CHEM 217 Lecture Notes - Lecture 9: Skeletal Formula, Electron Configuration, Diatomic Molecule
Unit 9 : Bonding
9. 1 BO ND I NG A ND A I D S DR U G S
Without HIV protease, AIDS can’t develop
With knowledge of structure of HIV-protease, pharmaceutical companies set to create a molecule that
would disable the active site by sticking to it
oUsed bonding theories to simulate the shape- companies developed several and they were
named protease inhibitors
9. 2 T Y PE S O F B O ND S
Chemical bonds form because they lower the potential energy between the charged particles that
compose atoms
oWhen two atoms approach each other, the electron of one atom are attracted to the nucleus of
another, according to coulomb’s law and vice versa
But at the same time, the electrons of each atom repel the electrons of the other and
the nucleus of one repels the nucleus of the other
Result is a complex set of interactions among a potentially large number of charged
particles
oIf these interactions lead to an overall net reduction of energy between charged particles, a
chemical bond forms
Metal and nonmetal -> ionic bonds -> electrons transfer -> anions and cations attract and lower potential
energy
Nonmetal and nonmetal -> covalent -> electrons shared -> shared electrons interact with both nuclei,
lowering potential energy
Metal and metal -> metallic bond -> electrons pooled
Can understand stability of covalent bond considering most stable arrangement (lowest potential energy)
of two positively charged particles separated by a small distance and a negatively sharged particle
oThe arrangement where negatively charged partiles lies between the two postivie ones has the
lowest potential energy because in this arrangement, the negative particle interacts most
strongly with both of the positively charged ones
In a sense, egatively charged particle holds the two positievs together by attracting the
positive charges of their nuclei
METALLIC BONDING
Metals lose electrons easily – in simplest model for metallic bonding – electron sea model – all atoms in a
metal lattice pool their valence electrons
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oThese pooled electrons are no longer localized on a single atom but delocalized over an entire
metal
The positively charged metal atoms are then attracted to the sea of electrons holding
metal together
9. 3 R E PR E SEN TIN G V A LE N C E EL E C T R O N S W ITH DO T S
Main group elements: valence electrons are those in outermost principal energy level
oSince valence electrons are held most loosely and since chemical bonding induces transfer/share
of valence electrons they are most important
A lewis diagram represents electrons as dots
oAtoms with 8 valence electrons are particularly stable – stable octet
Bonding atoms obtain stable electron configurations
oSince usually that means 8, this is known as the octet rule
Doesn’t try to calculate charges – focuses on octet rule because its simple. Octet rule is
a practical approach that does not accurately predict what happens for a large number
of compounds
9. 4 IO NIC B O ND I N G AN D L A TTI C E ENE R G I E S
Although Lewis models covalent bonding it also can be appied to ionic
IONIC BONDING AND ELECTRON TRANSFER
The lewis structure of an anion is usually written within brackets with a charge outside the upper right
oThen the positive and negative attract and result in compound
LATTICE ENERGY
Formation of ionic compound from its constituent elements is usually quite exothermic
oBut the transfer of electrons actually absorbs energy – based on only IE required, reactions would
be endothermic.
oLattice energy – energy associated with forming a crystalline lattice of alternating cations and
anions from the gaseous ions. Since the positive and negative charges meet to reduce the
potential energy and that emergy is emitted as the lattice forms
oAmount of energy released when lattice forms is difficult to calculate but best done through…
BORN HABER CYCLE
Hypothetical series of steps that represents formation of an ionic compound from constituent elements
Steps chosen so the change in enthalpy of each step is known except for the last one, which is the lattice
energy
o^ in enthalpy for overall process is also known
Using Hess’ law, we can etermine enthalpy change for unknown last step, lattice energy
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Trends in lattice energies: Ion size
oMagnitude of lattice energy decreases as we move down a column
We know that radius increases as we move down and that potential energy of
oppositely charged ions becomes less negative as distance between ions increases – the
size of alkali metal ions increases down the column and so does distance between metal
cations and chloride aions
The magnitude of the lattice energy of the chlorides decreases accordingly, making the
formation of the chlorides less exothermic
So, as ionic radii increase as we move down a column, the ions can’t get as
close to each other so they don’t release as much energy when lattice forms
Trends in lattice energies: Ion charge
oSometimes though even if th distance between two ions is greater, they can release more lattice
energy and it has to do with charge – magnitude of potential energy of two interacting charges
depends not only on the distance between the charges but also on the product of the charges
oE=(1/4πε0)(q1q2/r)
oFor NaF, energy is proportional to (1+)(1-)= 1- while CaO, E is proportional to (2+)(2-)=4-
So the relative stabilization for CaO relative to NaF is roughly 4x greater as observed in
the lattice energy
Summarizing trends in lattice energies:
oLattice energies become less exothermic (less negative) with increasing ionic radius
oLattice energies become more exothermic (more negative) with increasing magnitude of ionic
charge
9. 5 C O V A L EN T BO N D I N G: A N I N T RO TO L E WIS ST R UC T UR E S OF
M O LE C U L ES
A shared pair of electrons is called abonding pair while a pair tat is associated with one atom (not
involved) is called a lone pair (nonbonding electrons)
Represent shared electrons by a line between atoms
DRAWING LEWIS FOR MOLECULAR COMPOUNDS
Need to know two things
oWhat atoms are bonded together
oHow many valence electrons each needs to be represented
oA double bond is generallt shorter and stronger than a single bond
Remember in Lewis one dash always represents two electrons
Atoms can also share three pairs of electrons, as in N2 (N has 5 valence electrons)
oTriple bonds are even shorter and stronger than double bonds
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