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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North Carolina K-12 Science Standard Course of Study

9-12

Chemistry

Matter and its Interactions

PS.Chm.3.1

Electronegativity

I non metalli formano legami covalenti condividendo gli elettroni. Questi elettroni sono condivisi equamente tra i due atomi, o un atomo attrae gli elettroni più dell'altro? Il modello di Lewis descrive tutti i legami covalenti come elettroni equamente condivisi;tuttavia, questo non è sempre il caso.

Per esempio, se l'azoto gassoso è posto in un campo elettrico, si orienterà equamente fra i poli. Ma quando l'acido cloridrico gassoso, una molecola neutra, viene posto in un campo elettrico, l'idrogeno si orienta verso il catodo e il cloro verso l'anodo, indicando che l'idrogeno ha una carica positiva parziale e il cloro ha una carica negativa parziale. La capacità di un atomo di attrarre elettroni verso se stesso è chiamata elettronegatività.

Si dice quindi che il cloro sia più elettronegativo dell'idrogeno, attirando verso di se gli elettroni condivisi, mentre resiste alla rimozione dei propri elettroni. Questo, tuttavia, non rende il legame ionico. In un legame ionico, gli elettroni vengono trasferiti dai metalli ai non metalli, mentre in HCl gli elettroni sono condivisi in modo non eguo.

La densità elettronica è maggiore sul cloro che sull'atomo di idrogeno, che forma un legame covalente polare. Maggiore è la differenza di elettronegatività fra due atomi, più polare sarà il legame. Pertanto, oltre ai legami covalenti o ionici non polari, i legami covalenti polari si trovano in un'ampia varietà di composti.

Il chimico americano Linus Pauling studiò le energie necessarie per infrangere i legami in molecole come il cloro biatomico o l'idrogeno. Pauling stabilì una scala di elettronegatività basata su dati termochimici, che aiuta a prevedere i tipi di legame. L'elettronegatività è associata all'energia di ionizzazione e all'affinità elettronica degli atomi.

Nella tavola periodica, i valori di elettronegatività aumentano da sinistra a destra:i metalli sono meno elettronegativi rispetto ai non metalli, ad eccezione dei metalli di transizione. Inoltre, i valori di elettronegatività diminuiscono lungo la colonna e con l'aumentare della dimensione atomica, perché gli atomi sono meno in grado di attrarre elettroni a se stessi. Il fluoro, l'elemento più elettronegativo, ha un valore di elettronegatività assegnato arbitrariamente come 3, 98.

Il francio, d'altra parte, è l'elemento meno elettronegativo con un valore di elettronegatività di 0, 7. L'elettronegatività non ha unità;non può essere determinata sperimentalmente.

elettronegativit

Il fatto che un legame sia non polare o covalente polare determinato da una propriet degli atomi di legame chiamata elettronegativit.

I valori di elettronegativit degli elementi furono proposti da uno dei chimici pi famosi del XX secolo: Linus Pauling. Pauling studi le energie necessarie per rompere i legami nelle molecole eteronucleari come l'idrogeno e il fluoro. Sulla base dei valori, propose che l'energia richiesta per rompere un legame la media delle energie di legame di H_2 (436 kJ/mol) e F_2 (155 kJ/mol), cio 296 kJ/mol. Tuttavia, l’energia di legame dell’HF ottenuta sperimentalmente 565 kJ/mol, che molto pi alta del valore previsto. Per spiegare questa differenza, Pauling sugger che il legame dovesse avere un carattere ionico, che determinato dal concetto di elettronegativit.

L'elettronegativit una misura della tendenza di un atomo ad attrarre elettroni (o densit elettronica) verso se stesso.

L'elettronegativit determina come sono distribuiti gli elettroni condivisi tra i due atomi in un legame. Quanto pi un atomo attrae gli elettroni nei suoi legami, tanto maggiore la sua elettronegativit. Gli elettroni in un legame covalente polare vengono spostati verso l'atomo pi elettronegativo; quindi l'atomo pi elettronegativo quello con la carica negativa parziale. Maggiore la differenza di elettronegativit, pi polarizzata la distribuzione degli elettroni e maggiori sono le cariche parziali degli atomi.

Elettronegativit e tavola periodica

<ol style="list-style: disc;">
	<li>L'elettronegativit aumenta da sinistra a destra lungo un periodo della tavola periodica e diminuisce lungo un gruppo.</li>
	<li>I valori di elettronegativit derivati da Pauling seguono gli andamenti periodici prevedibili, con elettronegativit pi elevate verso l'angolo superiore destro della tavola periodica.</li>
	<li>Pertanto, i non metalli, che si trovano in alto a destra, tendono ad avere elettronegativit pi elevate con il fluoro, l'elemento pi elettronegativo di tutti (EN = 4,0).</li>
	<li>I metalli tendono ad essere elementi meno elettronegativi e i metalli del gruppo 1 hanno le elettronegativit pi basse.</li>
</ol>

I gas nobili sono esclusi dall'elenco dell'elettronegativit perch solitamente non condividono gli elettroni con altri atomi poich hanno un guscio di valenza completo. (Sebbene esistano composti di gas nobili come XeO2, possono formarsi solo in condizioni estreme e quindi non rientrano perfettamente nel modello generale dell’elettronegativit.)

Elettronegativit e affinit elettronica

Fare attenzione a non confondere elettronegativit e affinit elettronica. L'affinit elettronica di un elemento una quantit fisica misurabile, vale a dire l'energia rilasciata o assorbita quando un atomo isolato in fase gassosa acquisisce un elettrone, misurata in kJ/mol. L’elettronegativit, d’altra parte, descrive quanto strettamente un atomo attrae gli elettroni in un legame. una quantit adimensionale che viene calcolata, non misurata. Pauling deriv i primi valori di elettronegativit confrontando le quantit di energia richiesta per rompere diversi tipi di legami. Ha scelto una scala relativa arbitraria che va da 0 a 4.

Questo testo adattato da <a href="https://openstax.org/books/chemistry-2e/pages/7-2-covalent-bonding">Openstax, Chemistry 2e, Section: 7.2 Covalent Bonding.</a>

Elettronegativit

Il fatto che un legame sia non polare o covalente polare è determinato da una proprietà degli atomi di legame chiamata elettronegatività. 

Valori di elettronegatività di ...

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Chemical Bonding: Basic Concepts

Legami chimici: concetti fondamentali

KEY TERMS AND CONCEPTS

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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.

Electronegativity and Polar Bonds

types of chemical bonds

Chemical Bonding and Bond Properties

Chemical bonding explains how atoms and ions join to form molecules and compounds. American chemist Gilbert N. Lewis pioneered chemical bonding theory with the Lewis model. This model helps explain why bonds form, why some are stronger than others, and why compounds have fixed formulas like H2O instead of H4O. Ionic bonding involves ions, which are atoms or molecules with an electrical charge. A cation is a positive ion formed when a neutral atom loses one or more electrons from its valence...

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Valence Electrons, Lewis Symbols, and Bonding

Chemical bonds form when atoms or ions interact in ways that lower the potential energy of a molecule. Gilbert N. Lewis created a simple model to show bond formation using valence electrons, the electrons in the outer shell. In this model, electrons are shared or transferred to help atoms reach a stable electron configuration. The Lewis model helps describe two common bond types. An ionic bond forms when electrons move from a metal to a nonmetal. A covalent bond forms when two nonmetals share...

electronegativity

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...

ionic bonding and electron transfer

Ionic Compounds and Crystal Lattices

Ionic compounds form when positive and negative ions attract each other. A cation is a positive ion, and it forms when an atom loses one or more valence electrons. An anion is a negative ion, and it forms when an atom gains one or more valence electrons. Compounds made of these ions are called ionic compounds, or salts, and the force between them is an ionic bond. The properties of ionic solids help show how strong this attraction is. They usually form rigid, brittle crystals and have high...

trends in lattice energy: ion size and charge

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Lattice Energy Patterns in Ionic Solids

Lattice energy shows how strongly ions hold an ionic solid together. It is the energy required to separate one mole of a solid into its gaseous ions. For sodium chloride, this process requires 769 kJ per mole. The opposite convention may also be used, where lattice energy is the energy released when gaseous ions form the solid. In either case, a larger magnitude means a more stable ionic compound. For ionic crystals, lattice energy follows a pattern based on charge and ion size. The...

covalent bonding and lewis structures

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Covalent molecules form when atoms share electrons instead of transferring them. This bonding usually happens between nonmetal atoms with similar tendencies to attract electrons. Because the atoms pull on the electrons in a shared way, the bond is held together by mutual attraction. Covalent compounds often have physical properties that differ from ionic compounds. The attraction between neutral molecules is weaker than the attraction between charged ions. As a result, many covalent compounds...

bond polarity, dipole moment, and percent ionic character

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How Bond Polarity Shapes Molecular Dipoles

Bond polarity depends on electronegativity, the ability of an atom to attract shared electrons. The absolute value of the electronegativity difference, or ΔEN, gives a rough guide to bond type. When the difference is very small or zero, the bond is covalent and nonpolar. When it is larger, the bond may be polar covalent or ionic. Examples show how this works in real compounds. H–H has a ΔEN of 0, so it is nonpolar. H–Cl has a ΔEN of 0.9, so it is polar covalent. Na–Cl has a ΔEN of 2.1, so it...

lewis structures of molecular compounds and polyatomic ions

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Drawing Lewis Structures with Valence Electron Counts

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resonance

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Resonance forms in Lewis structures help explain molecules and ions that cannot be shown correctly with just one drawing. In nitrite ion, NO2−, the Lewis structure can be drawn in two ways. The N-O and N=O bonds switch positions in the two drawings, but the atoms stay in the same arrangement. If nitrite really had one single bond and one double bond, the two bond lengths should be different. A double bond is shorter and stronger than a single bond between the same atoms. Experiments show that...

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key terms and concepts

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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