Models of Bonding and Structure

what is ionic bonding?

the force of attraction between oppositely charged ions/species

ammonium formula and charge

NH4+

hydroxide formula and charge

OH-

nitrate formula and charge

NO3-

hydrogencarbonate formula and charge

HCO3-

carbonate formula and charge

CO3 2-

sulfate formula and charge

SO4 2-

phosphate formula and charge

PO4 3-

what is an ionic lattice?

a crystalline astrucutre formed by ions in a regular repeating pattern

what is lattice enthalpy?

the standard enthalpy change of the formation of 1mol of gaseous ions from the solid lattice

when does lattice enthalpy increase?

as the ionic charge increases and the ionic radius decreases

properties of ionic compounds

• strong electrostatic forces

• brittle

• high melting and boiling points

• not volatile (large amounts of energy required to overcome strong electrostatic forces of attraction)

• generally soluble as they form ion-dipole interactions

• only conduct electricity when molten or in solution

what are covalent bonds?

electrostatic attraction between a shared pair of electrons and the positively charged nuclei of two atoms

what happens in terms of orbitals in a covalent bond?

two orbitals overlap to form a molecular orbital

what is the octet rule?

atoms tend to gain a valence shell of 8 electrons

what is bond energy and what are its units?

the amount of energy required to break one mole of a covalent bond in the gaseous state

measured in kJmol^-1

what is a coordinate bond?

where both electrons in the covalent bond come from the same atom

what bond angle & shape results from two electron domains?

180º, linear

what bond angle & shape results from three electron domains and no lone pairs?

120º, trigonal planar

what bond angle & shape results from three electron domains with one being a lone pair?

~118º, bent

what bond angle & shape results from four electron domains and no lone pairs?

109.5º, tetrahedral

what bond angle & shape results from four electron domains with one being a lone pair?

107º, trigonal pyramidal

what bond angle & shape results from four electron domains with two being lone pairs?

104.5º, bent

what is electronegativity?

the ability of an atom to attract the shared pair of electrons in a covalent bond

what is a nonpolar covalent bond?

where the atoms have the same electronegativity so the electrons are shared equally

what is a polar bond?

a bond with uneven distribution of electron density due to atoms having different electronegativities so the electrons are drawn closer to the more electronegative atom

what are the charges of each atom in a polar bond?

• more electronegative = delta negative

• less electronegative = delta positive

what is a dipole?

where there is a separation of charge across a covalent bond

what is a dipole moment?

a measure of how polar a bond is, greater difference in electronegativity = greater dipole moment

what makes a molecule with more than two atoms polar?

if the dipoles do not cancel out (look at the spatial arrangement)

what is a giant covalent structure?

substances that form large networks of atoms joined by strong covalent bonds

properties of silicon covalent network

• each silicon atom covalently bonded to 4 others

• tetrahedral geometry

• bond angles ~109.5º

properties of silicon dioxide giant covalent structure

• each silicon atom covalently bonded to four oxygen atoms

• each oxygen bonded to two silicon atoms

• tetrahedral geometry

structure and bonding of diamond

• each carbon atom covalently bonded to four others

• tetrahedral geometry

• bond angles 109.5º

properties of diamond

• very strong

• very hard

• does not conduct electricity (all four electrons of each carbon used in bonding)

structure and bonding of graphite

• each carbon atom covalently bonded to three others in hexagonal rings arranged in flat layers

• trigonal planar geometry

• bond angles 120º

properties of graphite

• conducts electricity, there are electrons delocalised and free to move throughout the structure

• soft and slippery as layers are held together by weak intermolecular forces and can slide over each other

structure and bonding of buckminsterfullerene

• C₆₀

• spherical structure

• each carbon forms three covalent bonds

• forms interlocking hexagons and pentagons

properties of buckminsterfullerene

• one delocalised electron per carbon allowing limited electron movement through the structure

• so it is a semiconductor (cannot conduct as well as graphite or graphene)

structure and bonding of graphene

• single layer of carbon atoms arranged in a hexagonal lattice

• each carbon atom bonded to three others

• trigonal planar geometry

• 120º bond angles

properties of graphene

• conducts electricity (delocalised electrons)

• extends in two dimensions only

• effectively one atom thick

• strong and flexible

properties of giant covalent structures

• high melting and boiling points

• hardness depends on molecule

• most do not conduct electricity, only if there are delocalised electrons

• most are insoluble in water

what are the four types of intermolecular force?

• london dispersion forces

• dipole-dipole attractions

• dipole-induced dipole attractions

• hydrogen bonding

how do london dispersion forces work?

• as electrons are constantly moving this can lead to an uneven electron distribution

• this is a temporary dipole

• temporary dipoles can induce dipoles in neighbouring atoms or molecules (temporary induced dipoles)

• this causes a weak attractive force between atoms or molecules

what are london dispersion forces present between?

all atoms and molecules

what affects the strength of london dispersion forces?

• number of electrons

• surface area available for contact

what are dipole-dipole attractions?

attraction between the delta positive end of one polar molecule and the delta negative end of another

what is the effect of polarity on the boiling point of molecules?

polar molecules have higher boiling points than nonpolar molecules as dipole-dipole attractions increase strength of intermolecular forces

what is a dipole-induced dipole attraction?

• where a polar molecule is placed near a nonpolar molecule

• permanent dipole distorts electron cloud of nonpolar molecule

• creating a temporary dipole on the nonpolar molecule leading to a weak attractive force

what is the strongest type of intermolecular force?

hydrogen bonding

what is hydrogen bonding?

• when hydrogen is bonded to O, N or F (highly electronegative) the bond becomes strongly polarised

• the hydrogen becomes very delta positive and is attracted to a lone pair of electrons on O, N or F on another molecule

how are hydrogen bonds represented?

dotted line

which forces are van der waals forces?

• london dispersion

• dipole-dipole

• dipole-induced dipole

• all intermolecular forces because they act between molecules

general rule for solubility

like dissolves like

• nonpolar substances in nonpolar solvents

• polar substances in polar solvents

are giant covalent substances soluble?

nope because too much energy is needed to break strong covalent bonds in the lattice

do molecular covalent substances conduct electricity?

• no as they have no delocalised electrons

• some polar covalent substances can conduct when dissolved in water as they ionise

do giant covalent substances conduct electricity?

most do not except for graphite and graphene

how does chromatography work?

by passing a liquid mobile phase over a solid stationary phase

what is the Rf value?

how far a substance travels relative to the solvent front

how do you calculate the Rf value?

distance travelled by component/distance travelled by solvent

when does resonance occur?

where there are delocalised electrons so more than one valid lewis structure can be drawn for a molecule

what makes a molecule form resonance structures?

• when there is a pi bond that can occupy multiple positions

• atoms of equal electronegativity are adjacent

• lone pairs or double bonds can shift around the structure without breaking octet rules

what is the structure and bonding of benzene?

• all 6 carbon atoms are sp² hybridised and form 3 sigma bonds - two to carbons and 1 to a hydrogen

• there is a delocalised pi system above and below the ring

• all carbon-carbon bonds have equal length between single and double bonds

• planar

• bond angles of 120º

what is the experimental evidence for benzene?

• enthalpy changes of hydrogenation

• carbon-carbon bond lengths (are inbetween single and double)

• saturation tests

• infrared spectroscopy

what is the kekule structure of benzene?

alternating single and double carbon-carbon bonds

what does benzene show in saturation tests?

• does not decolourise bromine water via electrophilic addition → meaning it does not have isolated double bonds

why do some elements form molecules with an expanded octet?

they have vacant d-orbitals in their valence shell which can accomodate extra bonding pairs of electrons

what is the electron domain geometry of molecules with five electron domains around the central atom?

trigonal bypyramidal

what is the electron domain geometry of molecules with six electron domains around the central atom?

octahedral

what is the difference between molecular geometry and electron domain geometry?

• electron domain geometry = arrangement of electron pairs (bonding and lone) around the central atom

• molecular geometry = arrangement of atoms in space

what is the molecular geometry of a molecule with five bonding pairs and zero lone pairs?

trigonal bipyramidal

what is the molecular geometry of a molecule with four bonding pairs and one lone pair?

seesaw

what is the molecular geometry of a molecule with three bonding pairs and two lone pairs?

T-shaped

what is the molecular geometry of a molecule with two bonding pairs and three lone pairs?

linear

what is the molecular geometry of a molecule with six bonding pairs and zero lone pairs?

octahedral

what is the molecular geometry of a molecule with five bonding pairs and one lone pair?

square pyramidal

what is the molecular geometry of a molecule with four bonding pairs and two lone pairs?

square planar

what is the formula for formal charge?

FC = (number of valence electrons) - 1/2(number of bonding electrons) - (number of non-bonding electrons)

which resonance structure is preferred?

the one where the difference in formal charge is closest to zero

what is a sigma bond?

head on overlap of atomic orbitals with electron density along the bond axis

properties of sigma bonds

• strongest type of covalent bond

• always present in single, double and triple bonds

what overlaps form sigma bonds?

• s+s

• s+p

• p+p

what is a pi bond?

adjacent overlap of p orbitals creating electron density above and below the bond axis

what is hybridisation?

the mixing of atomic orbitals to form new hybrid orbitals

what is sp³ hybridisation?

one s orbital and three p orbitals from the same shell mix to form four sp³ hybrid orbitals and arrange with tetrahedral geometry

what is sp² hybridisation?

one s and two p orbitals from the same shell mix to form three sp² hybrid orbitals arranged with trigonal planar geometry

what is sp hybridisation?

one s orbital and one p orbital from the same shell mix to form two sp hybrid orbitals arranged with linear geometry

what is metallic bonding?

electrostatic attraction between positively charged metal ions and a sea of delocalised electrons

properties of metals

• malleable and ductile as layers of metal ions can slide over each other

• strong and hard due to being closely packed and strong electrostatic attraction

• can conduct electricity as solid and liquid

• good thermal conductivity

• high melting and boiling point

what is a transition metal?

an element with an incomplete d subshell or an element that can form an ion with an incomplete d subshell

structure and properties of transition metals

• metallic lattice structure

• transition metals can delocalise d-electrons to form metallic bonds

• high electrical conductivity

• high melting points

why do transition metals have high melting points?

• form metallic bonds with both s and d electrons

  • the more delocalised electrons the stronger the electrostatic forces of attraction

why do transition metals have such high electrical conductivity?

§large number of delocalised electrons meaning more electrons are able to move when a potential difference is applied

what is an alloy?

• mixture of metals or a metal mixed with a non-metal

• elements are physically combined but not chemically bonded

• atoms are held together by delocalised electrons

why do alloys have different properties to pure metals?

atoms of different sizes disrupt the regular pattern meaning layers cannot slide over each other as easily

properties of alloys

• increased hardness and strength to pure metals

• may be more corrosion resistant or more resistant to extreme temperatures

what are polymers?

molecules made by chemically linking many small molecules (monomers)

• polymers must be at least 50 units long

• each unit joined by strong covalent bonds

what is addition polymerisation?

where monomers join together without forming byproducts

typically a C-C double bond breaks

properties of polymers

• low density (loosely packed)

• unreactive

• water-resistant

• strong

two examples of natural and synthetic polymers

natural

• DNA

• starch

synthetic

• plastics (eg poly(ethene) )

• nylon