The Representative Elements
The main group elements — the s-block (Groups 1–2) and p-block (Groups 13–18) — constitute the large majority of the periodic table and include the most abundant elements in Earth's crust and atmosphere. While transition metals often steal the spotlight in inorganic chemistry, the chemistry of representative elements underlies agriculture, construction, atmospheric science, and biochemistry.
s-Block Chemistry: Alkali and Alkaline Earth Metals
Group 1: Alkali Metals (Li, Na, K, Rb, Cs, Fr)
Alkali metals are defined by their single valence electron (ns¹ configuration), which they surrender readily to form M⁺ ions. Key characteristics:
- Low ionization energies and high chemical reactivity — reactivity increases down the group
- React vigorously with water: 2M + 2H₂O → 2MOH + H₂. Cesium and rubidium react explosively.
- Form ionic oxides, peroxides, and superoxides depending on the metal: Li → Li₂O (oxide), Na → Na₂O₂ (peroxide), K/Rb/Cs → MO₂ (superoxide)
- Anomalous behavior of lithium: Li resembles magnesium more than sodium (the "diagonal relationship") — both form stable nitrides (Li₃N, Mg₃N₂), have high charge/radius ratios, and form less thermally stable carbonates
Sodium and potassium are the most biologically important alkali metals — Na⁺/K⁺ gradients across cell membranes drive nerve impulse propagation. Lithium carbonate is a first-line treatment for bipolar disorder.
Group 2: Alkaline Earth Metals (Be, Mg, Ca, Sr, Ba, Ra)
With two valence electrons (ns²), alkaline earth metals form M²⁺ ions. They are less reactive than Group 1 but still vigorous reductants:
- Beryllium is anomalous: very small Be²⁺ ion has extremely high charge density, leading to predominantly covalent bonding. BeCl₂ is polymeric (bridging chlorides) rather than ionic.
- Magnesium is the central metal of chlorophyll — the green pigment driving photosynthesis. It coordinates to a porphyrin-like ring (chlorin). Mg²⁺ also activates hundreds of enzymes, particularly ATP-utilizing kinases.
- Calcium is crucial for bone (hydroxyapatite, Ca₁₀(PO₄)₆(OH)₂), teeth, and cell signaling. Ca²⁺ ions act as second messengers in muscle contraction and nerve signaling.
- Barium sulfate (BaSO₄) is highly insoluble and X-ray opaque, used as a "barium meal" in GI tract imaging.
p-Block Chemistry
Group 13 (B, Al, Ga, In, Tl)
The Group 13 elements have ns²np¹ configurations and display the full range from metalloid (boron) to metal (thallium).
Boron is a unique element. It forms only covalent bonds (B³⁺ is too small and too highly charged to be stable) and has an inherent tendency toward electron deficiency — it has three valence electrons but needs eight for a full octet. This drives the formation of: - Boranes (boron hydrides): B₂H₆ (diborane) features three-center, two-electron (3c-2e) bonds — a fundamental example of electron-deficient bonding - Borax (Na₂B₄O₇·10H₂O): used in glass, ceramics, and as a flux - Boron nitride (BN): exists in hexagonal form (similar to graphite) and cubic form (hardness rivaling diamond)
Aluminum is the most abundant metal in Earth's crust. Al³⁺ has high charge density and tends to form covalent-character bonds. AlCl₃ is a powerful Lewis acid catalyst (Friedel-Crafts reactions). Aluminum metal is produced by the Hall-Héroult process: electrolysis of Al₂O₃ dissolved in molten cryolite (Na₃AlF₆).
Group 14 (C, Si, Ge, Sn, Pb)
Group 14 spans from the quintessential nonmetal (carbon) to a metal (lead). The chemistry of silicon deserves particular attention.
Silicon is the second most abundant element in Earth's crust (after oxygen), appearing mainly as SiO₂ (silica) and silicates. Unlike carbon's CO₂ (a gas), SiO₂ is an extended solid — silicon forms a three-dimensional covalent network with oxygen. Silicates are built from SiO₄ tetrahedra linked in chains, sheets, or three-dimensional frameworks: - Orthosilicates: isolated SiO₄⁴⁻ units (olivine, Mg₂SiO₄) - Pyroxenes: single chains (…SiO₃…)ₙ²ⁿ⁻ - Micas: sheet silicates with formula (AlSi₃O₁₀)ₙ⁵ⁿ⁻ - Feldspars and zeolites: 3D frameworks where Al substitutes for Si
Group 15: The Pnictogens (N, P, As, Sb, Bi)
Nitrogen dominates Earth's atmosphere (78% by volume) as the highly stable N₂ molecule (triple bond, bond energy ~945 kJ/mol). Breaking this bond — nitrogen fixation — requires either the industrial Haber process or the enzyme nitrogenase. Fixed nitrogen as ammonia and nitrates is essential for agriculture.
Phosphorus exists in several allotropes: white phosphorus (P₄ tetrahedra, highly reactive and toxic), red phosphorus (amorphous polymer, less reactive), and black phosphorus (layered structure resembling graphite). Phosphates are essential for life — DNA and RNA backbones consist of phosphate-sugar chains, and ATP (adenosine triphosphate) is the universal cellular energy currency.
Group 16: The Chalcogens (O, S, Se, Te, Po)
Oxygen participates in more compounds than any other element. Its high electronegativity (3.44 on the Pauling scale) and small size make it a uniquely powerful oxidizing agent.
Sulfur shows remarkable variety in oxidation states: −2 (sulfide, H₂S), 0 (elemental S₈), +4 (sulfite, SO₂), and +6 (sulfate, H₂SO₄). The Contact process for H₂SO₄ production is the world's largest industrial chemical process: 2SO₂ + O₂ ⇌ 2SO₃ (catalyst: V₂O₅, 450°C), then SO₃ + H₂O → H₂SO₄.
Group 17: The Halogens (F, Cl, Br, I, At)
Halogens are the most electronegative and reactive nonmetals. Fluorine is the strongest oxidizing agent known — it oxidizes even xenon and krypton. The reactivity follows: F₂ > Cl₂ > Br₂ > I₂.
Interhalogen compounds (e.g., ClF₃, IF₅, BrF₅) and polyhalides (e.g., I₃⁻) demonstrate that the halogens themselves can act as central atoms with expanded coordination. Fluorine's unique properties (small size, highest electronegativity) are exploited in Teflon (PTFE), fluorinated pharmaceuticals, and lithium-ion battery electrolytes.
Group 18: Noble Gases (He, Ne, Ar, Kr, Xe, Rn)
Once considered completely inert, the heavier noble gases form genuine chemical compounds. Xenon tetrафluoride (XeF₄), first synthesized in 1962, opened the field of noble gas chemistry. Xenon compounds include XeF₂, XeF₄, XeF₆, XeO₃, and XeO₄. Krypton forms KrF₂. These compounds are powerful fluorinating agents.
The chemistry of main group elements continues to yield surprises — from stable carbene analogs of heavier Group 14 elements to the rich coordination chemistry of polyphosphines — making this one of the most dynamic areas of modern inorganic research.