CHEM132 EXPERIMENT 4: Lewis Structures/VSEPR Theory

Purpose

1. construct Lewis structures given chemical formulas 2. predict type of bonding (ionic, nonpolar, polar) from a lewis structure 3. describe molecular shape and bond angles 4. predict molecular and electron geometry

Steps to drawing a lewis structure of an element

1. determine number of valence electrons (which will be the number of dots) 2. one electron on each side 3. once all four sides have an electron, remaining electrons pair around the atom

what causes a dipole?

If electrons are shared unequally, one atom of the bond has a higher electron density and holds electrons more tightly than the other, causing one electron to have a a partial negative charge and one to have a partial positive charge

What type of bond is CH?

nonpolar, C=2.5 H=2.1, 2.5-2.1= .4

Polar covalent bonds

occur between atoms with electronegativity differences between .4 and 2, the electrons are shared but they are shared UNEQUALLY, creating a partial positive charge on one atom and a partial negative charge on the other

nonpolar covalent bonds

occurs when the electronegativity difference is less than .4 and the electrons are shared equally

3D shape governs what?

physical and chemical properties of the molecule

What does the number of valence electrons in an atom correspond to?

placement/group on the periodic table

What type of bond is AlCl?

polar, Al= 1.5 Cl=3.0, 3.0-1.5= 1.5

How is polarity determined?

primarily by symmetry, if the molecule is symmetric the magnitude and direction of the charges pull in equal but opposite directions and we consider the molecule neutral overall and to have no net dipole

method for drawing lewis structures of a molecule or ion

1. determine the number of valence electrons, be sure to subtract or add for positive or negative charges 2. draw a skeletal structure by determining what atoms are bonded to one another, place the least electronegative atom in the center and make an effort to place the other atoms around the central atom SYMMETRICALLY 3. connect the central atom to the other atoms with a line representing a bond 4. complete the octets for the atoms surrounding the central atom 5. place the remaining electrons around the central atom (remember it may have more than 8 if it is in row 3 or beyond) --- count the electrons, if they match the number of valence electrons in the molecule you likely have the correct structure if not continue-- 6. if you have too many electrons, you need to include double or triple bonds (maintaining octet rule), convert electron pairs from a surrounding atom to a double or triple bond (remember some atoms may not form multiple bonds and may be left short of an octet)

Determining molecular geometry

1. draw correct lewis structure 2. count number of electron domains around the central atom 3. count the number of nonbonding electron pairs 4. consult "molecular geometry" table

main procedure steps

1. draw lewis structures of each compound 2. build models of each of the compounds

Determining electronic geometry

1. draw the correct lewis structure 2. count the number of domains around the central atom (single, double, triple bonds are all counted as one domain) 3. consult "predicting electron geometry table"

what is the most stable resonance structure?

1. there is a minimum number of formal charges 2. like charges are separated 3. negative formal charges are on the more electronegative atom and positive formal charges are on the less electronegative atom

What are the angles between the nonbonding pairs in ICl4-?

180

to predict if an ionic bond will form, the electronegativity of the atoms should differ by atleast how much?

2

What is the maximum number of dots around an atom?

8, forming an octet of electrons

Which resonance structure is "correct"?

For molecules with resonance, one single lewis structure does not accurately represent the bonding, the true bond is somewhere between the reasonable lewis structures (calculated by formal charges)

If a molecule is found to have bent molecular geometry, is it possible to determine the hybridization of the orbitals?

It is not possible to determine hybridization from molecular geometry alone. You would need to know the number of electron domains because bent molecular geometry can occur with 3 or 4 domains , which have different hybridizations.

Draw the lewis structure for BrF3, determine the number of electron domains, bond angles, electron geometry, hybridization, and molecular geometry.

Lewis Structure should look like picture attached 5 electron domains (three bonding domains around Br and 2 lone pairs on Br) 90, 120, 180 trigonal bipyramidal (because of the 5 electron domains) spd3 (because it's trigonal bipyramidal) T shaped (5 electron domains, 2 lone pairs)

Draw the lewis structure for SF6, determine the number of electron domains, bond angles, electron geometry, hybridization, and molecular geometry.

Lewis structure should look like the picture 6 electron domains 90, 180 octahedral sp3d2 octahedral

Draw the lewis structure for CO2, determine the number of electron domains, bond angles, electron geometry, hybridization, and molecular geometry.

Lewis structure should look like the picture attached 2 electron domains 180 linear sp linear

formal charge

The number of valence electrons in an isolated atom minus the number of electrons assigned to the atom in the Lewis structure

Why do valence electrons participate in bonding?

They are further away from the positively charged nucleus and are in a higher energy state, the electrons form bonds to become more stable.

Predict the molecular geometry of CO3^2-

Trigonal planar

What kind of structure stabilizes a molecule?

a structure that minimizes electrostatic interactions between electron pairs

resonance structures

atoms are arranged in the same way but the position of the electrons is different, typically encountered in molecules with double or triple bonds, represented by a double arrow **you MAY NOT have fewer than 8 electrons around the central atom in resonance structures**

octet rule

atoms tend to gain, lose, or share electrons until they are surrounded by eight valence electrons

what is the bond angle, number of domains, and hybridization for tetrahedral electron geometry?

bond angles: 109.5 electron domains: 4 hybridization: sp3

what is the bond angle, number of domains, and hybridization for trigonal planar electron geometry?

bond angles: 120 electron domains: 3 hybridization: sp2

what is the bond angle, number of domains, and hybridization for linear electron geometry?

bond angles: 180 electron domains: 2 hybridization: sp

what is the bond angle, number of domains, and hybridization for trigonal bipyramidal electron geometry?

bond angles: 90, 120, 180 electron domains: 5 hybridization: sp3d

what is the bond angle, number of domains, and hybridization for octahedral electron geometry?

bond angles: 90, 180 electron domains: 6 hybridization: sp3d2

bonding pairs vs lone pairs

bonding pairs are shared by atoms, lone pairs are non bonding

electronic geometry vs molecular geometry

electronic geometry describes the spatial arrangement of all pairs of electrons around the central atom, both bonding and lone pairs molecular geometry describes the spatial arrangement of only the bonding pairs of electrons

What happens sometimes to satisfy the octet rule?

electrons are added or removed from the molecule (an ion)

What are the two exceptions to the octet rule?

hydrogen and helium (accommodate two electrons)

What type of bond is NaCl?

ionic, Na= .9 Cl=3.0, 3.0-.9= 2.1

In an ionic bond, An atom with low ________ will lose it's electron(s) and transfer them to an atom with high __________.

ionization energy, electron affinity

generally ionic bonds form between

metals and nonmetals

VSEPR theory

regions of high electron density (electrons or lone pairs) will orient themselves as far apart from one another as possible in 3d space

when more than one acceptable lewis structure can be drawn, the molecule is said to have what?

resonance

covalent bond

results from the sharing of electrons of two nonmetals

Ionic Bond

results from transfer of electrons from a metal to a nonmetal, an electrostatic interaction between a positively charged cation and a negatively charged anion

Beyond what row can certain atoms expand their octet and accommodate more than eight electrons and more than four bonds?

row 3

An arrow with magnitude and direction can represent each dipole within a molecule, what is the magnitude equal to and what way does the arrow point?

the arrow points from the less electronegative atom to the more electronegative atom and the magnitude is equal to the difference in electronegativity

What happens to energy state when bonds are formed?

the energy state is lowered and the molecule becomes more stable than the parent atom

In a dipole, which atom bears the partial negative charge?

the more electronegative atom

True or False: atoms can share multiple pairs of electrons and form double or triple bonds

true