Table of Contents
Overview: What Makes Main Group Compounds Special?
Main group elements (s- and p-block) form compounds that dominate everyday inorganic chemistry: salts, oxides, acids, bases, many gases, and the backbone of organic molecules (C, N, O, P, S, halogens).
Here, the focus is on characteristic types of compounds and trends within groups and periods, not on detailed atomic structure or bonding models.
Key ideas that run through this chapter:
- Valence shell (outer electrons) largely determines what kinds of compounds form.
- Oxidation states follow clear patterns down groups.
- Acid–base and redox behavior of compounds often change systematically in the periodic table.
- Many “classic” reagents (acids, bases, oxidizing and reducing agents) are main group compounds.
We proceed group by group, concentrating on typical and technologically relevant compound classes.
Group 1: Alkali Metal Compounds
Alkali metals (Li, Na, K, Rb, Cs) form mainly ionic compounds in the oxidation state $+1$.
Halides and Oxides
- Halides: $M\text{X}$ (e.g. NaCl, KBr, LiF)
- High melting ionic solids.
- Highly soluble (except e.g. LiF is only sparingly soluble).
- Ubiquitous as electrolytes, in food (NaCl), and as starting chemicals.
- Oxides, Peroxides, Superoxides
- Oxides: $M_2\text{O}$ (Li$_2$O, Na$_2$O)
- Peroxides: $M_2\text{O}_2$ (Na$_2$O$_2$)
- Superoxides: $MO_2$ (KO$_2$, RbO$_2$, CsO$_2$)
- Trend: heavier alkali metals form peroxide and superoxide more easily.
- Superoxides are strong oxidizers and oxygen sources (e.g. KO$_2$ in breathing apparatus, regenerating O$_2$ and absorbing CO$_2$).
Hydroxides and Salts with Oxoanions
- Hydroxides: $M\text{OH}$ (NaOH, KOH)
- Strong, fully dissociated bases in water.
- Used in soap production, pH control, paper industry, etc.
- Salts with oxoanions:
- Carbonates: $M_2\text{CO}_3$ (Na$_2$CO$_3$ – soda; NaHCO$_3$ – baking soda).
- Nitrates: $M\text{NO}_3$ (KNO$_3$ – fertilizer, oxidizer in pyrotechnics).
- Phosphates, sulfates, etc. are very common industrial chemicals.
Complexes and “Organometallic-Like” Species
Although most complexes are discussed elsewhere, a few main group-related points:
- Alkali metals can form solvated cations (e.g. [Na(H$_2$O)$_n$]$^+$ in water).
- In non-aqueous media, they can form “sandwich”/cryptate complexes with crown ethers and cryptands; important for phase-transfer catalysis and separations.
- Organolithium and organosodium compounds (e.g. n-butyllithium) are strong bases/nucleophiles widely used in organic synthesis.
Group 2: Alkaline Earth Metal Compounds
Alkaline earth metals (Be, Mg, Ca, Sr, Ba) mainly form $+2$ compounds.
Oxides and Hydroxides
- Oxides: $MO$ (MgO, CaO – “quicklime”)
- Basic oxides, react with water to form hydroxides.
- CaO is used in cement, steelmaking, flue gas desulfurization.
- Hydroxides: $M(\text{OH})_2$ (Mg(OH)$_2$, Ca(OH)$_2$)
- Basic strength increases down the group.
- Ca(OH)$_2$ (“slaked lime”) is used for mortar, whitewashing, adjusting soil pH.
Carbonates and Sulfates
- Carbonates: $M\text{CO}_3$ (CaCO$_3$, MgCO$_3$, etc.)
- Poorly soluble (especially CaCO$_3$).
- Major minerals: limestone, chalk, marble (CaCO$_3$).
- Decompose on heating to oxides and CO$_2$ (basis of lime burning).
- Sulfates:
- CaSO$_4\cdot 2$H$_2$O (gypsum), used in plaster and construction.
- BaSO$_4$ is highly insoluble, used in medical imaging as a contrast agent.
“Hardness” of Water
- Dissolved Ca$^{2+}$, Mg$^{2+}$ form scale as carbonates/sulfates.
- Hard water affects soap performance and plumbing; softened by ion exchange or precipitation treatments.
Group 13: Boron, Aluminum, and Heavier Group 13 Compounds
Group 13 elements show a transition from covalent network chemistry (B) to more metallic/ionic behavior (Al, Ga, In, Tl).
Boron Compounds
- Boron oxide and borates:
- B$_2$O$_3$ is a glass-forming oxide.
- Borate glasses (borosilicate glass) are heat- and chemical-resistant.
- Boron hydrides (boranes): e.g. B$_2$H$_6$, B$_4$H$_{10}$
- Electron-deficient, with multi-center bonds (covered in more advanced bonding chapters).
- Historically important in bonding theory; some are used as specialty reagents.
- Borates:
- Borax (sodium tetraborate, Na$_2$B$_4$O$_7\cdot 10$H$_2$O) used in glass, detergents, and as a flux.
Aluminum Compounds
- Aluminum oxide: Al$_2$O$_3$ (alumina)
- Very hard, high-melting oxide.
- Used as abrasive, in ceramics, and as a catalyst support.
- Hydroxides and amphoterism:
- Al(OH)$_3$ is amphoteric: reacts with both acids and strong bases to form salts or aluminate complexes.
- Aluminum halides:
- AlCl$_3$ is a classic Lewis acid.
- Widely used as a catalyst in organic chemistry (Friedel–Crafts reactions).
- Aluminates and alum salts:
- AlO$_2^-$ salts in strong base (“aluminate”).
- “Alums” are double sulfates (e.g. KAl(SO$_4$)$_2\cdot 12$H$_2$O), used in water purification and as mordants in dyeing.
Heavier Group 13 (Ga, In, Tl)
- Form halides, oxides, and more metallic compounds.
- Gallium arsenide (GaAs) is an important semiconductor material (optical devices, high-speed electronics).
Group 14: Carbon, Silicon, and Related Compounds
Group 14 is central for both inorganic and organic chemistry.
Carbon Oxides and Oxo Acids
- CO and CO$_2$:
- CO: toxic, reducing gas.
- CO$_2$: linear, thermodynamically stable; forms carbonates and bicarbonates in water.
- Carbonates:
- Inorganic carbonates like CaCO$_3$, Na$_2$CO$_3$; important as building materials and in glass production.
- Oxo acids and their derivatives:
- Carbonic acid (H$_2$CO$_3$) exists in equilibrium with CO$_2$(aq); its salts (carbonates, hydrogencarbonates) are widely distributed.
Silicon Oxides and Silicates
- Silicon dioxide: SiO$_2$
- Exists in various forms (quartz, cristobalite, amorphous silica).
- Constitutes sand and many rocks; essential for glass manufacture.
- Silicates:
- Vast family of minerals (clays, feldspars, micas).
- Built from SiO$_4^{4-}$ tetrahedra linked into chains, sheets, or 3D frameworks.
Silicones (Organosilicon Compounds)
- Polysiloxanes (commonly called “silicones”):
- General backbone: –Si–O–Si–O– with organic groups on Si.
- Properties: thermal stability, flexibility, hydrophobicity.
- Used in sealants, lubricants, medical implants, and coatings.
Heavier Group 14 (Ge, Sn, Pb)
- Oxides and halides:
- SnO, SnO$_2$, PbO, PbO$_2$, and corresponding halides.
- Oxidation states often $+2$ and $+4$; stability of $+2$ increases down the group (inert pair effect).
- Tin and lead compounds are important in solders, glass, and older pigments; toxicity of many lead compounds has driven their replacement.
Group 15: Nitrogen, Phosphorus, and Their Compounds
Group 15 elements form key inorganic molecules and oxoanions.
Nitrogen Compounds
- Ammonia: NH$_3$
- Weak base; central in fertilizer and many industrial syntheses.
- Ammonium salts: NH$_4^+$X$^-$
- Behave chemically like alkali-metal cations in many respects.
- Key fertilizers: NH$_4$NO$_3$, (NH$_4$)$_2$SO$_4$.
- Oxides of nitrogen: NO, NO$_2$, N$_2$O, N$_2$O$_3$, N$_2$O$_5$
- Involved in atmospheric chemistry and acid rain formation.
- Basis for production of nitric acid.
- Nitric acid and nitrates:
- HNO$_3$: strong acid and oxidizer.
- Nitrates: oxidizing salts and major fertilizers.
Phosphorus Compounds
- Allotropes of elemental phosphorus (white, red, black) form various oxides and acids; details belong elsewhere. Here we focus on:
- Phosphorus oxides and oxo acids:
- P$_4$O$_6$, P$_4$O$_{10}$ leading to oxo acids like H$_3$PO$_4$ (phosphoric acid).
- Polyphosphates and metaphosphates important in detergents and fertilizers.
- Phosphates:
- Orthophosphate: PO$_4^{3-}$; hydrogen phosphates: HPO$_4^{2-}$, H$_2$PO$_4^-$.
- Central to energy transfer in living systems (ATP) and in fertilizers (e.g. Ca(H$_2$PO$_4$)$_2$).
Heavier Group 15 (As, Sb, Bi)
- Form oxides (As$_2$O$_3$, Sb$_2$O$_3$, Bi$_2$O$_3$), halides, and sulfides.
- Many arsenic and antimony compounds are toxic; some used historically as pigments and medicines.
- Bismuth compounds (e.g. bismuth subsalicylate) appear in pharmaceuticals and as nontoxic replacements for lead in some materials.
Group 16: Oxygen, Sulfur, and Chalcogen Compounds
Group 16 elements form many key oxides, sulfides, and oxo acids.
Oxides and Oxo Acids of Sulfur
- Sulfur oxides: SO$_2$, SO$_3$
- SO$_2$: gas with suffocating smell; reducing and bleaching agent.
- SO$_3$: reacts vigorously with water to form sulfuric acid.
- Sulfuric acid and sulfates:
- H$_2$SO$_4$: strong diprotic acid, major industrial chemical.
- Sulfates: SO$_4^{2-}$ salts (Na$_2$SO$_4$, CaSO$_4$, etc.) widely used.
- Sulfurous acid and sulfites:
- H$_2$SO$_3$ (in equilibrium with dissolved SO$_2$).
- Sulfites/bisulfites are reducing agents and preservatives.
Sulfides and Polysulfides
- Metal sulfides:
- Many ores are sulfides: FeS$_2$ (pyrite), ZnS, PbS.
- Often insoluble and colored, important in mineral processing and qualitative analysis.
- Hydrogen sulfide: H$_2$S
- Toxic, rotten-egg smell gas.
- Weak diprotic acid; its salts (sulfides, hydrogensulfides) are used in analysis.
- Polysulfides:
- Contain chains of S atoms (S$_n^{2-}$).
- Appear in vulcanized rubber and some specialized reagents.
Oxygen and Related Compounds
- Peroxides, superoxides, ozonides:
- Peroxides: O$_2^{2-}$; e.g. H$_2$O$_2$, Na$_2$O$_2$.
- Superoxides: O$_2^-$; e.g. KO$_2$.
- Ozone: O$_3$, powerful oxidizer, key in atmospheric chemistry.
- Oxides and oxo acids of Se and Te parallel sulfur chemistry but with increased metallic character and often higher toxicity.
Group 17: Halogen Compounds
Halogens (F, Cl, Br, I, At) are strong oxidizers as elements and form a wide range of salts and molecular compounds.
Halide Salts
- Metal halides: MX, MX$_2$, etc. (NaCl, CaCl$_2$, FeCl$_3$, etc.)
- Common, often high-melting and water-soluble.
- Used as electrolytes, in de-icing (CaCl$_2$), in lighting (NaCl in high-pressure sodium lamps).
- Hydrogen halides and hydrohalic acids:
- HF, HCl, HBr, HI.
- In water, form acids of varying strength (HF is weak, the others strong).
- HCl is widely used in metal cleaning and pH control.
Interhalogen Compounds and Polyhalides
- Interhalogens: species like ClF, BrF$_3$, ICl$_3$, ClF$_3$
- Often powerful fluorinating/oxidizing agents.
- Typically more reactive than the parent halogens.
- Polyhalide ions: I$_3^-$, I$_5^-$, etc.
- Formed by halide plus halogen (e.g. I$^-$ + I$_2$ → I$_3^-$).
- Stabilized by large, polarizable iodide; important in analytical chemistry and dye-sensitized solar cells.
Halogen Oxides and Oxo Acids
- Oxides: Cl$_2$O, ClO$_2$, BrO$_3$, etc., often unstable and reactive.
- Oxo acids: e.g. HClO, HClO$_2$, HClO$_3$, HClO$_4$ and their salts (hypochlorites, chlorites, chlorates, perchlorates).
- Hypochlorites (ClO$^-$) are used in bleaches and disinfectants.
- Perchlorates (ClO$_4^-$) are strong oxidizers used in propellants and pyrotechnics.
Group 18: Noble Gas Compounds
Noble gases were long thought to be completely inert; now some compounds, especially of heavier noble gases, are known.
Xenon and Krypton Compounds
- Xenon fluorides: XeF$_2$, XeF$_4$, XeF$_6$
- Strong fluorinating and oxidizing agents.
- React with water to give xenon oxides and oxo acids (e.g. H$_2$XeO$_4$).
- Xenon oxides: XeO$_3$, XeO$_4$
- Powerful oxidizers, some are explosive in pure form.
- Krypton difluoride: KrF$_2$
- Strong oxidizer formed under special conditions.
Clathrates and Physical “Compounds”
- Noble gases can be trapped in clathrate hydrates, where gas atoms sit in cages of water molecules.
- These are not chemical bonds in the usual sense but are often treated as “inclusion compounds.”
Trends and Patterns Across Main Group Compounds
Without re-deriving bonding or periodic trends in detail, some recurring patterns in main group compounds are useful to summarize:
- Oxidation states follow predictable maxima:
- Group number often equals maximum positive oxidation state (e.g. +5 for N, +6 for S, +7 for Cl).
- Lower oxidation states become more stable for heavier elements (inert pair effect).
- Acid–base character of oxides:
- Metals (left side) → basic oxides (Na$_2$O, CaO).
- Nonmetals (right side) → acidic oxides (SO$_3$, P$_4$O$_{10}$).
- Metalloids and some metals in higher oxidation states → amphoteric oxides (Al$_2$O$_3$, ZnO).
- Covalent vs ionic character:
- Higher electronegativity and smaller size (especially second period: B, C, N, O, F) favor covalent compounds.
- Larger, more electropositive elements favor ionic compounds in combination with nonmetals.
- Volatility and polymerization:
- Many small main group molecules (CO$_2$, SO$_2$, NH$_3$) are gases at ambient conditions.
- Others (SiO$_2$, (AlO(OH))$_n$, silicates, phosphates) form extended polymers or networks.
Understanding these patterns helps predict the composition, structure, and reactivity of main group compounds across the periodic table and connects them to their wide-ranging applications in materials, energy, environment, and technology.