Transcript 2. Chemical Bond

Introduction to chemical bond
Lewis Structure
Ionic bond
Covalent bond
Metallic bond
Intermolecular attractive force
H2 chemical bond formation
O-H bond formation
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MXn  M diletakkan di tengah sehingga terdapat n M-X
ikatan
H dan Halogen hanya memiliki 1 ikatan, group 16  2
ikatan, group 15  3 ikatan group 14  4 ikatan
Group2 dan group 13 electron deficient
Element di bawah Mg  hipervalence
C,N,O,P,S  effective -type overlap
Counting electrons
 The total number of electrons represented in a Lewis structure is equal to the sum
of the numbers of valence electrons on each individual atom. Non-valence
electrons are not represented in Lewis structures.
 The octet rule states atoms with eight electrons in their valence shell will be
stable, regardless of whether these electrons are bonding or nonbonding. The rule
applies well to acidic compounds. The 18-Electron rule is operative on atoms from
period 4, which have to achieve 18 electrons to their orbitals and achieve a stable
configuration which has the same electron configuration as a Noble gas. Similarly
from period 6, the atoms have to achieve 32 electrons to fill their orbitals.
Placing electrons
 Lewis structures for oxygen, fluorine, the hydrogen sulfate anion, and formamide
 Once the total number of available electrons has been determined, electrons must
be placed into the structure. They should be placed initially as lone pairs: one pair
of dots for each pair of electrons available. Lone pairs should initially be placed on
outer atoms (other than hydrogen) until each outer atom has eight electrons in
bonding pairs and lone pairs; extra lone pairs may then be placed on the central
atom. When in doubt, lone pairs should be placed on more electronegative atoms
first.
 Once all lone pairs are placed, atoms, especially the central atoms, may not have
an octet of electrons. In this case, the atoms must form a double bond; a lone pair
of electrons is
Examples
Tentukan struktur lewis dari :
O3 ,
NO3- ,
NCO ?
PO43-,
BrO3F,
IO2F2 ?
ClO4-,
SO42- ,
PF5 ,
SF4 ?
Hypervalence : P, S, Br
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Calculate number of bond
Jika aturan oktet dipenuhi :
number of bond = (total e- needed) –(valence e- available)
e- needed 2
= 5x8 = 40
e- available = 6 + 4(7) =34
Number of bonding = (40-34)/2 =3 ?
H-C≡N
e- needed = 1(2) +2(8) = 18
e- available = 1 + 4 + 5 = 10
Number of bonding = (18-10)/2 =4 ?
'Formal Charge’ is defined as the
number of valence electrons minus the
number of lone pair electrons minus
one half of the bonding electrons.
5-4-2=-1
6-2-3 = +1
6-2-3=+1
6-6-1=-1
The simplest Lewis theory cannot predict this without a 'fix' called
resonance.
6-6-1 =-1
Secondary resonance structure
S-O bond are chemically and symetrically
equivalent, average f.c = -1/2
5-4-2= -1
4-3 = +1
Muatan formal +  EN rendah
Muatan formal -  EN tinggi
Li-F
H-F
Both atoms have 0 formal charges
Ionic resonance
Mengapa sebuah struktur memiliki muatan parsial?
Apa perbedaan antara muatan formal dan muatan parsial?
Struktur molekul memiliki muatan parsial jika terbentuk
dari element element yang memiliki perbedaan
elektronegativitas
Intermolecular attractive force
Hydrogen bond vs covalence bond
Hydrogen bond effect
CHCl3
61.2°C
CCl4
76.8°C
The boiling points of ethanol and methoxymethane show the dramatic effect that the hydrogen bonding has on the
stickiness of the ethanol molecules:
The hydrogen bonding in the ethanol has lifted its boiling point about 100°C.
It is important to realise that hydrogen bonding exists in addition to van der Waals attractions. For example, all the following
molecules contain the same number of electrons, and the first two are much the same length. The higher boiling point of the
butan-1-ol is due to the additional hydrogen bonding.
ethanol (with hydrogen
bonding)
78.5°C
methoxymethane (without
hydrogen bonding)
-24.8°C