chemical bonding

Ionic and Covalent Bonding

Atoms rarely exist in nature, in their pure un-bonded state. Why?

• Atoms by themselves have higher potential

energy being in an unbonded state than a

bonded state; therefore, they bond with other

atoms to attain a lower potential energy state.• Atoms tend to seek a natural lower energy state.• It is the valence electrons in any atom that

determines whether or not a particular atom will

bond with other atoms and in what ratio.

The Bond Mechanism• A chemical bond is formed when the nuclei of

two atoms mutually attract valence electrons of the other atom. This occurs by the attraction of the positive nuclei and the negative valence electrons.

• Remember for this to occur one atom has to have a higher electronegativity than the other atom. This higher electronegative atom will attract the other atoms valence electrons, while the other atom will have a natural tendency to lose electrons. Some atoms can bond together with do difference in electronegativity. Ex Hydrogen-Hydrogen

• In all cases, atoms will attain a lower potential energy and become stable like a noble gas.

Two types of chemical bonds

Ionic bonds occur . . . Covalent bonds occur . . .

when anions and when atoms share

cations attract by valence electrons.

virtue of their

opposite charges

between metal and between two non metals

non metals

How do Cations & Anions Form?

• Metals lose electrons and form positive cations.

• Non metals gain electrons and form negative anions.

• Remember the value of the charge is equal to the number of electrons lost or gained; the magnitude of the charge will be positive if the atoms loses electrons and negative if the atom gains electrons.

Ionic Bonds• The mechanism in an ionic bond is the

attraction of the metal cation (+) to the nonmetal anion (-).

• The ratio of cations to anions is determined by the cancellation of positive and negative charges so that the overall charge of the compound is neutral or zero.

• The name of the cation-anion complex in its lowest whole number ratio is a formula unit and not a molecule.

Lewis Structures

• Also known as electron dot notation.

• Lewis structures are representations of the atom and its valence electrons. In a Lewis structure the nucleus is represented by the symbol of the atom and the valence electrons are represented by dots placed around the atomic symbol

Review of how to draw Lewis Dot Diagrams for the atoms.

Let’s say we want to draw the Lewis structure for

Sodium (Na)First we must find Na’s column number on the periodicTable. We find it and all alkali metals are in group 1.

This means we need to show 1 dot which stands for the1 valence electron of the alkali metals and thus for Na.

So we write down the symbol for sodium and then putthe dot anywhere by the symbol, top, side or bottom, itdoes not matter.

Na

How about an Alkaline Earth Metal like Calcium (Ca)?

Looking at the periodic table of elements, we findthat Ca is in group 2 so it will need 2 valenceelectrons.

We will there for begin placing the first electron dot atany position around the atom, (top, bottom or either side,it does not matter). The 2nd electron is placed either clockwise or counterclockwise in another position fromwhere we placed the first electron dot

Ca

Try Aluminum

It’s a group 3 metal so it needs 3 dots for 3

valence electrons.

Don’t forget to place a dot in any position around theatomic symbol (Al) and then follow with the next two dots, in 2 other positions around the atoms in a clockwise or counterclockwise rotation.

Al

Try carbon on for sizeCarbon is a group 4 element so we will

place 4 dots around carbon, 1 dot at the 1st

position and the next three at the three other

positions around the symbol C in a

clockwise or counter clockwise rotation.

C

Notice that carbon has 4 unpaired electrons

Do the Lewis Structure for NitrogenNitrogen is a group 5 element. It has 5

valence electrons. We will need to place 5

dots around the symbol N. Each dot

represents a valence electron.

N

Notice that nitrogen has 1 pair and three single electrons. It is the single electrons that will become involved in a covalent bond. Paired electrons can be part of a covalent bond but that is beyond the scope of this course.

Can you do oxygen? You should have determined that since

oxygen is a group 6 element, you will need to place 6 dots around the symbol O. Each dot stands for a valence electron and all group 6 elements have 6 valence electrons.

O

Did you notice that there are 2 pairs and2 singles? This means oxygen will form 2 bonds in most cases. Why?

Because pairedelectrons usuallyaren’t involved in bonding.

Let’s do a Halogen Since the Halogens are in group 7, they all

have 7 valence electrons. This means they will all have 7 dots around their atomic symbol, 1 dot representing each valence electron.

Cl

Notice that there are 3 pairs ofelectrons and only 1 single. This means a halogen will have only 1 bond because the pairs rarely are involved in bonding.

Examples

• Alkali Metals Li. Na. K. Rb. Fr.• Alkaline Earth .Be. .Mg. .Ca. .Sr.

Metals .Ra.Cations they form: Alkali Metals Li+ Na+ K+ Rb+ Fr+

Alkaline Earth Be2+ Mg 2+ Ca 2+ Sr 2+ Metals Ra 2+

Halogens Atoms/Ions

Notice how each halogen above has 7 valence electrons and just needs 1 more valence electron to make an octet as shown in the anions that are formed from halogens below.

In each case above there is 1 more electron in allcombined energy levels than there is protons in the nucleus. This makes the new ions anions.

Ratios of Cations & Anions in Ionic Compounds

• For Ca2+ and F1- the ratio will be:

CaF2 1(+2 Ca) + 2(-1 F) =0• For Mg2+ and O2- the ratio will be: MgO 1 (+2 Mg) + 1(-2 O) = 0• For Al3+ and S2- the ratio will be:• Al2S3 2(+3 Al) + 3(-2 S) = 0• For determining these ratios, there is an

alternative method, called the crisscross rule. Try looking this one up.

Differences in Electronegativities of elements

• Its easy to see why metals and nonmetals form ionic compounds; its because metals are electropositive and non metals are electronegative; they have large differences in electronegativity.

• But bonding between 2 non metals results in bonding between elements that don’t have a large difference in electronegativity. This type of bonding results in sharing of electrons and creates covalent bonds which hold together covalent compounds or molecular compounds.

Bond CharacterIf the differences in electronegativity of twoelements is greater than 1.7 then the bond is ionic.You can look up electronegativity values.If the difference is 1.7 or less then the bond issaid to be covalent; however, covalent bonds can be classified as nonpolar covalent bonds (with electronegative differences of 0 to 0.3) or polar covalent bonds (with electronegativity differences above 0.3 to below 1.7)

Ionic 3.3 to > 1.7

Polar Covalent 1.7 to > .3

Covalant .3 to 0

Difference in electronegativity

Covalent Bonds

• The idea of sharing electrons between two non-metals, in order to obtain an octet is what covalent bonding is all about.

Lewis structures showing Covalent Bonds

In a Hydrogen-Hydrogen bond we begin byshowing each atom’s electron dot structure anddrawing them side by side.

Then we draw a circle around each hydrogen and include in that circle two electrons, since that is what each hydrogen needs for it to become stable like helium

The number of pairs of electrons in the vin ofThe vin diagram shows the type of bond

H H.. Put the electron dots between the 2 atoms,Since that is where the bond will be!

Let’s see that in Click by click animation

Single Bond

Single, Double & Triple Bonds

• 1 shared pair is a single bond

• 2 shared pairs is a double bond

• 3 shared pairs is a triple bond

• See examples of these Lewis Structures

• Single bonds have the longest bond length

have the weakest bond energy for covalent bonds; triple bonds have the shortest bond length and have the strongest bond energy for covalent bonds.

Coordinate Covalent Bond

• A coordinate covalent bond occurs when one nonmetal donates both electrons in the bond to the shared pair.

• Look for a coordinate covalent bond example on your own on the internet or in your book

VSEPRValence Shell Electron Pair Repulsion Theory

This theory states that the 4 pairs of valence electrons, around a bonded atom, all repel each other such that they position themselves equal distances, and as far as possible from each other; so it is the positions of these electron pairs affects the shapes of molecules.

Linear example: any diatomic molecule H-H

Triatomic Linear example: carbon dioxide O C O

Triatomic Bent example: Water H O

Pyramidal example: Ammonia N

Tetrahedral example: Carbon Tetrachloride

Understand these basic shapes

H

H

H H

CCl

Cl

Cl

Cl

Since nitrogenis in group 5 of the periodic table show 5 valence electrons foreach atom.

Some More Covalent Bonding

Problem: Draw the covalent bonds in diatomic Nitrogen

1) Draw the Lewis structure for 2 Nitrogen atoms.

N N

Next place all the single electrons (there are 3 singles and onePair) between both atoms, because they need to share 3 each,to make an octet (8) of valence electrons





NN






NN


Finally circle 8 electrons for each nitrogen atom.

Notice that you must have 3 pairs or 6 electrons in the vin diagram

Since Fluorineis in group 7 of the periodic table show 7 valence electrons foreach atom.


Problem: Draw the covalent bonds in diatomic Fluorine

1) Draw the Lewis structure for 2 Fluorine atoms.

Next place the single electrons between both atoms,(there is 1 single and three pairs). Place the single electrons between the two atoms because that is where the bond will beand because they need to share 1 each, to make a pair and tomake an octet (8) of valence electrons

F F

Since Fluorineis in group 7 of the periodic table show 7 valence electrons foreach atom.



1) Draw the Lewis structure for 2 Fluorine atoms.

F F

Next place the single electrons between both atoms,(there is 1 single and three pairs). Place the single electrons between the two atoms because that is where the bond will beand because they need to share 1 each, to make a pair and tomake an octet (8) of valence electrons



F F

Finally circle 8 electrons for each fluorine atom.

Notice that you must have 1 pair or 2 electrons in the vin diagram

Since carbon is in group 4, show 4 valence electrons, and sinceoxygen is in group 6 show 6 valence electrons for each atom.

Problem: Draw the covalent bonds for a molecule of Carbon Dioxide

1) Draw the Lewis structures for 1 carbon & 2 oxygen atoms.

Next place the single electrons between the oxygen and carbonatoms, (there are 4 singles in carbon and 2 singles in oxygen). Place the single electrons between the atoms because that is where the bonds will be and because all atoms need to make pairsWhich will be shared to make an octet (8) of valence electrons.

O O

Place carbon as the central atom since there is only 1 carbonand there are 2 oxygens.

C





O O


C





O O


C





O O


C





O O


C


O O C

Now circle an octect for each atom.


O O C



O O C


Notice that there are 2 pairs (4) of electrons in the vin diagram between each oxygen and the carbon atom.

This means that there are 2 double bonds holding the carbondioxide molecule together.

1) Draw the Lewis structures for 1 carbon & 4

chlorine atoms.

Draw the Lewis Structure showing the covalent bonding in Carbon Tetrachloride (CCl4)

C Cl

Cl Cl

Cl

Carbon is in group 4 so it has 4valence electrons and chlorine is in group 7, so it has7 valence electrons.


C Cl

Cl Cl

Cl

Since there is only 1 carbon and there are 4 chlorine atomsmake the carbon atom the central atom.


Cl

Cl Cl

ClC



Cl

Cl Cl

C

Cl



Cl

Cl

CCl

Cl



ClCCl

Cl

Cl



CCl

Cl

Cl

Cl

Since there is only 1 carbon and there are 4 chlorine atomsmake the carbon atom the central atom.Now place each single electron from chlorine betweencarbon and the chlorine so that each single with chlorineand carbon can make a pair. Place the singles making pairsbetween the atoms because that is where the bond will be.


CCl

Cl

Cl

Cl

Now place each single electron from chlorine betweencarbon and the chlorine so that each single with chlorineand carbon can make a pair. Place the singles making pairsbetween the atoms because that is where the bond will be.


CCl

Cl

Cl

Cl



CCl

Cl

Cl

Cl



CCl

Cl

Cl

Cl



CCl

Cl

Cl

Cl



CCl

Cl

Cl

Cl


Now circle an octet for each atom of chlorine and for thecarbon atom. Notice that there are 4 single bonds holding the moleculetogether. 1 electron in each shared pair is from carbon and1 is from chlorine.

Lewis Structures in Ionic Bonding

Remember in ionic bonds the metal gives its electrons to the non-metal, and the non-metal takes electrons from the metal so that each is able to have an octect of electrons in its outer most energy level.

Q: I don’t understand, how does a metal get 8 electrons in its valence if it gives electrons away?

A: When it gives its few electrons in its valence awaya new energy level or layer of 8 electrons lies underneath the electrons that were given away.

Lets show the Lewis structure for NaCl (Sodium Chloride)

Na + Cl Na+

-

Cl

Ionic Bonding for Aluminum Bromide (AlBr3)

Al Br

Br

Br

+

-

+1

-+1

-+1

1st Draw 1 Al & 3 BrElectron Dot Structures

Next give 1 electronFrom Al to each of The Br atoms; noticeThe change in charge Of the atoms to formIons.The overall (-) and (+)Charges balance for aNet charge of 0

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chemical bonding