Ch. 19:Representative Elements
Groups 1A-4A
Representative Elements
Groups 1A  8A
– Chemical properties determined by valence
electrons in level s & p
Transitions metals (center of table)
– Result from filling d- orbitals
Lanthanides & Actinides
-- result from filling of 4f and 5f orbitals respectively
• Heavy black line divides the metals from
nonmetals
• Elements on either side of line exhibit metallic
AND nonmetallic properties
• These are called metalloids or semimetals (Si,
Ge)
Big Ideas
Representative Elements
a. Same column, same family, similar chemical
properties
b. Typically, FIRST element is most different than the
others
H, Be, B, C, N, O, F
c. Metallic properties increase down column(esp 4A)
d. Other properties vary regularly up/down column
e. Majority of naturally occurring elements are in
compounds
Group 1A
• ns1 valence e• Easily lose valence e- (except for hydrogen)
• Alkali Metals
• Form M+ cations
• React vigorously with H2O to form M+ and OH- ions
AND H2 gas
Group 1A- Hydrogen
• H = most abundant element in the universe (@3/4 of all
mass!)
• H2 (g) is rare in Earth’s atmosphere
• H = 3rd most abundant element in human body (10% by mass)
• Commercial production of H2 from methane (CH4), water@
700C, 1300F
Group 1A cont’d
• Metals (Li, Na, K…), Alkali Metals
• Form oxides/hydrides that are all basic in water;
• (Metal + H2O  OH- (aq)…
– NaH (hydride)  H2(g) + ….
– Na2O (oxide)
– Na2O2 (peroxide)
– NaO2 (superoxide)  O2(g) + ….
Group 2A
•
•
•
•
ns2
Oxides are also basic in water;
Metal + H2O  OH-(aq)….
Ca2+ & Mg2+ are most common metals in human
body
• Ca2+ & Mg2+ are most common ions I “hard” water
• Ca2+ precipitates fluoride out of water (Ksp = 4x10-11)
• Marble CaCO3, converted to CaSO4 by acid rain
Group 3A
• ns2np1
• Shows increasing metallic character moving down
the group
• Boron-hydrogen covalent compounds = “boranes”
– VERY reactive, electron deficient
– Unusual B-H-B bonds (fig 19.8, p889)
– example
Group 3A-Aluminum
• Al = 3rd most abundant element in Earth’s crust,
oceans, air (8% by mass)
• Al = most abundant metal on earth (Al2O3)
• Expensive to recover, consumes @ 5% of all
electricity in US!
• Al is very reactive, forms oxide “skin” that stops
reaction
Group 4A
• ns2np2
• Silicon
– Si = 2nd most abundant element in Earth’s crust,
oceans, air (26% by mass)
– Si as oxides (silica, silicates)
Group 4A- Carbon
C = 2nd most abundant element in human body (@18%)
Carbon- Diamond
Carbon-Graphite
Carbon monoxide bonds to hemoglobin 100x more
than oxygen
Carbon dioxide dissolved in water is acidic
Sugar (CH2O) ferments with yeast to form ethanol
Group 4A- Lead
Lead
Poisonous!!
Lead in gasoline was tetrahedral lead
#1 use: batteries
Ch 20 –Group 5A
Nitrogen
1. N2 = most abundant element in air(78% by
volume)
2. Very unreactive as N2; triple bonded
3. Nitrogen fixation: N2  compounds containing N
(bacteria, human activity)
a. “fixed” nitrogen encourages algae growth
b. amount of “fixed” nitrogen is currently
increasing.
Nitrogen cont’d
4. Explosives: Many compounds with N (+H, C, and/or
O)  N2, H2O and/or CO2 gases –EXOTHERMIC!
a. airbags
b. Trinitrotoluene, TNT and nitroglycerin
5. Smog; at high T& P: N2 +O2  2NO
(in air  NO2 = brown gas)
Group 5A cont’d
Phosphorus
1. Biological buffer system:H2PO4-  HPO42- + H+
2.
3.
ATP + H2O  ADP + HPO42- + Energy
Reaction structures
Group 6A
Group 6A-Oxygen
O = most abundant element in human body
O = most abundant element in earth’s crust,
oceans, air (49% by mass)
O2 = 2nd most abundant element in air (21% by
volume)
Ozone = O3
1. ozone layer absorbs U.V. light in upper
atmosphere
2. highly toxic; antibacterial properties
Group 6A cont’d
Sulfur
Acid rain:
1. S (fuel, esp. Diesel & Coal) + O2  SO2
2. SO2 + O2  SO3 (catalyzed by dust & particles in air)
3. SO3 + H2O  H2SO4! ACID rain !
Selenium & cancer ?
Polonium
alpha particle emitter; murdered Russian ex-spy Litvinenko
chemically like Oxygen!
Group 7A
Halogens
I. Found in nature as X2 or XII. X + X  X2 (quickly)
III. Strong oxidizers; want to remove electrons from other
atoms:
Cl + Cu+ -- Cl- + Cu2+
Fluoride
1. strong acids: HCL, HBr, HI, but NOT HF
2. Generally soluble in salts: chlorides, bromides, but NOT
Fluorides
3. Entropy does not favor F-(aq)
Group 8A
“Noble Gases”
a. generally unreactive; NO compounds containing
He, Ne, or Ar
b. recently, found compounds of Xe and Kr with F
and O