals in non-stoichiometric proportions)
•!steel is an interstitial carbide: harder than pure Fe
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TRENDS IN GROUP IV
Going down the periodic table in Group IV:
1)! The +2 oxidation state becomes more stable than +4 due to the “inert pair”
effect.
+2 is rare for C, Si, Ge.
+2, +4 is common for Sn.
+4 is unstable for Pb
" strong oxidizing agent
" prefers to be +2.
2)! Basicity of oxides and hydroxides increases
CO2, SiO2, GeO2 are weakly acidic.
SnO, SnO2, PbO are amphoteric.
3)! Hydrides become less stable
THOUSANDS of stable hydrocarbons (compounds of C and H)
SiH4 is stable but is spontaneously flammable.
Ge, Sn, Pb hydrides are very unstable.
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Group 5-6: Hydrolysis of nitrogen oxides
Hydrolysis = reaction with water
N is a non metal: oxides are acidic.
N-oxide + H2O = oxyacid
N2O5 + H2O # 2HNO3 (nitric acid)
3NO2 + H2O # 2HNO3 + NO
N2O3 + H2O # 2HNO2 (nitrous acid)
HNO3 is colorless, corrosive; turns yellow
in sunlight (photochemical decomposition)
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Trends for GROUP V ELEMENTS;
N, P, As, Sb, Bi
Trends Going down the periodic table:
1)! Electronegativity decreases.
2)! Size increases
3)
Switch from non-metallic to metallic
4)
Hydroxides and oxides start acidic, become more basic.
5)! Hydrides become less stable.
NH3 is stable.
PH3 is stable but burns in air.
AsH3 decomposes easily.
SbH3, BiH3 are very unstable.
7)! “Inert pair effect” becomes more pronounced: +3 becomes more
stable than +5.
P: +5 dominates (lower EN than N)
As3+, As5+ are equally common.
Sb: +3 dominates.
Bi: +3 dominates.
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Group VI Elements: O, S, Se, Te (Po)
O2: Most abundant element on earth (21% of air)
50% of earth’s crust is oxygen (but not O2)
General Properties
Most widely used oxidizing agent
O-O bond in O2 is strong (495 kJ/mol), as are many X-O bonds
ns2np4 configuration;
can gain 2 e- or share 2 e- to fill the shell
" Oxidation states
-2 (O in all compounds except with F; O2F2, OF2 )
-1 (only in peroxide ion, O22-)
+2, +4
+6 (NOT O, only larger members of group)
Allotropes of oxygen:
Light or
electrical discharge
3O2
Strong oxidizing agent
O3 # O2 + O
2 O3
decomposition
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Anhydrides and Dehydration
Anhydride: compound formed by loss of ___________________
2 NaOH ! H2O + Na2O
base anhydride
H2SO4 ! H2O + SO3
acid anhydride
H3PO4 ! H2O + H4P2O7 acid anhydride
Anhydrides are good
dehydrating agents;
They react with water (hydrolysis rxn)
Sucrose + H2SO4
product:
(C12H22O11) (removes H2O)
carbon
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Halogens (Group 7)
•!
Have valence of 1 (same as H)
Can be used to replace H in many compounds thus influencing the properties.
•!
High reactivity of halogens WHY??
•!
Can also have positive oxidation states when combined with more electronegative
elements
(e.g. oxygen to form oxyhalides: ClO4–, IO3–, etc.)
Selected uses
Fluorine – prepare polymers, fluorocarbons (Teflon,
CFC’s), glass etching
Chlorine – chlorinated organics/plastics (PVC), bleaching
agent (NaClO), water treatment (destroy bacteria)
Bromine – photographic film (AgBr)
Iodine – iodized salt
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INTERHALOGEN COMPOUNDS:
Combine halogens; central atom is least electronegative
(it has to share electrons with surrounding atoms)
1:1 ratio
BrF, ClF, ICl, BrCl
1:3 ratio
ClF3, BrF3, IF3 , ICl3 (the only non-F)
1:5 ratio
BrF5, IF5 (ClF5 formed with difficulty)
Only Br & I large enough to be central atoms with 5 surrounding atoms
1:7 ratio IF7
Only I is large enough to be a central atom with 7 surrounding atoms
Central atom is always bigger in size and less electronegative
Extremely Reactive: superb oxidizing agents
Oxidation state of central atom decreases to 0 or -1
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Chemistry of Noble Gases; Group 8
React only under rigorous conditions
Heavier elements have lower ionization energy, can more easily share eXe Ionization E =1176 kJ
O2 Ionization E = 1171kJ
"!Xe forms compounds with F and O!
Xe + n F2 # XeF2
OR XeF4
OR XeF6
Oxidation states of +2, +4, +6
XeF6(s) + H2O(l) # XeOF4(l) + 2HF(g)
XeF6(s) + 3H2O(l) # XeO3(l) + 6HF(aq)
Kr forms only one compound; KrF2
Only one known “compound” made with He; Endohedral complex of He
inside a C60 cage
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