Categories of elements—a traditional approach

1) Representative elements
—groups 1A through 7A: with outermost s or p sublevels (orbitals) partially filled with electrons,
e.g., Na (No. electrons = 11): 1s2 2s2 2p6 3s1
C (No. e = 6): 1s2 2s2 2p2

—group 0: These are the noble (inert) gases which are quite resistant to chemical reactions, and with outermost s (and p) orbitals filled
e.g., He (No e = 2): 1s2;
Ne (No. e = 10): 1s2 2s2 2p6

Example of inert gas compounds: XeF2(or 4 or 6), XeO3, KrF2, and so on

2) Transition metals (B-group elements)
—Electrons start filling in d orbitals.
Fe (No. e = 26): 1s2 2s2 2p6 3s2 3p6 4s2 3d6

3) Inner transition metals (lanthanides and actinides)
—electrons filling in f orbitals
Pm (No. e = 61): [54Xe] 6s2 4f5

 "New" named elements
104     Rutherfordium     Rf
105     Dubnium     Db
106     Seaborgium     Sg  (The only element named after a scientist who is still alive, whic was proposed by the American Chemical Society!)
107     Bohrium     Bh
108     Hassium     Hs
109     Meitnerium     Mt  (The second "female" element, named after the physicist Liz Meitner.)

Blocks of elements: determined by e in outermost orbital
s block: 1A and 2A groups—Na, [Ne] 3s1; Mg, [Ne] 3s2
p block: 3A through 7A groups—O, [He] 2s2 2p4
d block: transition metals—Fe, [Ar] 4s2 3d6
f block: inner transition metals—Pm, [Xe] 6s2 4f5

Representative elements
Noble gases/Inert gases (group 0):
• Sub-shell orbitals are filled by electrons
He: 1s2
Ne: [He] 2s2 2p6
Ar: [Ne] 3s2 3p6
• Extremely unreactive (does not form ions easily)
• Applications: providing inert atmosphere for the preparation of air sensitive compounds.

Alkali metals (group 1A)
• One electron in s orbital
Li: [He] 2s1 Na: [Ne] 3s1
• Oxidize extremely easily (e.g. by O2 to form Na2O or by Cl2 to give NaCl)—to form M+ ion easily due to their low first ionization energy and high 2nd IE (to give inert gas electron configuration).

Alkaline earth metals (group 2A)
• Two electrons in s orbital
Be: [He] 2s2 Mg: [Ne] 3s2
• Form M2+ ions easily (due to low first two IE and high 3rd IE)—CaCO3, Ca3(PO4)2, MgSO4, etc.

Aluminum (boron) group (group 3A)
• Electrons start occupy the p orbitals
B: [He] 2s2 2p1
Al: [Ne] 3s2 3p1
• Form M3+ ions (removal of the outermost electrons): e.g., Al2O3
• Al2O3 (corundum)—a very hard material
    rubies—some Al atoms in Al2O3 replaced by Cr3+
    sapphire— Fe2+ and Ti4+
• Applications—Ga as thermometer (m.p. 30 °C, b.p. 2403 °C) which has a wide range of liquid form.

Carbon group (group 4A)
• Two electrons in the p orbitals
C: [He] 2s2 2p2
Si: [Ne] 3s2 3p2
Sharing 4 electrons to form covalent bonds (which will be discussed later)
e.g.: methanol, CH3OH
• C—the key element in organic chemistry
• Si and Ge—the key elements in electronics
• Other applications—Sn coating, Pb(CH2CH3)4 as antiknock reagent in gasoline (but has been phased out due to Pb toxicity)

Nitrogen group (group 5)
• Three electrons in the p orbitals
N: [He] 2s2 2p3
P: [Ne] 3s2 3p3
Sharing 3 or 5 electrons to form covalent bonds
e.g.: ammonia, NH3 (3) and nitrate NO3– (5)
• N—essential to living organism
Gaseous form (N2) cannot be used by plants, and has to be "fixed" to ammonia (by industrial process or by nitrogenase in microorganisms)
• P—DNA backbone (see book cover); bones and teeth, ATP (bioenergy storage), hydroxyapatite (Ca5(PO4)3OH)—"synthetic bone"
Sb/Bi—in metal casting

Oxygen group (group 6)
• Four electrons in the p orbitals
O: [He] 2s2 2p4
S: [Ne] 3s2 3p4
Accepting 2 electrons to form covalent /ionic bonds
e.g.: water, H2O and metal oxides MxOy
• O2—produced by plants in photosynthesis, and used in our body to "burn" food to produce energy.
• S—the culprit of acid rain (caused by H2SO4 from burning S-containing coal), the "linker" in rubber to make rubber stronger (otherwise rubber would become too soft when temperature raises, and could not be used as tires!)
• Se—a toxic but essential trace element

Halogens (group 7A)
• Five electrons in the p orbitals
F: [He] 2s2 2p5
Cl: [Ne] 3s2 3p5
Accepting 1 electrons to form covalent /ionic bonds
e.g.: table salt, NaCl and CF2 unit in Teflon
• Toxic as elemental form, but important in biological systems as "anion" form.
• Applications—bleach (sodium hypochlorite), AgBr (in films), Teflon, etc.

Hydrogen—The most abundant element in the universe.
• can gain or lose one electron to form compounds,
e.g., H2O, HCl, NaH, etc.

Transition metals
• Filling electrons in the d orbitals
Fe: [Ar] 4s2 3d6
Mo: [Kr] 5s2 4d4
• Several bio-essential elements: Mn, Fe, Cu, Zn
(Cr, Co, and Mo in trace amount)
• Key roles in industrial catalysis
e.g. hydrogenation, oxidation, polymerization
(Ru, Rh, Ni, Pd, and Pt compounds)
• Key roles in "metalloproteins" (Read more here!)
e.g. hemoglobin/myoglobin, proteases, cytochrome c oxidase, phospholipases