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: XeF_{2(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—...by 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 NO^3– (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 M_xO_y^z±
• 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