1 Mr. ShieldsRegents Chemistry U08 L05

2 Ionization energy Ionization Energy (IE) – The amount of energy needed to remove an electron from an atom or ion. Every electron in any atom or ion has a specific ionization energy. First Ionization Energy – The amount of energy needed to remove the 1st electron from the outermost shell of a neutral (uncharged) atom.

3 Rb + energy  Rb + + 1e - Ionization energies are measured in KJ/mole 1 2 3

4 Ionization Energy Some atoms have 2 or more electrons in the valence Shell. For example, for example alkaline earth elements such as Mg or Sr Removing the 1 st electron (first ionization energy) will always take less energy than the energy necessary to Remove the 2 nd electron (second ionization energy). This trend continues. The energy necessary to remove a 3 rd electron is greater than the energy necessary to Remove the 2 nd and so on

5 Ionization 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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7 Ionization Energy - Groups So… Why do ionization energies display these trends 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 (IE = 496 KJ/Mol) Rb (IE = 403 KJ/mol)

8 Ionization Energy - Periods Ionization energies across a Period increase: As we move across a period the principle energy level of 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=5 Rb (IE=403 KJ/mol) Ag (IE=731KJ/mol)

9 Ionization Energy – Noble Gases Noble Gases: The valence shell octet found in the NOBLE GASES is a particularly stable electron configuration. Any change from this config will take unusually high energy since the net result is “destabilizing” But the trend of decreasing ionization going down a group is also true even for the Noble Gases Ne:2-8IE:2081 KJ/mol Kr: Rn:

10 Ionization Energy trends

11 2 nd or 3 rd IONIZATION ENERGY: Once an e - is removed the positivenuclear charge per electron is now stronger. So the I.E. to remove the next electron increases Secondly, enough electrons are lost to achieve an octet, removal the next electron would destroy the very stableoctet and the I.E. necessary to remove the next electron jumps significantly 1 st IE(KJ/Mol)2 nd IE3 rd IE Na: ,560 Mg: ,4507,730

12 Electronegativity When we discussed Intermolecular forces we needed To be able to specify whether a molecule was polar or Non-polar. We had a set of guide lines we used to help determine Non-polarity. What are they? So… What is the definition of a polar and a non-polar molecule? Electronegativity is a property of an element that Will help us quantify how polar a molecule is.

13 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) F Cs FF vs Low EN Hi EN Same or Simialar EN (polar) (non-polar) δ+ δ-

14 Electronegativity ALL ELEMENTS have IE values even if they have negative Oxidation 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)

15 Electronegativity Since all Elements have electronegativity values we can Use these values to: 1) Help determine what molecules are polar 2) 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!

16 Electronegativity Trends When considering Electronegativity, the trends are Very similar to those observed for Ionization Energy hold true for Electronegativity - Electronegativity decreases down a Group - Electronegativity increases across Periods

17 Electronegativity Trends increasing decreasing Noble Gases Have Zero EN