S2.4: From Models To Materials Flashcards
Bonding types and bonding models
Bonding types are used to explain chemical and physical properties of a substance
Bonding is best thought of as a continuum of three different bonding types -> like equilateral triangle
-> decide by using a bonding model
-> has its limitations -> over simplification
Basics of the bonding triangle/arkel ketelaar triangle
Location of an element/compound -> determined by electronegativity values of elements present
2 axes ->
difference in electronegativity (y axis)
Average electronegativity (x axis)
Bonding triangle -> help for more accurate assessment of bonding type and prediction of associated properties
Where would common compounds/molecules be found on the bonding triangle?
Elements:
0 difference in EN
Along x axis
Ionic compounds:
Large difference in EN
Top center or apex
Covalent compounds:
Low difference in EN
Bottom right
Polar covalent bond -> between ionic and covalent
How can percentage of bonding type be found?
Other side of y axis -> % covalent/ionic
-> used to calculate the percentage of bonding type
-> can help explain the difference in properties
Properties: melting point
Solid -> melts when attractive forces overcome and particles are free to move
-> influenced by how particles are packed
Metals -> wider range
Network covalent -> higher than ionic
Simple covalent -> low
Properties: electrical conductivity
Ionic compound -> do not conduct in solid state
-> molten or aqueous -> chemically decomposed -> allow movement of ions
When solid is conductive -> movement of electrons, no decomposition
-> metals: delocalized ions
Allotropes of non-metals (ex: graphite) that contain delocalized electrons -> good conductors
Covalent compounds -> poor conductors -> electrons localized in fixed orbitals
Electrical conductivity in semi-conductors
Conductivity of semi-conductors (ex: silicon) -> higher temp -> increases conductivity
This is because:
Small energy gap between highest occupied energy level and lowest occupied energy level
-> higher temp -> electron easily excited to conduction band
In silicon -> only possible by doping (adding other atoms: this case group 15 or 13)
= n- or p- type semi-conductors
Properties: elasticity
Ability of a material to resist a distorting influence and return to its original size and shape when the distorting force/influence is removed
Returns to initial shape when force is removed -> applied force > modulus of elasticity -> permanent deformation
Metals: atoms can slide over each other -> force applied -> revert back to original shape when force removed
Polymers: polymer chain can be stretched without being permanently broken
Unless force > modulus of elasticity
Properties: brittleness
Many covalent solids -> brittle
-> Fracture when subject to stress rather than undergo
deformation -> bonds cannot be reformed after broken
Metals: layers can slide over each other without breaking -> ductile and malleable
-> when impurities added (alloys) -> lattice disturbed -> less malleable/harder
Ionic solid: brittle -> ionic lattice break without being deformed when sufficient force
Summary of properties:
100% metallic
100% covalent
>90% ionic
METALLIC:
-> Good conductor of heat and electricity (liquid and solid state)
-> high mp and bp (alkali metals are exception)
-> malleable and ductile
COVALENT:
-> poor conductors of heat and electricity (-graphite)
-> generally low mp (- network covalent solids)
-> brittle
IONIC:
-> Good electrolytes (conduct electricity when molten/aqueous and are decomposed in the process)
-> poor conductors in solid state
-> relatively high mp
Summary of properties: 50% ionic-50% covalent
Properties vary depending on the compound
Ex: aluminum chloride
Anhydrous AlCl3 -> intermediate melting point (193°C)
Dimerizes to Al2Cl6 when liquid -> reverts back to AlCl3 when vapor
Solid -> poor conductor
Aqueous solution -> electrolyte -> [Al(H2O)6]3+ ions which are acidic
What are alloys?
Mixture of metals - physical mixture
Can also be metal + nonmetal
-> most commonly carbon
Possible due to nondirectional nature of metallic bonds
What are the two main types of alloys?
Substitutional alloys:
Element added to base metal replaces the metal ions in the lattice
Interstitial alloys:
Element added occupies vacant space in the metallic lattice of the base metal
-> both non-directional bonding
Why do the properties of alloys vary?
Alloys -> different properties than metal they contain
Different packing of cations in the lattice
-> distort regular arrangement of cations
-> different radius -> distort crystalline structure -> less directional bonding/properties altered
Can make it:
-> harder for layers to slide -> harder metal
-> metallic bonds affected -> decrease melting point
What are some common alloys and their:
Elements present
Properties
Uses
Brass:
Copper + zinc
Strong, resistant to corrosion
Door handles/hinges, musical instruments
Steel:
Iron + carbon/chromium/vanadium/molybdenum
Very strong
Constriction/bridges, cars
Stainless steel:
Iron + chromium + nickel + carbon
Corrosion resistant
Cutlery, cookware, surgical instruments
Solder:
Lead + tin
Low melting point
Joining metals in electrical circuits and jewelry
Bronze:
Copper + tin
Hard, strong, resistant to corrosion
Medals, sculptures, ship fittings
