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Ch 12 Chemical Bonding
Bonds formed betw atoms depend
on e- config & attraction the atoms
have for e-’s
- varies systematically
12.1 Electronegativity
• - the relative tendency of an atom to attract e’s to itself when it’s bonded to another atom
– Values assigned to elems on the basis of
experiments
12.1 Electronegativity
• Electroneg diff betw atoms incr as bond
strength incr
• Electroneg.’s are influenced by same factors
that affect ionization energies & e- affinities
– Follow the same trends
– Fr – lowest electroneg, F – highest
• \ in bonds w/ F, bonding e-’s are drawn closer
to F than the other atom
12.1 Electronegativity
• In a rxn betw 2 elems, relative attraction for e’s determines how they react
– Electroneg scale determines this attraction
12.2 Bond Character
• If electroneg diff is high, atoms transfer e-’s to
form ionic bonds
– Held together by electrostatic (+ & - ) attraction
• If electroneg diff is small, atoms share e-’s to form
covalent bonds.
– Amt of transfer depends on electroneg diff betw 2
atoms
• If electroneg diff < 1.67, for covalent bonds
• If electroneg diff > or = 1.67, form ionic bonds
– All bonds have some charact. of both ionic & covalent
12.3 Ionic Bonds
• When e-’s are tranferred from 1 atom to the
other, (+) & (-) ions are formed
– When brought close together, the attractive force
betw them holds the ions together
• Ionic Bond – the electrostatic force that holds
2 ions together due to differing charges
12.3 Ionic Bonds
• Ionic comps have high melting pts, can
conduct electricity in molten state, are usually
soluble in water, & usually crystallize as
sharply defined particles.
12.4 Ionic Radii
• - found from experimental data & simplifying
assumptions
• Internuclear Distance – in a crystal – found by
adding the radii of 2 ions in a comp
– Can also be used as bond length in a molec
• Or bond axis
• Radii are not fixed values
– 1. fuzziness of e- cloud
– 2. effect ea ion has on neighboring ions
12.5 Covalent Bonds
• - shared pair of e-’s
• Covalent comps have low melting pts, do not
conduct electricity, & are brittle
• Molecule – particle resulting from 2 or more
atoms bonding covalently
• Bond Axis – line joining the nuclei of 2 bonded
atoms in a molecule
• Bond Angle – angle betw 2 bond axes in a
molec.
12.5 Covalent Bonds
• Bond Length – distance betw nuclei along
bond axis
– Not fixed – bond acts like a stiff spring
– Atoms vibrate as though bonds were strectching &
shrinking
– Bonds also undergo bending, wagging, &
rotational vibrations
• All cause bond angles & lengths to vary
– \ bond lengths & angles are average values
12.5 Covalent Bonds
• Chemists use infra-red spectroscopy to
determine molecular structure & vibrations of
molecules.
– Using IR spectrum, can compute bond strengths &
determine much about how & where specific
atoms are bonded in a molec.
• Microwave radiation affects rotation of molecs
– Can also determine bond lengths & bond angles
12.6 Covalent Radii
• It’s possible to determine internuclear dist
betw 2 bonded atoms
– For ICl it is 230
– Cl2 is 199, I2 is 266 – ½ of ea of these may be used
for the radii of ea atom & their sums are the bond
dist of ICl.
• Covalent radii are only approximate
– Useful in predicting bond lengths in molecs
12.7 Polyatomic Ions
• - consist of 2 or more atoms covalently
bonded together and possessing an overall
charge
– Form ionic bonds like other ions
– Most are negative (exception – NH4)
12.8 Van der Waals Radii
• There is a certain minimum distance
maintained betw atoms which are not bonded
to ea other
– E- clouds of ea atom repel ea other
– When free atoms or molecs collide, they act as if
they had a rigid outer “shell”.
• The “shell” limits how close other atoms or molecs may
come
– Bonded atoms are closer together than unbonded
atoms.
12.8 Van der Waals Radii
• The radius of this imaginary rigid shell is called
Van der Waals Radius
– The minimum dist maintained betw nonbonded
atoms or atoms on adjacent molecs
– Larger than covalent radius of bonded atoms
12.9 Summary of Radii
• Studied 4 types of radii:
1.
2.
3.
4.
Atomic
Ionic
Covalent
Van der Waals
12.9 Summary of Radii
• Atomic Radii – measured in 2 ways
– 1. Meas on individual atoms in gaseous state
• Atoms unaffected by neighboring atoms
– 2. Meas on atoms in metallic crystals
• Neighboring atoms have large effect
– These 2 methods won’t give the same results
12.9 Summary of Radii
• Ionic Radii – differ from atomic radii bec of
loss or gain of e-’s
12.9 Summary of Radii
• Covalent & Van der Waals Radii – vary greatly bec
of wide range of atoms which can be bonded
together
• Covalent radii is expected to be less than atomic
radii
– When an atom is bonded to more than 1 other atom,
e- cloud may be distorted
• May make covalent radius larger than atomic radius
– Same thing happens w/ Van der Waals radii
• Radii are used to predict internuclear dist betw
atoms.
12.10 Special Properties of Metals
• Metals form crystals in which ea metal atom is
surrounded by 8 or 12 neighboring metal
atoms
– Form when atoms crowd together and outer level
orbitals overlap
– E-’s can easily move from 1 atom to another
• These e-’s are called delocalized electrons.
• Not held in 1 place – “float” around the crystal
12.10 Special Properties of Metals
• If an electric field is applied, e-’s will flow thru
the metal, creating electric current
• Delocalized e-’s interact w/ light creating
luster.
• Metalic Bond – constituted by the delocalized
e-’s holding metallic atoms together
12.10 Special Properties of Metals
• Properties of metals are determined by the #
of outer e-’s available
– Group 1 - 1 e- available - soft
– Group 2 - 2 e-’s available - harder
– Transition metals – d e-’s take part in metallic
bond
• Many are very hard & stron
• Groups 3-6 – 3 to 6 delocalized e-’s
• Groups 7-10 - 6 delocalized e-’s
– Not all d e-’s are involved in the bond
12.10 Special Properties of Metals
• It is possible to strengthen some metals by
combining them w/ others to form alloys.