medelyn VSEPR
-
Upload
gringgo222 -
Category
Documents
-
view
225 -
download
0
Transcript of medelyn VSEPR
7/29/2019 medelyn VSEPR
http://slidepdf.com/reader/full/medelyn-vsepr 1/6
VSEPR-Valence shell electron pairrepulsion
Valence shell electron pair repulsion (VSEPR) rules are a model in chemistry used to predict the shape of individual molecules based upon the extent of electron-pair electrostatic
repulsion.[1]
It is also named Gillespie – Nyholm theory after its two main developers. The
acronym "VSEPR" is sometimes pronounced "vesper" for ease of pronunciation; however, the
phonetic pronunciation is technically more correct.
The premise of VSEPR is that the valence electron pairs surrounding an atom mutually repel
each other, and will therefore adopt an arrangement that minimizes this repulsion, thus
determining the molecular geometry. The number of atoms bonded to a central atom plus the
number of lone pairs of its nonbonding valence electrons is called its steric number.
VSEPR theory is usually compared and contrasted with valence bond theory, which addresses
molecular shape through orbitals that are energetically accessible for bonding. Valence bondtheory concerns itself with the formation of sigma and pi bonds. Molecular orbital theory is
another model for understanding how atoms and electrons are assembled into molecules and
polyatomic ions.
VSEPR theory has long been criticized for not being quantitative, and therefore limited to the
generation of "crude", even though structurally accurate, molecular geometries of covalent
molecules. However, molecular mechanics force fields based on VSEPR have also been
developed.
Valence Shell Electron Pair Repulsion Theory
Valence shell electron pair repulsion theory, VSEPR, is a super-simple technique for predictingthe shape or geometry of atomic center in small molecules and molecular ions:
7/29/2019 medelyn VSEPR
http://slidepdf.com/reader/full/medelyn-vsepr 2/6
Crucially, atomic center with VSEPR determined geometry can be joined together into molecular
entities like cyclohexane and glucose:
This molecular building-block logic can be extended, enabling large bio molecular structures like
DNA to be modelled and understood:
The VSEPR Technique
Six or so steps are required to generate the VSEPR geometry of an atomic centre such as:
Carbon in methane, CH4
Nitrogen in ammonia, NH3Xenon in xenon tetra fluoride, XeF4
Iodine in the iodide di fluoride ion, [IF2] –
First, determine the number of electrons in the outer (valence) shell about the central
atom (C, N, Xe, I, etc.):
7/29/2019 medelyn VSEPR
http://slidepdf.com/reader/full/medelyn-vsepr 3/6
Carbon, for example has four valence electrons, nitrogen 5, etc.
Second, find valency and number of electrons associated with the ligand X:
Third, construct a valid Lewis structure of the molecule in question showing all of
the bonds and all of the lone pairs (nonbonded pairs) of electrons.
If the structure is a molecular ion, add one valence electron for each negative chargeand remove one valence electron for each positive charge.
7/29/2019 medelyn VSEPR
http://slidepdf.com/reader/full/medelyn-vsepr 4/6
Not all Lewis structures have eight electrons about the central atom A (as emphasized
by very simple Lewis octet theory). For example,
Sulfuric acid, H2SO4, has two monovalent OH functions and two doubly bonded
oxygen that behave as single ligands:
Phosphorus pentachloride, PCl5, has 10 electron
7/29/2019 medelyn VSEPR
http://slidepdf.com/reader/full/medelyn-vsepr 5/6
Valence- Shell Electron- Pair Repulsion
(VSEPR) Models
The 3-dimensional structure of BF3 is different from PF3, and this is difficult tocomprehend by considering their formulas alone. However, the Lewis dot structure for them are
different, and the electron pair in :PF3 is the reason for its structure being different from BF3 (no
lone pair).
Three-dimensional arrangements of atoms or bonds in molecules are important properties as are bond lengths, bond angles and bond energies. The Lewis dot symbols led us to see the non- bonding electron pairs, whose role in determining the shape of a molecule was examined by N.V.
Sidgwick and H.E. Powell in 1940, and later by R.S. Nyholm and R.J. Gillespie. They have
developed an extensive rationale called valence-shell electron-pair repulsion (VSEPR) model of
molecular geometry.
The Valence-Shell Electron-PairRepulsion (VSEPR) models consider theunshared pairs (or lone electron pairs) and the
bonding electrons. This considerations of lone
and bonding electron pairs give an excellentexplanation about the molecular shapes. The
VSEPR model counts both bonding and
nonbonding (lone) electron pairs, and call the
total number of pairs the steric number (SN ). If the element A has m atoms bonded to it
and n nonbonding pairs, then
SN = m + n
SN is useful for predicting shapes of molecules.
If X is any atom bonded to A (in single, double,or triple bond), a molecule may be represented
by AXmEn where E denote a lone electron pair.
This formula enable us to predict its geometry.The common SN , descriptor, and examples are
given in the table on the right.
Note that the SN is also called the number of VSEPR pairs or number of electron pairs. The
VSEPR model has another general rule: lone pairs of electrons take up more space than bondedpairsmaking the bond angle, say H-O-H for water less than the tetrahedral angle of 109.5 °.
Actually, the H-O-H angle in water is 105 °.
The geometry of the molecules with their SNs equal to 2 to 6 are given in the Table. The firstline for each is the shape including the lone electron pair(s). If the lone electron pairs are
ignored, the geometry of the molecule is given by another descriptor.
Molecular shapes and steric numbers (SN)
Example SN Descriptor
BeCl2, CO2 2 Linear
BF3, SO3 SO2E, OO2E
3Trigonal planar bent
CH4
NH3E
H2OE2
4
Tetrahedral
pyramidal
bent
PF5
SF4EClF3E2
5
Trigonal bypyramidal
butterflyT-shape
SF6, OIF5 BrF5E
XeF4E2
6octahedral pyramidal
square planar
E represents a lone electron pair.
SN is also called the number of VSEPR pairs
or number of electron pairs.
7/29/2019 medelyn VSEPR
http://slidepdf.com/reader/full/medelyn-vsepr 6/6
The VSEPR theory assumes that each atom in a molecule will achieve a geometry that
minimizes the repulsion between electrons in the valence shell of that atom. The five compounds
shown in the figure below can be used to demonstrate how the VSEPR theory can be applied tosimple molecules.