achm 111,week 8 octet rule and chemical bonding

7/29/2019 ACHM 111,Week 8 Octet Rule and Chemical Bonding

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Octet rule / Chemical bonding

The octet rule is a simple chemicalrule of thumb

that states that atoms tend to combine in such a way

that they each have eight electrons in their valence

shells, giving them the same electronic configurationas a noble gas. The rule is applicable to the main-

group elements, especially carbon, nitrogen, oxygen,

and the halogens, but also to metals such as sodium

or magnesium. In simple terms, molecules or ionstend to be most stable when the outermost electron

shells of their atoms contain eight electrons
http://en.wikipedia.org/wiki/Chemistryhttp://en.wikipedia.org/wiki/Rule_of_thumbhttp://en.wikipedia.org/wiki/Atomshttp://en.wikipedia.org/wiki/Electronshttp://en.wikipedia.org/wiki/Valence_shellhttp://en.wikipedia.org/wiki/Valence_shellhttp://en.wikipedia.org/wiki/Electronic_configurationhttp://en.wikipedia.org/wiki/Noble_gashttp://en.wikipedia.org/wiki/Carbonhttp://en.wikipedia.org/wiki/Nitrogenhttp://en.wikipedia.org/wiki/Oxygenhttp://en.wikipedia.org/wiki/Halogenshttp://en.wikipedia.org/wiki/Sodiumhttp://en.wikipedia.org/wiki/Magnesiumhttp://en.wikipedia.org/wiki/Electron_shellhttp://en.wikipedia.org/wiki/Electron_shellhttp://en.wikipedia.org/wiki/Electron_shellhttp://en.wikipedia.org/wiki/Electron_shellhttp://en.wikipedia.org/wiki/Magnesiumhttp://en.wikipedia.org/wiki/Sodiumhttp://en.wikipedia.org/wiki/Halogenshttp://en.wikipedia.org/wiki/Oxygenhttp://en.wikipedia.org/wiki/Nitrogenhttp://en.wikipedia.org/wiki/Carbonhttp://en.wikipedia.org/wiki/Noble_gashttp://en.wikipedia.org/wiki/Electronic_configurationhttp://en.wikipedia.org/wiki/Valence_shellhttp://en.wikipedia.org/wiki/Valence_shellhttp://en.wikipedia.org/wiki/Electronshttp://en.wikipedia.org/wiki/Atomshttp://en.wikipedia.org/wiki/Rule_of_thumbhttp://en.wikipedia.org/wiki/Chemistry


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Octet rule

The octet rule states that atoms are most stable when they

have a full shell of electrons in the outside electron shell. The

first shell has only two electrons in a single s subshell.

Helium has a full shell, so it is stable, an inert element.

All the other shells have an s and ap subshell, giving them at

least eight electrons on the outside. The s andp subshells

often are the only valence electrons, thus the octet rule is

named for the eight s andp electrons


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Chemical reactions

Chemical reactions take place because atoms

need to achieve a more stable electronic

configuration.

Chemical reactions involve the re-distribution

of the electrons in the outter shell.

Outter shell electrons are called valence

electrons


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Octet rule / Periodic table

All atoms ,except the group 8 atoms need to

achieve 8 electrons in the outter shell.

In order to do these atoms have two choices,

ie to loose electrons or to gain electrons.

Atoms in groups 1-3 will loose ,ie donate their,

electrons when they react.

Atoms in groups 5-7 will accept, ie gain

electrons, when they react.


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Valence shells

The valence shell is the outermost shell of anatom. It is usually said that the electrons in thisshell make up its valence electrons, that is, theelectrons that determine how the atom behaves

in chemical reactions. Atoms with complete valence shells,Group 8 or 0

= noble gases) are inert (unreactive).

Those with only one electron in their valence

shells (alkalis) or just missing one electron fromhaving a complete shell (halogens) are the mostreactive.
http://en.wikipedia.org/wiki/Atomhttp://en.wikipedia.org/wiki/Atomhttp://en.wikipedia.org/wiki/Atomhttp://en.wikipedia.org/wiki/Valence_electronshttp://en.wikipedia.org/wiki/Valence_electronshttp://en.wikipedia.org/wiki/Alkalishttp://en.wikipedia.org/wiki/Alkalishttp://en.wikipedia.org/wiki/Halogenshttp://en.wikipedia.org/wiki/Halogenshttp://en.wikipedia.org/wiki/Halogenshttp://en.wikipedia.org/wiki/Alkalishttp://en.wikipedia.org/wiki/Valence_electronshttp://en.wikipedia.org/wiki/Atom


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Periodic table and valence

On the Periodic Table with shell totals you can easily see theoctet rule.

A valence is a likely charge on an element ion.

All of the Group 1 elements have one electron in the outsideshell and they all have a valence of plus one.

Group 1 elements will lose one and only one electron, to

become a single positive ion with a full electron shell of eight

electrons (an octet) in the s andp subshells under it. Example Na --- Na+ + e-

2,8,1 2,8


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Group 2

Group 2 elements all have two electrons in the

outer shell and all have a valence of plus two.

Beryllium can be a bit different about this, but

all other Group 2 elements can lose two

electrons to become +2 ions. They do not lose

only one electron, but two .

Example Ca --- Ca2+ + 2e-

2,8 ,8,2 2 , 8 , 8


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Group 3

Group 3 elements have a valence of plus

three. Boron is a bit of an exception to this

because it is so small it tends to bond

covalently. Aluminum has a valence of +3.

Example Al --- Al 3+ + 3e-


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Group 4

The smallest Group 4 elements, carbon and

silicon, are non-metals because it is difficult

to lose the entire four electrons in the outer

shell.

Small Group 4 elements tend to make only

covalent bonds, sharing electrons. Larger

Group 4 elements have more than onevalence, usually including +4.

We will explain this later


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Group 5

Small Group 5 elements, nitrogen and

phosphorus, are non-metals.

They tend to either gain three electrons to

make an octet or bond covalently. The larger

Group 5 elements have more metallic

character.

We will explain these later.


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Group 6

Small Group 6 elements, oxygen and sulfur,

tend to either gain two electrons or bond

covalently. The larger Group 6 elements have

more metallic character.

Example O + 2e- --- O2-

2 ,6, 2 , 8,


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Group 7

Group 7 elements all have seven electrons in

the outer shell and either gain one electron to

become a -1 ion or they make one covalent

bond.

Example F + 1e- -- F-

2 ,7, 2 , 8,

The Group 7 elements are diatomic gases dueto the strong tendency to bond to each other

with a covalent bond.


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Group 0/8 = Inert

All of the inert elements, the noble gases,

have a full octet in the outside shell (or two in

the first shell).

They do not naturally combine chemically

with other elements.

Inert = chemically un-reactive.


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Chemical bonding: ionic

Two basic types: ionic or covalent

Ionic bonding: When atoms react they loose or gain

electrons. Atoms from groups 1-3 all react by

loosing/or donating their electrons. All metals behave this way, including Fe,Cu,Zn. On

the other hand atoms in groups 6 and 7 react by

accepting /or gaining electrons.

These are the non-metals such as F, Cl, I, O, and S.


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Ionic bonding/ ions

All metals loose electrons to form positive

ions. These are called cations ,

i.e Na+, Ca 2+, Fe2+, Al3+

All non-metals gain electrons to form

negatively charged anions,

i.e.Cl- ,S2- and O2-


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Ionic bonds


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Ionic bonding

Ionic Bonding

Some atoms gain electrons to become anions

Others lose electrons to become cations

Ions are attracted by their opposing charges

Electrical Neutrality Maintained

Most Important Bonding in Rocks and Minerals


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Ionic bonds

The Ionic Bond: Ionic bonds are formed when there

is a complete transfer of electrons from one atom to

another, resulting in two ions, one positively

charged and the other negatively charged. Forexample, when a sodium atom (Na) donates the

one electron in its outer valence shell to a chlorine

(Cl) atom, which needs one electron to fill its outer

valence shell, NaCl (table salt) results. Ionic bondsare often 4-7 kcal/mol in strength


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Chemical bonding: covalent

Covalent bonding is when atoms share their valence electrons

in order to have 8 in their outer shells.

This kind of bonding takes place between non-metals groups

4, 5 and 6.

This bonding does not form ions.

Such bonds lead to stable molecules if they share electrons in

such a way as to create a noble gas configuration for each

atom.

Each atom donates one electron to form a single covalent

bond


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Lewis structure

The Lewis structures are just an attempt to show thesevalence electrons in a graphic manner as they are used tocombine with other elements.

The element symbol is in the center and as many as four

groups of two electrons are shown as dots above, below, tothe right and left of the element symbol to show the valenceelectrons.

All of the inert gases (noble gases) have all eight of theelectrons around the element symbol, except for helium,

which has only two electrons even with a full shell. Below is ademonstration of the noble gases written in Lewis structure.Notice the electrons are in red just to emphasize them.


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Lewis structure


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Covalent bonding = sharing of

electrons
http://upload.wikimedia.org/wikipedia/commons/1/17/Covalent.svg


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Dot/ cross diagram


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Water: bond pair and lone pair


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Diatomic molecules/elements

Why are some elements diatomic?

Hydrogen gas forms the simplest covalent

bond in the diatomic hydrogen molecule.

The halogens such as chlorine also exist as

diatomic gases by forming covalent bonds.

The nitrogen and oxygen which makes up the

bulk of the atmosphere also exhibits covalent

bonding in forming diatomic molecules.
http://hyperphysics.phy-astr.gsu.edu/hbase/molecule/hmol.htmlhttp://hyperphysics.phy-astr.gsu.edu/hbase/molecule/hmol.html


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Covalent bonds


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Covalent bonding

Covalent Bonding

Electrons share electrons to fill incomplete shells

Most Important Bonding in Organic Materials (and

Organisms)


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Covalent bonds

The Covalent Bond: Covalent Bonds are the strongest

chemical bonds, and are formed by the sharing of a pair of

electrons.

The energy of a typical single covalent bond is ~80

kilocalories per mole (kcal/mol). However, this bond energy

can vary from ~50 kcal/mol to ~110 kcal/mol depending on

the elements involved.

Once formed, covalent bonds rarely break spontaneously.


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Octet rule


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Ionisation energy


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Ionization energy

Definition: The first ionisation energy is the energy

required to remove the most loosely held electron

from one mole of gaseous atoms to produce 1 mole

of gaseous ions each with a charge of 1+. This is more easily seen in symbol terms.

It is the energy needed to carry out this change per

mole of Na.

Na (g) --- Na+ (g) + e-


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Ionisation energy

The state symbols - (g) - are essential. When

you are talking about ionisation energies,

everything must be present in the gas state.

Ionisation energies are measured in kJ mol-1

(kilojoules per mole). They vary in size from

381 (which you would consider very low) up

to 2370 (which is very high).

All elements have a first ionisation.


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Ionization energy

It is the amount of energy required to remove

one electron form the outter shell of an atom.

An atom can have as many ionization energies

as it has electrons.

Sodium has 11 electrons so it can have 11

ionization energies. First ,second third and so

on till eleven.


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Ionisation energies for Na


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Patterns

The first 20 elements

First ionisation energy showsperiodicity. That means that it

varies in a repetitive way as you move through the Periodic

Table. For example, look at the pattern from Li to Ne, and then

compare it with the identical pattern from Na to Ar.

These variations in first ionisation energy can all be explained

in terms of the structures of the atoms involved.


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Trends in ionisation energy


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Ionization energy

Notice that the highest ionization energies are

seen in the Group O elements, known more

commonly as the inert, or noble gases.

This is because the atoms of these elements

have p-orbitals that are full of electrons. This

electronic configuration is particularly stable,

so that the atoms of these elements areextremely reluctant to lose any electrons.

as Na+ and K+.


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Ionization energy

At the other extreme, the lowest ionization

energies are seen in the Group 1 elements,

known as the alkali elements.

These elements only have one electron in

their outermost orbital, and loss of this one

electron will give an ion with a stable, noble

gas-like, electronic configuration. Hence, theyreadily give up this outer electron. This means

they readily form singly charged cations.


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Explaining the pattern in the first few elements

Hydrogen has an electronic structure of 1s1. It is a very small

atom, and the single electron is close to the nucleus and

therefore strongly attracted. There are no electrons screening

it from the nucleus and so the ionisation energy is high (1310

kJ mol-1). Helium has a structure 1s2. The electron is being removed

from the same orbital as in hydrogen's case. It is close to the

nucleus and unscreened. The value of the ionisation energy

(2370 kJ mol-1

) is much higher than hydrogen, because thenucleus now has 2 protons attracting the electrons instead of

1.


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Ionization energy

It is influenced:

By distance of outer electron from nucleus

The further away the less energy.

The balance between remaining electrons and

protons in the nucleus .


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Factors affecting the size of ionisation energy

A high value of ionisation energy shows a high attraction

between the electron and the nucleus.The size of that

attraction depends:

The charge on the nucleus.The more protons there are in the

nucleus, the more positively charged the nucleus is, and themore strongly electrons are attracted to it.

The distance of the electron from the nucleus.

Attraction falls off very rapidly with distance. An electron

close to the nucleus will be much more strongly attractedthan one further away.


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Trends in ionisation energy down a group

As you go down a group in the Periodic Table

ionisation energies generally fall. Taking Group

1 as a typical example:

Why is the sodium value less than that of

lithium?

I i i f G 1


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Ionisation energy of Group 1


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Trends in Group 1

There are 11 protons in a sodium atom but

only 3 in a lithium atom, so the nuclear charge

is much greater. You might have expected a

much larger ionisation energy in sodium, butoffsetting the nuclear charge is a greater

distance from the nucleus and more

screening. Li 1s22s1 1st I.E. = 519 kJ mol-1

Na 1s22s22p63s1 1st I.E. = 494 kJ mol-1

E l i i f i i i


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Explaining pattern of ionisation

energy

E l i i tt f i i ti


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Explaining pattern of ionisation

energy


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Ionisation energy and reactivity

I i ti i d ti it


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Ionisation energies and reactivity

The lower the ionisation energy, the more

reactive is the atom/element.

You can explain the increase in reactivity of

the Group 1 metals (Li, Na, K, Rb, Cs) as you go

down the group in terms of the fall in

ionisation energy. Whatever these metals

react with, they have to form positive ions inthe process, and so the lower the ionisation

energy, the more easily those ions will form.


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Sodium: very reactive

Sodium ((Latinnatrium), is a soft, silvery

white, highly reactive element and is a

member of the alkali metalswithin "group 1

It has only one stable isotope, 23Na.

Sodium quickly oxidizes in air and is violently

reactive with water, so it must be stored in an

inert medium, such as kerosene. Sodium ispresent in great quantities in the earth's

oceans as sodium chloride (common salt).
http://en.wikipedia.org/wiki/Latinhttp://en.wikipedia.org/wiki/Silverhttp://en.wikipedia.org/wiki/Whitehttp://en.wikipedia.org/wiki/Alkali_metalshttp://en.wikipedia.org/wiki/Isotopehttp://en.wikipedia.org/wiki/Airhttp://en.wikipedia.org/wiki/Waterhttp://en.wikipedia.org/wiki/Kerosenehttp://en.wikipedia.org/wiki/Sodium_chloridehttp://en.wikipedia.org/wiki/Sodium_chloridehttp://en.wikipedia.org/wiki/Kerosenehttp://en.wikipedia.org/wiki/Waterhttp://en.wikipedia.org/wiki/Airhttp://en.wikipedia.org/wiki/Isotopehttp://en.wikipedia.org/wiki/Alkali_metalshttp://en.wikipedia.org/wiki/Whitehttp://en.wikipedia.org/wiki/Silverhttp://en.wikipedia.org/wiki/Latin


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Sodium: ionization energy

Na (1s22s22p63s1) ----> Na+ (1s22s22p6) + e-

First ionization energy for sodium

Can have a maximum of 11 ionization

energies.


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Sodium Na


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Chemical bonding


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Ionic bonding


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C-H Covalent Bond

Carbon Hydrogen Covalent


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Carbon-Hydrogen Covalent

BondingBond Number Example

Energy

(kcal/mol)

single

H|

H--C--H|

H

~80

double

H H| |

H--C==C--H| |

H H

~150

triple

H|

C|||C|

H

~200


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Covalent bonds

achm 111,week 8 octet rule and chemical bonding

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