Molecular Structures

Ch. 9

Types of Chemical Bonds

• Ionic bond- the taking of electrons between atoms

• Covalent bond- the sharing of electrons between atoms

• Why do some elements take electrons and some share?

Electronegativity Difference (ΔEN)• Electronegativity 电负性 - the pull an atom has on

electrons while bonding• Periodic trend:– Increase moving up a group and across the periodic table

from left to right– Shielding effect 屏蔽效应 - valence electrons feel less pull

by the nucleus because lower level electrons shield them– Most electronegative element is Fluorine

• ΔEN- difference in electronegative values of the elements forming the bond

Calculating ΔEN• Always subtract the small EN value from the

larger EN valueHF ---> H= 2.1; F= 4.0 ---> 4.0-2.1= 1.9

HF= very polar covalent bondOF2---> ?

Fe2O3 --->?

O= 3.5; F= 4.0 ---> 4.0-3.5= 0.5 ; moderately polar covalent

O= 3.5; Fe= 1.8 ---> 3.5-1.8= 1.7; very polar covalent

Polar Bonds• Polar covalent bond 极性共价健 - a covalent

bond with unequal sharing of the electrons–Creates a polar molecule (H2O)–Molecule has dipoles 偶极子 (δ+ and δ–

ends)• Non-polar covalent bonds- a covalent bond

with equal sharing of the electrons–All diatomic molecules (H2, F2, Cl2, etc…)

Intermolecular Forces• Forces between molecules but not actual bonds• van der Waals forces 范德华力 - weak

attractions between molecules– London dispersion forces 伦敦色散力– Weakest connecting force between non-polar

molecules• Attraction/repulsion between nucleus and electrons

– Dipole-dipole forces 取向力• Weak bonds between dipoles of polar molecules

• Hydrogen bonds 氢键 - weak dipole force between hydrogen and electronegative elements– Strongest intermolecular force; strength in numbers

Molecular Shapes• VSEPR Theory 价层电子对互斥理论 :– Valence Shell Electron Pair Repulsion– Valence electrons repel each other as much as

possible –Using the # of lone pairs (non-bonding

electrons) and # of bonds around the central atom, one can predict the 3D shape of a molecule

– Double and triple bonds only count as 1 bond– Lewis-dot diagrams help organize the electrons

VSEPR Models• Linear–2 bond pairs of electrons–0 lone pairs–Bond angle of 180o

–Common compounds• CO2, CaH2, HgCl2 180o

VSEPR Models• Bent (120o)–2 bond pairs of electrons–1 lone pairs–Bond angle of 120o

–Common compounds• O3, SO2, etc…

120o

VSEPR Models• Bent (105o)–2 bond pairs of electrons–2 lone pairs–Bond angle of 105o

–Common compounds• H2O, OF2, etc…

105o

VSEPR Models• Trigonal Planar 平面三角形–3 bond pairs of electrons–0 lone pairs–Bond angle of 120o

–Common compounds• BF3, SO3, NO3

-, etc…

120o

VSEPR Models• Trigonal Pyramidal 三角锥–3 bond pairs of electrons–1 lone pairs–Bond angle of 107o

–Common compounds• NH3, PCl3, etc…

107o

VSEPR Models• Tetrahedral 四面体的–4 bond pairs of electrons–0 lone pairs–Bond angle of 109.5o

–Common compounds• CH4, SO4

2-, etc…109.5o

VSEPR Models• Trigonal Bipyramidal 三角双锥–5 bond pairs of electrons–0 lone pairs–Bond angle of 120 and 90o

–Common compounds• PF5, PCl5, etc…

90o

120o

VSEPR Models• Octahedral 八面体的–6 bond pairs of electrons–0 lone pairs–Bond angle of 90o

–Common compounds• SF6, SeF6 etc…

90o