ch4.12.bonding molgeo

1Chapter 4© 2013 Pearson Education, Inc.

Chemical Bonding: The Ties that Bind

Carbon exists commonly as charcoal, coal, peat and soot. When soot is subjected to high temperature and pressure, it can form diamond. This process can be explained by understanding the chemical bonds that hold the atoms together.


Stable Electron Configurations

Fact: Noble gases, such as helium, neon, and argon are inert, they undergo few if any, chemical reactions.Theory: The inertness of noble gases results from their electron structures; each (except helium) has an octet of electrons in its outermost shell.Deduction: Elements become less reactive when they alter their electron structures to that of a noble gas.


Sodium can lose a valence electron. After doing so, its core electrons are configured like the noble gas neon.



Chlorine can gain an electron, and in doing so, its electron structure becomes like argon.



Lewis (Electron Dot) SymbolsG. N. Lewis developed a method of visually representing the valence electrons as dots around the symbol of an atom.

1)What is a valence electron?

2)Why do some atoms “lose” valence e-s while others “gain” them?q


Lewis (Electron Dot) Symbols


Sodium Reacts with Chlorine (Fact)


Sodium Reacts with Chlorine (Theory)


Na+ ions and Cl- have opposite charges and attract each other. The resulting attraction is an ionic bond.Ionic compounds are held together by ionic bonds and exist as crystal lattice.

Sodium Reacts with Chlorine (Theory)


Atoms and Ions: Distinctively Different


Octet Rule

In chemical reactions, atoms tend to gain, lose, or share electrons so as to have eight valence electrons. This is known as the octet rule.

A little bit of relevant history about Dmitri Mendeleev:

http://web.lemoyne.edu/~giunta/ea/mendeleevann.html


Octet Rule

Metals lose electrons to take on the electron structure of the previous noble gas. In doing so, they form positive ions (cations).

Nonmetals tend to gain electrons to take on the electron structure of the next noble gas. In doing so, they form negative ions (dogions)..uh nope, that would be anions.


Octet Rule


Formulas and Names of Binary Ionic Compounds

Cation Charge: The charge of a cation from the representative elements is the same as the family number.

The name of a cation is simply the name of the element.

Examples:

Na+ = sodium ion

Mg2+ = magnesium ion


Anions: The charge of an anion from the representative elements is equal to the family number minus eight.The name of an anion is the root name of the element plus the suffix –ide.Examples:

Cl- = chloride ionO2- = oxide ion



To name binary ionic compounds, simply name the ions.

Examples:

NaCl = sodium chloride

MgO = magnesium oxide



Many transition metals can exhibit more than one ionic charge. Roman numerals are used to denote the charge of such ions.

Examples:

Fe2+ = iron(II) ion

Fe3+ = iron(III) ion

Cu2+ = copper(II) ion

Cu+ = copper(I) ion



Commonly Encountered Ions



Covalent Bonds

Many nonmetallic elements react by sharing electrons rather than by gaining or losing electrons.

When two atoms share a pair of electrons, a covalent bond is formed.

Atoms can share one, two, or three pairs of electrons, forming single, double, and triple bonds.


Binary covalent compounds are named by using a prefix to denote the number of atoms.

Names of Binary Covalent Compounds


Binary covalent compounds have two names:

2.Second name = prefix + root name of second element + suffix –ide.

1. First name = prefix + name of 1st element(Note: If the first element has only one atom, the prefix mono- is dropped.)



Easy Examples:

SBr4

sulfur tetrabromide

P2O3

diphosphorus trioxide



ElectronegativityElectronegativity is a measure of an atom’s attraction for the electrons in a bond.


Polar Covalent Bonds

When two atoms with differing electronegativities form a bond, the bonding electrons are drawn closer to the atom with the higher electro-negativity. Such a bond exhibits a separation of charge and is called a polar covalent bond.


Bond Polarity

Bond polarity can be represented on a Lewis structure with either the partial symbol or with the arrow as shown at the right.


The difference in electronegativity between two bonded atoms can be used to determine the type of bond. Use the adjacent table as a rule of thumb.

Δ EN Type of Bond

< 0.5 Nonpolar covalent

Between 0.5 and 2.0

Polar covalent

Greater than 2.0

Ionic

Bond Polarity


Polyatomic Ions

Polyatomic ions are groups of covalently bonded atoms with a charge.


Writing Formulas Using Polyatomic Ions

When writing formulas for compounds containing polyatomic ions, it may be necessary to use parentheses to denote the proper number of the ions.

Example: calcium nitrate

Ca2+ NO3-

Ca(NO3)2


Naming Compounds with Polyatomic Ions

When naming compounds with polyatomic ions, simply name the ions in order.

Example: (NH4)2SO4

ammonium sulfate


Rules for Sketching Lewis Structures

1. Count valence electrons.2. Sketch a skeletal structure.3. Place electrons as lone pairs around outer atoms to

fulfill the octet rule.4. Subtract the electrons used so far from the total

number of valence electrons. Place any remaining electrons around the central atom.

5. If the central atom lacks an octet, move one or more lone pairs from an outer atom to a double or triple bond to complete an octet.


Sketching Lewis Structures


Odd Electron Molecules: Free Radicals

An atom or molecule with an unpaired electron is known as a free radical.

Examples include:

NO NO2 ClO2


Molecular Shapes: The VSEPR Theory

The Valence Shell Electron Pair Repulsion (VSEPR) theory predicts the shape of molecules and polyatomic ions based on repulsions of electron pairs on central atoms.


….and then there was Spaceballs with Princess Vespa!


VESPA


Shapes and Properties: Polar and Nonpolar Molecules

In order for a molecule to be polar, two conditions must be met:

1. It must have polar bonds.

2. The bonds must be arranged such that a separation of charge exists.


Ammonia

Ammonia Manufacturing Plant

• What is NH3(g)?

• Hmmm…what is (NH4)1+(aq)

• Which one smells bad?

Only the lone pairs know the way I feel today…Only the lone pairs…Know this feelin’ ain’t right…


Common Molecular Geometries…..with a Central Atom


Determining Bond Angles in a Simple Molecule…a Rule of Thumb

• What about when there are lone pairs of electrons, (non-bonding pairs) on the central atom in a molecule of a part of a molecule?

• In general a bond angle is compressed 2o for each pair of electrons.


Chemical Vocabulary


Water: The Case of a Bent Molecule!

• Example: WaterThe ideal H-O-H bond angle is 109.5o

The experimental H-O-H bond angle is 104.5o

• Why does the bond angle change?


Meanwhile in Elizabethan England…Dickens ponders a sequel novel… “A Tale of Three

Molecules: Carbon Dioxide, Tetrachloromethane and Formaldehyde”

http://images.google.com/imgres?imgurl=http://wps.prenhall.com/wps/media/objects/602/616516/Media_Assets/Chapter10/Text_Images/FG10_0103UN.JPG&imgrefurl=http://wps.prenhall.com/wps/media/objects/602/616516/Chapter_10.html&h=434&w=1600&sz=81&tbnid=HFA_cJDb76cJ:&tbnh=40&tbnw=147&start=2&prev=/images%3Fq%3Dcarbon%2Bdioxide%2Bdipole%26hl%3Den%26lr%3D

Take it from me, “Mr. Hyde”…formaldehyde is really dreadful stuff…ycchh!

Chuck


Predicting Molecular Polarity: Carbon Dioxide, Tetrachloromethane &

Formaldehyde

1) For each of the molecules in the previous slide identify each bond in the molecule as either polar or non-polar. (BTW: If the difference in electronegativity for the atoms in a bond is greater than 0.4, we consider the bond polar. If the difference in electronegativity is less than 0.4, the bond is essentially non-polar.)

2) If there are no polar bonds, the molecule is non-polar. If the molecule has polar bonds, move on to Step #4.

3) If there is only one central atom, examine the electron groups around it.4) If there are no lone pairs on the central atom, and if all the bonds to the

central atom are the same, the molecule is non-polar. If the central atom has at least one polar bond and if the groups bonded to the central atom are not all identical, the molecule is probably polar. Move on to Step #7.

5) Draw a geometric sketch of the molecule. (3-D wedge, line & dotted line)


Predicting Molecular Polarity…continued!

7) Determine the symmetry of the molecule using the following steps:a) Describe the polar bonds with arrows pointing toward the more

electronegative element. b) Use the length of the arrow to show the relative polarities of the

different bonds. (A greater difference in electronegativity suggests a more polar bond, which is described with a longer arrow.)

c) Decide whether the arrangement of arrows is symmetrical or asymmetrical If the arrangement is symmetrical and the arrows are of equal length, the molecule is non-polar. If the arrows are of different lengths, and if they do not balance each other, the molecule is polar. If the arrangement is asymmetrical, the molecule is polar.

d) Try your skills…goto…ChemTeam…

http://dbhs.wvusd.k12.ca.us/webdocs/Bonding/Molecular-Polarity.html

http://www.mpcfaculty.net/mark_bishop/molecular_polarity_study_sheet.htm


Polarity: The TruthIntroduction:• The polarity of a molecule is the sum of all of the bond

polarities in the molecule. • Since the dipole moment (m, measured in Debyes (D)) is a

vector (a quantity with both magnitude and direction), the molecular dipole moment is the vector sum of the individual dipole moments.

• Remember Dicken’s sequel? Well…if we compare the molecular dipole moments of formaldehyde and carbon dioxide, both containing a polar carbonyl (C=O) group, we find that formaldehyde is highly polar while carbon dioxide is non-polar. Since CO2 is a linear molecule, the dipoles cancel each other.

• Now…Draw a reasonable Lewis structure for these molecules.

FYI: If you’re really motivated to learn about how dipole moments are calculated..goto this link:

http://www.chemistry.mcmaster.ca/esam/Chapter_7/section_3.html

John Roberts, Chief Justice, Supreme Court


Quick, let’s make our getaway or we’ll be Lost in the Ozone Again!!!

• Decomposition of Ozone movies: CL2F2; NO

• http://cwx.prenhall.com/petrucci/medialib/media_portfolio/15.html

• Great Ozone 3-D Geometry site:• http://www.elmhurst.edu/~chm/

vchembook/206bent.html

• http://earthobservatory.nasa.gov/Newsroom/NewImages/images.php3?img_id=5189


Expanded Octets…(uh duz that mean like 9 or 10?)

• OK…there is this compound called Xenon tetrafloride.• Is it covalent? • How do you know?• What is the central atom in the structure?• How do you know?• Is the Lewis Structure at the right “correct”? Why/ Why not?

Consider the following: 1) When you make a Lewis Structure for a molecule of a

compound you first determine the sum of the valence electrons for each atom represented in the empirical formula of the compound. For XeF4 that would be (8)Xe + (4 x 7)4F = 36e-s.

2) Doing some simple math, XeF4 requires four bonds, one each for each Xe-F bond, (4 total). That requires 4 x 2 = 8 e-s

3) This gets complicated because Xe has a full valence shell, with no single electrons available for forming bonds. However, if you split two of the pairs on Xe you get four, single electrons available for forming bonds.

Lewis Structure

3-D Structure


XeF4 & Expanded Octets continued….

4) Following this logic, make a single bond with each F atoms connected to a central Xe atom. Recall that each F atom has 7 valence e-s, (three pairs and one single). Sharing a single e- with Xe results in four covalent bonds and four octets, (one octet around each F atom).

5) Doing simple e- math again we calculate 4F atoms x 8e-s = 32e-s for the F atoms. We have two pairs, (or four e-s), remaining around our central Xe atom. What the heck do we do with them??? Besides, we already have an octet around Xe and each F atom!

6) Aha!! Many atoms expand their octet. Only atoms with d orbitals can expand their octet. This requires that the atom have a principal quantum number, (n), of 3 or more. Therefore these atoms will be in the third or higher period of the periodic table and have an atomic number of 12 or more.

Note: Although these atoms can expand their octet, they do not always do so. Only the central atom will expand its octet. After drawing a Lewis structure in the normal way, if the formal charges on the molecule are decreased by creating a double bond, the double bond will form.


Formal ChargeIntroduction

A hydrogen atom is made up of one proton and one electron. The formal charge of the atom, the sum of the charge of the proton and the charge of the electron, is zero. The formal charge on any atom is zero when the number of protons (the atomic number) and the number of electrons that "belong" to that atom are equal. We have seen that it requires 13.6 kcal/mol to separate an electron from a hydrogen atom. The resulting hydrogen nucleus, the proton, has a formal charge of +1.Assigning formal charges to isolated atoms and ions is easy. So is assigning formal charges to atoms that are covalently bonded within molecules.

Calculating Formal ChargesTo determine the formal charge of an atom within a molecule, separate the atom from its bonding partner(s), dividing all bonding electrons equally between the bonded atoms. Then compare the number of electrons that "belong" to each atom to the atomic number of that atom. Figure 1uses color coding to illustrate the procedure for methane, CH4.


Formal Charge Example: Methane

1) Each hydrogen is assigned one of the two electrons it shares with the central carbon atom; the formal charge on each hydrogen atom in methane is zero.

2) The central carbon is assigned one of the two electrons it shares with each of the four hydrogens. These are its four valence electrons. But the carbon atom also has two inner shell electrons to consider.

3) The total number of electrons assigned to the carbon is six; this is the same as the atomic number of carbon, and the formal charge on the carbon atom is zero.

http://www.usm.maine.edu/~newton/Chy251_253/Lectures/Formal%20Charge/FormalCharge.html

Chem Team formal charge tutorial:http://dbhs.wvusd.k12.ca.us/webdocs/Bonding/FormalCharge.html


Hmmm…Back to XeF4

1) What is the formal charge on each F atom? (0, +1)2) What is the formal charge on Xe? (Did you get 0? -2?)3) If the formal charges are zero…there is no reason to alter the

suggested structure. If the formal charges are not zero, then you should attempt making double bonds to reduce formal charges on the atoms in the strucrure, more especially the central atom.

4) Now…consider the polyatomic ion, (ClO3)1-.

5) Which structure at the left, “a” or “b”, is correct? Explain6) What is the geometry of the correct structure?7) Check this web site for the answer:

http://www.up.ac.za/academic/chem/mol_geom/mol_geometry.htm

a)

b)


Mr. “T” & Your Microwave Oven• So you just put the food into the microwave, press

the “start” button in and PRESTO! it heats it up. But why does it heat the food yet it doesn't heat the dish, and why is the inside of the oven always cold?

• Mr. “T” sez…”I pity the fool who doesn’t know that a microwave oven has a magnetron in it. (A magnetron is actually a type of radio transmitter. If it was on a radio mast, (antenna), (don't try this), it would be able to send radio signals a long way. But it is inside a metal box, (your microwave oven), which keeps the signal in.”

• Mr. “T” sez…”Microwaves can put bad megahertz on you if you mess around with them. So kids…if you don’t want megahertz from me….don’t mess around with the microwave oven!”

• The frequency of the transmitter is 2450MHz (megahertz), which is a wavelength of 12cm (that's why it's micro waves, rather than short waves (several meters), medium waves (hundreds of meters) or long waves (thousands of meters). There's a good reason for the frequency being 2450 Megahertz, which I'll explain.

Mr. “T”, AKA, “Mr. Science” asks us to consider: How does the microwave in your kitchen work?


Microwave Oven II• Hey kids…food has lots of water in it, you

know… H2O. • A water molecule has the O (Oxygen) in the

middle, and the two H's (Hydrogen) stuck on it like Mickey Mouse ears at a particular angle… (105o).

• The H's are positive and the O is negative, so the molecule has a + and - end. It has "polarity".

• Hey…how come this is true?• True or False?: Polar molecules line themselves

up in an electrical field.• In your microwave oven the electrical field is

changing 2,450 million times a second!• The water molecules don't quite have time to line

up one way before they have to try to line up the other way!

• So, anything with water in it has all these molecules being moved this way and that way by the electrical field, and heated up. WHY??? The dishes, walls of the oven, etc, don't pick up radio waves, so they don't get heated up.

http://www.zyra.org.uk/microw.htm

http://images.google.com/imgres?imgurl=http://library.thinkquest.org/C004535/media/water_diagram.gif&imgrefurl=http://library.thinkquest.org/C004535/properties_of_water.html&h=161&w=166&sz=3&tbnid=U7Ursf5iscJ:&tbnh=90&tbnw=92&start=13&prev=/images%3Fq%3DMickey%2Bmouse%2Bwater%26hl%3Den%26lr%3D

Hey you..yup you there in the back row. Pay attention!

Mickey Mouse


You Write the Captions…

• http://cwx.prenhall.com/bookbind/pubbooks/hillchem3/medialib/media_portfolio/text_images/CH10/FG10_08a.JPG


Water

http://home.online.no/~slunde/gbess1199.jpg

The Bessegan, Norway

Old Time Hockey, Milt Schmidt

Striper Fishin’ at Lobstahville

What is the maximum density of water?

What would happen if water was most dense at 0oC?

What would happen if water was non-polar?

ch4.12.bonding molgeo

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