first ionization potentials (ip1) vary systematically … 2060 lecture 7: ip and ea l7-6 element z...

CHEM 2060 Lecture 7: IP and EA L7-1

Periodic Trends In Ionization Potentials First ionization potentials (IP1) vary systematically through the periodic table.


General Points • Down any group, IP1 decreases↓ as Z increases↑ (Z = atomic number).

Largest change He (24.587 eV) → Rn (10.748 eV) (noble gases) Smallest change Li (5.392 eV) → Cs (3.894 eV) (alkali metals)

• Across a period, generally IP ↑ as Z increases↑ (e.g. Na → Ar). …but some exceptions due to extra stability of filled or half-filled shells. e.g., ns2, ns2p3, ns2p6


Another Look at the Exceptions in IP1 Trends Across a Period Be (9.322 eV) > B (8.298 eV) and N (14.534 eV) > O (13.618 eV) Atoms with half full shells e.g., N ([He]2s22p3) and full shells e.g., Be ([He]2s2) have larger IP1 values than the trend (i.e., harder to remove an electron). ***NOTE: A good (accurate) explanation for this phenomenon is not possible until we talk about radial probability distributions, shielding and penetration. When studying, come back and re-read this section.***


Electron Affinity (EA) • Chemical reactivity is controlled by electrons. One of the most important

factors in determining chemical reactivity is how easily an atom/molecule is oxidized (electron removed) and easily it is reduced (electron added).

• Silly 1st year mnemonic: LEO the lion says GER. • We have looked at ionization potentials (the removal of an electron), now let’s

look at electron affinities (the addition of an electron). [Def] The electron affinity (EA) of an atom A (or molecule) is the energy required to remove an electron from the negative ion A–, in the gas phase. A–

(g) → A(g) + e- ΔE = EA NOTE: Atomic IP values are always positive (it always requires energy to remove an electron from a neutral atom), but EA values can be either negative or positive (it might require energy to remove an electron from the anion or the process might release energy).


CAUTION: Be Very Careful With Signs if energy is required for A-

(g) → A(g) + e- EA is +ve if energy is released A-

(g) → A(g) + e- EA is –ve

…meaning that A(g) is more stable than A-(g).

• Obviously, the inverse process is the actual reduction of the atom A(g), so if

one wanted to know how much energy is required or released when adding an electron to the atom, the answer is –EA.

TAKE HOME MESSAGE:

A large, positive electron affinity value means that the anion is very stable. This means that the neutral atom his a “high affinity for electrons”.


Element Z EA (eV) Element Z EA (eV) Hydrogen H 1 0.754209 Gallium Ga 31 0.3 Helium He 2 -0.5 Germanium Ge 32 1.233 Lithium Li 3 0.6180 Arsenic As 33 0.81 Beryllium Be 4 -0.5 Selenium Se 34 2.0206 Boron B 5 0.277 Bromine Br 35 3.365 Carbon C 6 1.2629 Nitrogen N 7 -0.07 Rubidium Rb 37 0.48592 Oxygen O 8 1.461125 Strontium Sr 38 -0.3 Fluorine F 9 3.399 Neon Ne 10 -1.2 Indium In 49 0.30 Sodium Na 11 0.547930 Tin Sn 50 1.112 Magnesium Mg 12 -0.4 Antimony Sb 51 1.07 Aluminum Al 13 0.441 Tellurium Te 52 1.9708 Silicon Si 14 1.385 Iodine I 53 3.0591 Phosphorus P 15 0.7465 Xenon Xe 54 -0.8 Sulfur S 16 2.077120 Cesium Cs 55 0.471630 Chlorine Cl 17 3.617 Argon Ar 18 -1.0 Potassium K 19 0.50147 Calcium Ca 20 -0.3 Elements with high EA are highlighted in blue. Elements with negative EA are highlighted in magenta.


General Trends…

Halogens (F, Cl, Br, I) − large, (+)ve electron affinities

Adding an electron completes a closed shell s2p6 configuration! Noble Gases (He, Ne, Ar, (Kr), Xe) – negative electron affinities

This means that the anion is unstable and it is energetically favorable to lose an electron from A-

(g). Note that the neutral atom A has a closed shell s2p6 configuration.

Alkaline Earth Metals (Be, Mg, Ca, Sr) – negative electron affinities These also have a closed shell configuration, ns2.

Question: Which elements have the highest IP1 values? Lowest EA1 values? Why?


Periodic trends in EA1


Electronegativity (χ) – a very important concept! • IP and EA values are very useful thermodynamic quantities (energies). They

can be measured, although as we’ve seen, sometimes the measurements are not very precise (e.g., negative EA values).

• IP and EA values can only be measured for isolated atoms (or molecules) in

the gas phase. They cannot be measured for individual atoms within a molecule.

• However, it is often desirable to understand electron distribution within

molecules. This involves the question of the relative tendencies of each atom to acquire “control” of shared electrons.

The atomic property of electronegativity is invoked to be able to rationalize observed molecular properties/reactivities. [Def] Electronegativity (χ) is the relative ability of an atom to attract electrons to itself in a chemical bond.


A little story…

My grandmother had a Boston Terrier that was obsessed with playing tug of war. You couldn't sit down in her house without having him push a nasty damp sock into your hand. If you were bored enough to accept it, he'd growl (ferociously, he thought) and pull. He was a little dog, though, and you could easily pick up the sock with dog still attached. He'd dangle like a little Christmas tree ornament until he got tired and let go. The contest was futile because my mass was about ten times his. If I'd been a Boston Terrier, the match would have been different. A tug-of-war also goes on between atoms involved in a chemical bond. The bonding electrons are the sock. The atom that can pull on the bonding electrons more strongly will get them. The winner should be the atom with the higher effective nuclear charge.


Examples: Sodium chloride, NaCl

§ ionic bond, meaning effectively a complete transfer of an electron from Na to Cl resulting in Na+ Cl-

§ χNa = 0.93; χCl = 3.16 (Pauling electronegativity values)

Hydrogen fluoride, HF

§ polar covalent bond, meaning that the probability of locating the bonding electrons close to F is higher that of locating them near H.

§ χH = 2.20; χF = 3.98

Bromine, Br2

§ non-polar covalent bond, meaning that the probability density of the bonding electrons is larges exactly between the two Br nuclei.

§ χBr = 2.96

NOTE THAT χ HAS NO UNITS!


Different “flavors” of χ Electronegativity is a very handy concept, but unlike IP and EA it isn’t an energy value that can be definitively measured. As a result, there are several specific definitions of electronegativity resulting in several similar, but non-interchangeable, sets of numericals values for χ. Mulliken Electronegativity (χMULL) Mulliken proposed that the electronegativity of a species should be defined as proportional to the sum of the IP and EA for that species.

χMULL = c(IP + EA) (c = proportionality constant) IP – ability of atom to hold on to an electron EA – ability of atom to attract an electron …seems to make sense.


Example: Cl large IP and large EA ∴ large χ However this doesn’t always work that well, in part because EA values are not known very accurately for a lot of the elements. Pauling Electronegativity (χPAULING) Uses comparison of bond energies… …which can be measured.


• Pauling noticed that the bond energies of polar bonds (A-B) were greater than the average bond energies of the two homonuclear species (avg. A-A & B-B).

• Pauling attributed this “excess” bond energy to the “ionic contribution” A2 molecule and B2 molecule versus AB molecule. Example: HF

Bond Energies HF 567 kJmol-1 H2 432 F2 155 Average bond energy of H2 and F2 261 Extra bond energy assumed to be a consequence of ionic bonding because of electronegativity differences in H and F.


Pauling then set the electronegativity difference as χA – χB = 0.208Δ1/2 [eV] ⇑ converts to units of eV where Δ = DEAB -

€

[(DEA2 )(DEB2 )]1/2 or average bond energy

DE = Bond Dissociation Energy Pauling set χF = 3.98 and χH = 2.20, then calculate all other values using data • Pauling and Mulliken, χ values agree fairly well. • one way to convert them is

€

χ P = 1.35 χM

1/2−1.37

Question: Fluorine is the most electronegative element, and yet chlorine has a larger EA. WHY? (Hint: Think about the relative atomic sizes.)

=[DEA2

+DEB2]

2

first ionization potentials (ip1) vary systematically … 2060 lecture 7: ip and ea l7-6 element z...

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