11Mr. Shields Regents Chemistry U08 L05

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Ionization energyIonization energy

Ionization Energy (IE) –

The amount of energy needed to remove an electronfrom an atom or ion. Every electron in any atom or ionhas a specific ionization energy.

First Ionization Energy –

The amount of energy needed to remove the 1st electronfrom the outermost shell of a neutral (uncharged) atom.

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Rb + energy Rb+ + 1e-

Ionization energies are measured in KJ/mole

1 2 3

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Ionization EnergyIonization EnergySome atoms have 2 or more electrons in the valenceShell. For example, for example alkaline earth elementssuch as Mg or Sr

Removing the 1st electron (first ionization energy) willalways take less energy than the energy necessary to Remove the 2nd electron (second ionization energy).

This trend continues. The energy necessary to remove a3rd electron is greater than the energy necessary toRemove the 2nd and so on

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Ionization Energy TrendsIonization Energy Trends The energy necessary to remove a valence electron Varies from atom to atom in predictable ways:

1) Ionization energy increases as each successive electron is removed

2) Ionization energy DECREASES going DOWN a GROUP

3) Ionization energy INCREASES as you go ACROSS a PERIOD

4) The noble gases have the highest Ionization energies

5) The alkali metals have the lowest Ionization energies

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Ionization Energy - GroupsIonization Energy - GroupsSo… Why do ionization energies display thesetrends in groups and periods? Let’s see …

Ionization energies decrease going down a Group:

Moving down a group the principle energy level increases

- distance of electron from the positive nucleus increases and there are more electrons providing inner shell shielding of the nucleus

Therefore the valence electrons are less strongly held

Na 2-8-1 (IE = 496 KJ/Mol) Rb 2-8-18-8-1 (IE = 403 KJ/mol)

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Ionization Energy - PeriodsIonization Energy - Periods

Ionization energies across a Period increase:

As we move across a period the principle energy levelof the valence electron doesn’t change

- However Nuclear charge increases across the period while the distance of the valence electron from nucleus remains the same

- Therefore valence electrons feel the inc. nuclear charge more strongly and it takes more energy to remove them from the atom

+37 n=5 +47 n=5Rb 2-8-18-8-1 (IE=403 KJ/mol) Ag 2-8-18-18-1 (IE=731KJ/mol)

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Ionization Energy – Noble GasesIonization Energy – Noble Gases

Noble Gases:

The valence shell octet found in the NOBLE GASESis a particularly stable electron configuration.

Any change from this config will take unusuallyhigh energy since the net result is “destabilizing”

But the trend of decreasing ionization going downa group is also true even for the Noble Gases

Ne: 2-8 IE: 2081 KJ/molKr: 2-8-18-8 1351Rn: 2-8-18-32-18-8 1037

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Ionization Energy trends

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2nd or 3rd IONIZATION ENERGY:

Once an e- is removed the positive nuclear chargeper electron is now stronger. So the I.E. to removethe next electron increases

Secondly, enough electrons are lost to achieve anoctet, removal the next electron would destroy the very stable octet and the I.E. necessary to removethe next electron jumps significantly

1st IE(KJ/Mol) 2nd IE 3rd IENa: 2-8-1 496 4,560

Mg: 2-8-2 736 1,450 7,730

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ElectronegativityElectronegativityWhen we discussed Intermolecular forces we needed To be able to specify whether a molecule was polar orNon-polar.

We had a set of guide lines we used to help determineNon-polarity. What are they?

So… What is the definition of a polar and a non-polar molecule?

Electronegativity is a property of an element thatWill help us quantify how polar a molecule is.

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ELECTRONEGATIVITY:

- A measure of how strongly an element pulls electrons towards it in a chemical bond

Electronegativity (unlike I.E.) has no units:

EN has an arbitrary scale that ranges from 0 – 4.0

Fluorine’s EN is the highest (4.0) Francium’s EN is the lowest (about 0.7)

FCs F Fvs

Low EN Hi EN Same or Simialar EN (polar) (non-polar)

δ+ δ-

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ElectronegativityElectronegativityALL ELEMENTS have IE values even if they have negativeOxidation Numbers (gain electrons).

The same is true for electronegativity.

- elements that tend to lose electrons (metals) still have an electronegativity value (though smaller than the non-metals)

IE(KJ/mol) Electronegativity

Fr 393 (lowest) 0.7 (lowest)F 1681 (very hi) 4.0 (highest)

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ElectronegativityElectronegativitySince all Elements have electronegativity values we canUse these values to:

1) Help determine what molecules are polar2) What molecules are more polar than others

For example: Consider these two molecules

H-Cl and H-I … both are polar molecules.

Now, which one is MORE polar? (Hint you’ll need to use EN values)

We’ll talk more about this in the next unit!

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Electronegativity TrendsElectronegativity Trends

When considering Electronegativity, the trends areVery similar to those observed for Ionization Energyhold true for Electronegativity

- Electronegativity decreases down a Group

- Electronegativity increases across Periods

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Electronegativity Trends

increasing

decre

asin

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Noble GasesHave Zero EN