Chem Lecture 13 - Chemical bonds define materials properties

bond type influences ___ properties

The nature of bonding (ionic, metallic, covalent) determines hardness, melting point, conductivity, and ductility of a material

Why ionic solids are hard and brittle

Fixed lattice of oppositely charged ions maximizes attraction, but displacement brings like charges adjacent causing strong repulsion and fracture

Why ionic solids conduct electricity only when molten or dissolved

In solid form ions are locked in lattice

Relationship between lattice energy and melting point

Higher lattice energy → stronger ionic attraction → higher melting point and lower solubility

How ion size affects lattice energy

Larger ion → greater internuclear distance → weaker attraction → lower lattice energy and melting point

How ion charge affects lattice energy

Larger ionic charge → stronger electrostatic force → higher lattice energy, charge effect dominates over size

Metallic bond, what defines it and how it differs from ionic/covalent

Metallic bonding arises from delocalized valence electrons shared among positive metal cations forming a lattice immersed in an electron sea

Why metals conduct electricity

Mobile delocalized electrons carry electric charge freely through the lattice

Why metals conduct heat efficiently

Free electrons transfer kinetic energy rapidly across the lattice, resulting in high thermal conductivity

Why metals are malleable and ductile

The non-directional metallic bonding allows ions to slide past each other while remaining surrounded by the electron sea, preventing bond breakage

Why metals are shiny

Delocalized electrons absorb and re-emit a broad range of light wavelengths, reflecting visible light and producing metallic luster

Trend of metallic melting points across a period

Melting point increases left→right as metal cation charge and number of delocalized electrons rise strengthening attraction, then falls in post-transition metals

Trend of metallic melting points down a group

Decreases down the group as atomic radius increases, weakening electrostatic attraction between cations and electron cloud

Band theory explanation of metallic conductivity

Overlapping valence and conduction bands in metals allow electrons to move to higher energy states easily under an electric field

How band structure distinguishes metals semiconductors insulators

Metals: overlapping bands

Effect of temperature on metal conductivity

Higher temperature increases ion vibrations, scattering electrons and decreasing conductivity

Intrinsic semiconductor definition and mechanism

Pure semiconductor with moderate band gap where thermal energy excites some electrons from valence to conduction band, leaving mobile holes behind

Extrinsic semiconductor and doping types

Doping introduces donors (n-type) adding electrons or acceptors (p-type) creating holes to enhance conductivity

How a diode allows current in one direction

p–n junction permits electron flow from n→p under forward bias but blocks it under reverse bias due to depletion region potential

Why bonding type affects mechanical behavior summary

Ionic lattices brittle, covalent networks hard and high-melting, metallic lattices ductile and conductive, molecular solids soft and low-melting

Why covalent network solids like diamond are extremely hard

Each atom forms strong directional covalent bonds in a 3D network, requiring breaking many bonds simultaneously to deform

Difference between conductor, semiconductor, and insulator using band gap scale

Conductor: 0–few×kBT, semiconductor: ~50×kBT, insulator: >200×kBT (e.g. diamond)

Organic bond and its importance

Organic bonds are covalent C–C and C–H frameworks forming the basis for molecular materials, polymers, and bio-compounds

Hydrocarbons general features

Contain only C and H, nonpolar, insoluble in water, interact via London dispersion, boiling point increases with chain length

Difference between saturated and unsaturated hydrocarbons

Saturated (alkanes) have single C–C bonds sp3

Aromatic hydrocarbons key feature

Contain conjugated benzene ring with delocalized π electrons, giving stability and unique chemical behavior distinct from alkenes

Why alkenes and alkynes are more reactive than alkanes

π bonds are weaker and more accessible to attack, so addition reactions easily occur across multiple bonds

Effect of chain length on hydrocarbon physical properties

Longer chains have stronger London forces leading to higher melting and boiling points and viscosity

Effect of branching on hydrocarbon properties

Branched molecules have lower surface area, weaker dispersion forces, and therefore lower boiling and melting points than straight chains

Why hydrocarbons are good fuels

Combustion releases large exothermic energy as C–H and C–C bonds convert to stronger C=O bonds in CO2 and O–H in H2O

Classification of hydrocarbons by bond type

Alkanes single bonds (CnH2n+2), alkenes double (CnH2n), alkynes triple (CnH2n−2), aromatics delocalized π ring

Common uses of hydrocarbons by chain length

C1–C4 gases (fuel), C5–C7 liquids (solvents, gasoline), C12–C24 liquids (jet, diesel), C50+ solids (waxes, lubricants)

Isomerism in hydrocarbons and effect on properties

Structural isomers have same formula but different connectivity

Difference between constitutional and stereoisomers

Constitutional differ in atom connectivity, stereoisomers differ only in spatial arrangement of same connections

Why rotation about single bond does not produce isomers

Rotation about σ bonds does not change connectivity or relative atomic positions permanently, so same molecule not isomer