Understandings 1.Lewis (electron dot) structures show all the valence electrons in a covalently bonded species 2.The “octet rule” refers to the tendency of atoms to gain a valence shell with a total of eight electrons. 3.Some atoms, like Be and B, might form stable compounds with incomplete octets of electrons. 4.Resonance structures occur when there is more than one possible position for a double bond in a molecule. 5.Shapes of species are determined by the repulsion of electron pairs according to the valence shell electron pair repulsion (VSEPR) theory. 6.Carbon and silicon form covalent network (giant covalent) structures.
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4.3 Covalent Structures IB Chemistry SL Mrs. Page Essential Idea
Lewis (electron dot) structures show the electron domains in the
valence shell and are used to predict molecular shape. Nature of
Science Scientists use models as representatives of the real world
the development of the model of molecular shapes (VSEPR) to explain
observable properties. Understandings 1.Lewis (electron dot)
structures show all the valence electrons in a covalently bonded
species 2.The octet rule refers to the tendency of atoms to gain a
valence shell with a total of eight electrons. 3.Some atoms, like
Be and B, might form stable compounds with incomplete octets of
electrons. 4.Resonance structures occur when there is more than one
possible position for a double bond in a molecule. 5.Shapes of
species are determined by the repulsion of electron pairs according
to the valence shell electron pair repulsion (VSEPR) theory.
6.Carbon and silicon form covalent network (giant covalent)
structures. Application & Skills 1.Deduction of Lewis (electron
dot) structures of molecules and ions showing all valence electrons
for up to four electron pairs on each atom. 2.The use of VSEPR
theory to predict the electron domain geometry and the molecular
geometry for species with two, three, and four electron domains.
3.Prediction of bond angles from molecular geometry and presence of
non-bonding pairs of electrons 4.Prediction of molecular polarity
from bond polarity and molecular geometry 5.Deduction of resonance
structures, examples include but are not limited to C 6 H 6 CO 3 2-
and O 3 6.Explanation of the properties of covalent networks (giant
covalent) compounds in terms of their structures. U1 &2: LEWIS
STRUCTURES Multiple ways to show the same molecule N H H H NH H H
NH H H NH H H X U1& 2: Lewis Structures & Ions We can use
Lewis structures of ionic compounds Must use brackets around cation
and anion (electrostatic force bonds ions) NH 4 ClNH 4 NO 3 U1
& 2: Rules for Lewis Structures Add up the total number of
valence electrons in the molecule. Draw the skeletal structure. Use
a line between each element to symbolize an electron pair.
Distribute the remaining electrons around the elements in pairs to
form octets. (Hydrogen can only ever have 2 electrons.) If you do
not have enough to form octets, make double or triple bonds. Ions
must have square brackets around them with the charge notated in
the top right hand corner. To be a correct Lewis structure, ALL
electrons must be shown. U1 &2: Lewis Structures Tell us about
covalent bonds Bonding pairs vs. Lone pairs Single, double or
triple bonds They do not tell us about the actual shape of the
molecules U5: VSEPR Valence Shell Electron Pair Repulsion theory.
States that pairs of valence electrons repel each other and are
therefore arranged as far apart from each other as possible. So far
we have dealt with structural formulas which only show the types of
atoms, bonds and lone pairs of electrons. They do not show the
shape of the molecule. VSEPR is a model that allows us to look at
shapes of molecules U5: VSEPR Basic Shape To determine the shape
you must look at the electron domains (areas where there are pairs
of electrons) U5: VSEPR Basic Shape Linear: central atom with two
electron domains (no lone pairs) Bond angle 180 Ex: BeCl 2, CO 2
https://www.boundless.com/ U5: VSEPR Basic Shape Trigonal Planar:
central atom with three electron domains (no lone pairs) Bond angle
120 Ex: BeF 3, NO 3 - https://www.boundless.com/ U5: VSEPR Basic
Shape Tetrahedral: central atom has four electron domains (no lone
pairs) Bond Angle 109.5 Ex: CH 4, [NH 4 ] +
https://www.boundless.com/ Solid Wedge = bond in front Dashed Wedge
= bond toward back Effect of Lone Pairs Molecular Geometry gives
the shape of the molecule Electron Domain Geometry is based on the
number of electron domains Electron domains can be bonded pairs or
lone pairs Lone pair electrons occupy more space than bonding pairs
and therefore alter the bond angles from molecular geometry The
more lone pairs, the greater the repulsion and therefore the
greater the impact on the bond angle U5: VSEPR V-Shaped (Bent) e-
Geometry: Trigonal Planar Molecular Geometry: (V-shaped or bent) :
central atom has three electron domains (1 lone pair) Bond angle
is
