Name: electronegativity and polar bonds
Uploaded: 2020-09-03T13:38:28.000Z
Duration: 2 min 54 s
Channel: JoVE

K12 Standards

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Chemistry

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PS.Chm.3.1

Electronegativity

Niemetale tworzą wiązania kowalencyjne poprzez współdzielenie elektronów. Ale czy te elektrony są równo dzielone między oba atomy, czy też jeden atom przyciąga elektrony bardziej niż drugi?

Model Lewisa przedstawia wszystkie wiązania kowalencyjne jako równo współdzielone elektrony, jednak nie zawsze tak jest. Na przykład, jeśli gazowy azot zostanie umieszczony w polu elektrycznym, będzie on orientował się równomiernie między biegunami. 

Ale kiedy gazowy chlorowodór, obojętna cząsteczka, jest umieszczony w polu elektrycznym, wodór orientuje się w kierunku katody, a chlor w kierunku anody, co wskazuje, że wodór ma częściowy ładunek dodatni, a chlor ma częściowy ładunek ujemny. 

Zdolność atomu do przyciągania elektronów do siebie nazywana jest elektroujemnością. Mówi się zatem, że chlor jest bardziej elektroujemny niż wodór, przyciągając do siebie wspólne elektrony, jednocześnie opierając się usunięciu własnych elektronów.

To jednak nie sprawia, że wiązanie jest jonowe. W wiązaniu jonowym elektrony są przenoszone z metali na niemetale, podczas gdy w HCl elektrony są nierównomiernie dzielone. Gęstość elektronów jest wyższa na chlorze niż na atomie wodoru, tworząc polarne wiązanie kowalencyjne.

Im większa różnica elektroujemności między dwoma atomami, tym bardziej polarne będzie wiązanie. Tak więc, oprócz niepolarnych wiązań kowalencyjnych lub jonowych, polarne wiązania kowalencyjne znajdują się w wielu różnych związkach. 

Amerykański chemik Linus Pauling badał energie potrzebne do zerwania wiązań w cząsteczkach takich jak dwuatomowy chlor czy wodór. Na podstawie danych termochemicznych stworzył skalę elektroujemności, która pomaga przewidywać typy wiązań. 

Elektroujemność jest związana z energią jonizacji i powinowactwem elektronowym atomów. W układzie okresowym pierwiastków wartości elektroujemności rosną od lewej do prawej — metale są mniej elektroujemne w porównaniu z niemetalami, z wyjątkiem metali przejściowych. 

Dodatkowo wartości elektroujemności zmniejszają się w dół kolumny i wraz ze wzrostem wielkości atomu, ponieważ atomy są mniej zdolne do przyciągania elektronów do siebie.

Fluor, pierwiastek najbardziej elektroujemny, ma arbitralnie przypisaną wartość elektroujemności 3,98. Z drugiej strony frank jest najmniej elektroujemnym pierwiastkiem o wartości elektroujemności 0,7. 

Elektroujemność nie ma jednostki, nie można jej wyznaczyć doświadczalnie.

elektroujemność

To, czy wiązanie jest niepolarne czy polarne kowalencyjne, zależy od właściwości atomów wiążących zwanej elektroujemnością.

Wartości elektroujemności pierwiastków zaproponował jeden z najsłynniejszych chemików XX wieku: Linus Pauling. Pauling badał energie wymagane do rozerwania wiązań w cząsteczkach heterojądrowych, takich jak wodór i fluor. Na podstawie tych wartości stwierdził, że energia potrzebna do rozerwania wiązania będzie średnią energii wiązań H2 (436 kJ/mol) i F2 (155 kJ/mol), tj. 296 kJ/mol. Jednakże uzyskana eksperymentalnie energia wiązania HF wynosi 565 kJ/mol, czyli jest znacznie wyższa od wartości przewidywanej. Aby wyjaśnić tę różnicę, Pauling zasugerował, że wiązanie musi mieć charakter jonowy, co jest określone przez koncepcję elektroujemności.

Elektroujemność jest miarą tendencji atomu do przyciągania elektronów (lub gęstości elektronów) do siebie.

Elektroujemność określa, w jaki sposób wspólne elektrony są rozmieszczone pomiędzy dwoma atomami w wiązaniu. Im silniej atom przyciąga elektrony w swoich wiązaniach, tym większa jest jego elektroujemność. Elektrony w polarnym wiązaniu kowalencyjnym są przesunięte w stronę atomu bardziej elektroujemnego, zatem atomem bardziej elektroujemnym jest ten z częściowym ładunkiem ujemnym. Im większa różnica elektroujemności, tym bardziej spolaryzowany rozkład elektronów i większe ładunki cząstkowe atomów.

<h4>Elektroujemność i układ okresowy</h4>

Elektroujemność wzrasta od lewej do prawej w całym okresie układu okresowego i maleje w dół grupy.

Wartości elektroujemności uzyskane przez Paulinga są zgodne z przewidywalnymi trendami okresowymi, przy czym wyższe elektroujemności znajdują się w prawym górnym rogu układu okresowego.

Zatem niemetale, które znajdują się w prawym górnym rogu, mają zwykle najwyższe elektroujemności, przy czym fluor jest najbardziej elektroujemnym pierwiastkiem ze wszystkich (EN = 4,0).

Metale są zwykle pierwiastkami mniej elektroujemnymi, a metale grupy 1 mają najniższą elektroujemność.

Gazy szlachetne są wyłączone z listy elektroujemności, ponieważ atomy te zwykle nie dzielą elektronów z innymi atomami, ponieważ mają pełną powłokę walencyjną. (Chociaż związki gazów szlachetnych takie jak XeO2 istnieją, mogą powstawać jedynie w ekstremalnych warunkach i dlatego nie pasują do ogólnego modelu elektroujemności.)

<h4>Elektroujemność a powinowactwo elektronowe</h4>

Należu uważać, aby nie pomylić elektroujemności i powinowactwa elektronowego. Powinowactwo elektronowe pierwiastka to mierzalna wielkość fizyczna, czyli energia uwolniona lub pochłonięta, gdy izolowany atom w fazie gazowej przejmuje elektron, i jest mierzona w kJ/mol. Eelektroujemność opisuje natomiast, jak mocno atom przyciąga elektrony w wiązaniu. Jest to wielkość bezwymiarowa, którą się oblicza, a nie mierzy. Pauling wyprowadził pierwsze wartości elektroujemności, porównując ilości energii potrzebnej do rozerwania różnych typów wiązań. Wybrał dowolną skalę względną od 0 do 4.

Elektroujemność

To, czy wiązanie jest niepolarne, czy polarne kowalencyjne, zależy od właściwości atomów wiążących zwanej elektroujemnością. 

Wartości elektroujemności ...

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electronegativity and polar bonds

Electronegativity helps explain why some covalent bonds are nonpolar and others are polar covalent. It is a measure of how strongly an atom attracts shared electrons toward itself. That pull controls how electron density is distributed in a bond.

Linus Pauling proposed the first electronegativity values by studying bond energies in heteronuclear molecules such as hydrogen fluoride. He expected the bond energy of HF to be about the average of the bond energies of H2 and F2, or 296 kJ/mol. The experimental bond energy of HF was much higher, at 565 kJ/mol, which suggested the bond had some ionic character.

In a polar covalent bond, the shared electrons shift toward the more electronegative atom. That atom carries the partial negative charge. The larger the electronegativity difference between two atoms, the more the electrons are pulled to one side, and the larger the partial charges become.

Electronegativity follows clear periodic trends. It increases from left to right across a period and decreases down a group. As a result, the highest values are found toward the upper right of the periodic table, especially among nonmetals. Fluorine is the most electronegative element, with a value of 4.0, while group 1 metals have the lowest values. Noble gases are usually left off electronegativity lists because they already have full valence shells and usually do not share electrons. Some noble gas compounds, such as XeO2, can form only under extreme conditions.

Do not confuse electronegativity with electron affinity. Electron affinity is the measurable energy released or absorbed when an isolated gas-phase atom gains an electron. Electronegativity is different. It describes how strongly an atom attracts electrons in a bond, so it is calculated rather than measured. Pauling placed these values on an arbitrary relative scale from 0 to 4.

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Learn how electronegativity explains polar covalent bonds, Pauling’s bond-energy idea, periodic trends, and the difference from electron affinity.

Electronegativity and Polar Bonds | JoVE K12

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Electronegativity and Polar Bonds

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