Ch. 18 Main Group Elements

Atomic Size and Group Anomalies

• difference in radii between 1st and 2nd elements of a group cause 1st element to exhibit different properties

  • 2A: basic metal oxides, except BeO amphoteric

  • 3A: boron nonmetal, other active metals; Al most abudant metal on earth

  • 4A: C and Si diff chem properties bc C-C bonds vs Si-O bonds (more reactive than C-C)

  • 5A: N exist diatomic, P forms aggregates (clusters) of atoms

  • 6A: O exist diatomic, S aggregates; O most abundant nonmetal on earth (not atm) & body

  • 7A: F less affinity than Cl bc repulsion

Abundance and Preparation

• ¼ all elements exist in free states, others are salts

• C is cheapest industrial red. agent

Alkali Metals (1A)

• react greatly w/ water, so found under inert solvents usually

• form different oxides depending on radius:

Chemistry of Hydrogen

• H₂ colorless, odorless

• very flammable in air that has high percent of H by volume → explosive

• H sources: CH_{4 +} H₂O → H₂

Hydrogen & Hydrides

• H non-metal behavior (covalent w/ nonmetals, salts with metals)

• Hydrides form when H combines with active metals (1A & 2A)

  • Hydride ion strong reducing agent (strong base b/c H^- wants H⁺)

  • Covalent Hydrides have positive H (like HCl); H₂O most important hydride b/c properties of water

  • Metallic (interstitial) Hydrides made when TM reacted w/ H₂ → one way to store H for fuel

Alkaline Earth Metals (2A)

• reactive w/ H₂O but less than alkali

• oxides very basic except BeO

• rxn w/ H₂O results in H₂

Alkaline Earth Metals Application

• Ca makes bones, teeth

• Mg helps metabolism + muscles; structural material b/c low density

Group 3A (Boron Family)

• exhibit increasing metallic character down a group

Boron

• metalloid, but gen. act as nonmetal → makes covalent

• Boranes (Boron hydrides): very reactive (e^- deficient) + react exo. w/ O

Aluminum

• metallic properties

  • conducts heat + electricity

  • shiny, reflective (make lustrous look)

  • covalents bonds (Al₂O₃ amphoteric, Al(H₂O) acidic))

Gallium

• low melting point (29.8°C) & high boiling point (2400°C) → highest liquid range of any metal NEED MEMORIZE

• thermometers

• similar to Al in chemistry

Indium (In)

• chem similar to Al + Ga

• +1 to +3 ions

Thallium (Tl)

• completely metallic character

3A Reactions

• Reacts w/ halogens (X₂) [2M + 3X₂ → 2MX₃] except Tl

Group 4A Elements (Carbon Group)

• has C, fundamental constituent of molecule for life

• has Si, basis of geology

4A Properties

• four covalent bonds to nonmetals (eg. SiF₄, CH₄, SnCl₄)

• C form pi bonds, Si can’t

4A Use and Source

• C - allotropes (like graphic, diamond)

• SI - semimetal found in silica

• Ge - rare semimetals in semiconductors

• Sn - soft, silvery metal

  • used as alloy & protective coat → steel

• Pb - melts at low temps

  • toxic in nature (lead poisoning)

• reacts w/ halogens to have 4 single bonds (MX₄)

5A elements

• varying properties, metallic down group

  • N & P nonmetals (+3 in salts), while Bi and Sb (antimony) metallic (cations)

• 3, 5, or 6 covalent bonds

N Chemistry

• all elemental nitrogen is diatomic, triple bond

• N₂ unreactive & can coexist with most elements without reacting

Nitrogen-Based Explosives

• N₂ stability allows for contribution to explosives

• TNT (trinitrotoluene) is solid at room temp and releases great energy, resulting in explosion

Nitrogen Fixation

• process of transforming N₂ into other compounds w/ Nitrogen

  • eg. Haber process used to make ammonia

• fixation can result from high-temperature combustion in automobile engines

• some bacteria found in plants make N2 → NH₃

Ammonia (NH₃)

• toxic, colorless, pungent (eg. smelling salts, glass cleaners)

• in fertilizer

• weal base

Nitrogen Hydrides - Hydrazine (N₂H₄)

• strong reducing agent & pesticide

• pyrophoric (catch fire instantly when exposed to air)

N Oxides

• series of oxides, ox state +1 to +5

• Dinitrogen Monoxides (N₂O)

  • known as laughing gas (mild aesthetic)

  • [ ] increasing in atm b/c of microorganisms in soil

• Nitrogen Monoxide (NO)

  • regulator in biological systems

  • neurotransmitter & vasodilator (widens blood vessels for better flow)

  • normally colorless gas

Oxyacids of Nitrogen (eg. HNO₃)

• used to create nitrogen-based explosions

• colorless, fuming liquid with pungent odor

• aq HNO₃ made from Ostwald process (68% via distillation, 95-99 using H₂SO4)

P vs. N

• different chem properties b/c N stronger (pi) bonds, P single (sigma) bonds

• N more EN, while P bigger

Phosphides

• ionic substances w/ P^-3 anions

• react w/ H₂O → make phosphine (PH₃, acid)

Phosphorus Oxides

• react w/ O₂ to form oxides w/ +3 to +5 ox. states

Phosphorus Oxyacids

• aka orthophosphoric acid

  • made from P₄H₁₀ + H₂O

  • aq H₃PO₄ weak acid (poor ox. agent)

Phosphorus in Fertilizers

• essential for plant growth

• contains soil that has soluble phosphorus, as soil much insoluble P

Group 6A (Chalcogens)

• all behave nonmetals

  • react w/ metal to attain noble gas configuration (become -2 anions)

• form covalents bonds w/ nonmetal

Selenium

• shows inverse relationship between cancer and selenium levels in soil (more selenium in soil = less cancer)

Polonium

• highly toxic and radioactive

• natural contaminant of tobacco

Chemistry of Oxygen

• found everywhere (soil and rocks, body, water, atm.)

• reacts w/ carbon-containing molecules → combustion to provide energy for survival

• 21% of the volume of Earth’s atm.

• paramagnetic in nature

Ozone (O₃)

• made by passing electrical discharge through pure O₂ gas

• less stable than Oxygen at standard condition → toxic

• natural in atm and blocks UV light

• powerful ox. agent that kills bacteria and for cleaning (can recontaminate)

• 2 forms of elemental O

Chemistry of Sulfur

• found in deposits of free element

• recovered using Frasch process (super heated H₂O pumped into deposit, sulfur forced to surface)

Sulfur Aggregates and their Stability

• exist as S₂ gas at high temps

• most stable sulfur forms is S₈ rings

Sulfur Oxides

• Sulfur monoxide (SO)

  • unstable

  • weak pi bonding compared to O₂ pi bond

• Sulfur dioxide (SO₂)

  • colorless pungent gas made from burning Sulfur in air

• Sulfur trioxide (SO₃)

  • made from SO₂ + O₂

• antibacterial agent w/ uses in preserving fruit

Oxyacids of Sulfur

• H₂SO₃ (made from SO₂ + H₂O)

• H₂SO₄ (made from SO₃ + H₂O) → diprotic in nature

  • dehydrating agent b/c high water affinity → can remove water in chem. rxns

Group 7A (Halogens)

• nonmetals whose properties vary smoothly going down group (s → l → g)

  • F₂ exception b/c e^- repulsion

• High reactivities, so can’t be found as free elements in nature

• High EN values

Hydrogen Halides

• F₂ + H₂ → 2HF and “explosive vigor”

• Cl + H react slow in dark, fast in UV

• Br & I react w/ H slowly

• act as acids (like HF)

Halogen Oxyanions & Oxyacids

• all halogens react w/ O to form series oxyacids (except F)

• Chlorate salts

  • weed killers and in fireworks + explosives

Group 8A (Noble Gases)

• highly unreactive

Helium

• does not form compounds

• uses in MRI machines, pressurizing gas for rocket fuel

Neon

• not form compounds

• neon signs

Argon

• forms bonds under certain circumstances

• creates noncorrosive atmosphere in incandescent light bulbs (emit light when heated)

Krypton and Xenon

• form many stable compounds

  • eg. XF₄