Name: ionic compounds and crystal lattices
Uploaded: 2020-09-03T13:38:28.000Z
Duration: 2 min 48 s
Channel: JoVE

K12 Standards

north-carolina-k-12-science-standard-course-of-study

North Carolina K-12 Science Standard Course of Study

9-12

Chemistry

Matter and its Interactions

PS.Chm.3.1

Ionic Bonding and Electron Transfer

I legami ionici sono formati dal trasferimento di elettroni dai metalli ai non metalli. In confronto ai legami covalenti, in cui gli elettroni sono condivisi, nei legami ionici i metalli tendono a perdere e i non metalli tendono ad accettare gli elettroni. Ma perché?

La configurazione elettronica più stabile di un atomo è con un ottetto completo. Il raggiungimento di questo stato, insieme all'abbassamento delle energie potenziali, è una forza trainante importante per la formazione dei legami. Ma come prevedere quando gli elettroni verranno trasferiti e non condivisi?

Per questo, esaminiamo le energie di ionizzazione e le affinità degli elettroni degli atomi. Nella tavola periodica, i metalli del blocco s hanno basse energie di ionizzazione. Ciò facilita la perdita più rapida di elettroni, per raggiungere un ottetto, formando contemporaneamente cationi.

Al contrario, i non metalli del blocco p, ad eccezione dei gas nobili, hanno elevate affinità per gli elettroni e li accettano prontamente per formare anioni. I cationi e gli anioni con carica opposta sperimentano forti interazioni elettrostatiche, che si attraggono a vicenda per formare legami ionici. I composti ionici spesso formano strutture cristalline dure e ben definite con punti di fusione elevati, a causa della forte attrazione elettrostatica tra gli ioni.

Considerate il fluoruro di litio, un composto ionico a base di litio e fluoro. Quando il litio perde un elettrone per diventare un catione di litio con un duetto, l'elettrone viene assorbito dal fluoro, per formare l'anione con un ottetto. Secondo la legge di Coulomb, queste due particelle cariche sono attratte l'una dall'altra per formare fluoruro di litio, un composto neutro.

Il modello di Lewis può essere utilizzato per descrivere e prevedere la formula chimica dei composti ionici. Il simbolo di Lewis mostra le cariche nell'angolo in alto a destra e raffigura anioni e cationi con il simbolo scritto fra parentesi. Considerate il cloruro di stronzio, un sale comunemente usato per ottenere il colore rosso brillante nei fuochi d'artificio.

Il simbolo di Lewis per lo stronzio mostra due elettroni spaiati mentre il cloro possiede un elettrone spaiato. Lo stronzio deve perdere due elettroni, mentre il cloro deve guadagnare un elettrone per raggiungere un ottetto. Pertanto, un catione di stronzio si lega a due anioni cloruro, nel rapporto di 1:2.

legame ionico e trasferimento di elettroni

Gli ioni sono atomi o molecole dotati di carica elettrica. Un catione (uno ione positivo) si forma quando un atomo neutro perde uno o pi elettroni dal suo guscio di valenza, mentre un anione (uno ione negativo) si forma quando un atomo neutro acquista uno o pi elettroni nel suo guscio di valenza. I composti formati da ioni sono chiamati composti ionici (o sali) e i loro ioni costituenti sono tenuti insieme da legami ionici: forze elettrostatiche di attrazione tra cationi e anioni con carica opposta.

Propriet dei composti ionici

Le propriet dei composti ionici fanno luce sulla natura dei legami ionici.

<ol style="list-style-type: disc;">
	<li>I solidi ionici presentano una struttura cristallina e tendono ad essere rigidi e fragili; tendono anche ad avere punti di fusione e di ebollizione elevati, il che suggerisce che i legami ionici sono molto forti.</li>
	<li>I solidi ionici sono anche cattivi conduttori di elettricit per lo stesso motivo: la forza dei legami ionici impedisce agli ioni di muoversi liberamente nello stato solido.</li>
	<li>La maggior parte dei solidi ionici, tuttavia, si dissolve facilmente in acqua. Una volta disciolti o fusi, i composti ionici sono ottimi conduttori di elettricit e calore perch gli ioni possono muoversi liberamente.</li>
</ol>

La formazione di composti ionici

Molti elementi metallici hanno potenziali di ionizzazione relativamente bassi e perdono facilmente elettroni. Questi elementi si trovano a sinistra in un periodo o nella parte inferiore di un gruppo sulla tavola periodica. Gli atomi non metallici hanno affinit elettroniche relativamente elevate e quindi guadagnano facilmente gli elettroni persi dagli atomi metallici, riempiendo cos i loro gusci di valenza. Gli elementi non metallici si trovano nell'angolo in alto a destra della tavola periodica.

Poich tutte le sostanze devono essere elettricamente neutre, il numero totale di cariche positive sui cationi di un composto ionico deve essere uguale al numero totale di cariche negative sui suoi anioni. La formula di un composto ionico rappresenta il rapporto pi semplice tra il numero di ioni necessari per dare un numero identico di cariche positive e negative.

I composti ionici formano strutture tridimensionali disposte regolarmente

importante notare, tuttavia, che la formula di un composto ionico non rappresenta la disposizione fisica dei suoi ioni. Non corretto riferirsi alla “molecola? di cloruro di sodio (NaCl) perch non esiste un singolo legame ionico, di per s, tra qualsiasi coppia specifica di ioni sodio e cloruro. Le forze attrattive tra gli ioni sono isotrope – le stesse in tutte le direzioni – il che significa che ogni particolare ione ugualmente attratto da tutti gli ioni vicini di carica opposta. Ci fa s che gli ioni si dispongano in una struttura reticolare tridimensionale strettamente legata. Il cloruro di sodio, ad esempio, costituito da una disposizione regolare di un numero uguale di cationi Na+ e anioni Cl–. La forte attrazione elettrostatica tra gli ioni Na^+ e Cl– li tiene strettamente insieme nel NaCl solido. Sono necessari 769 kJ di energia per dissociare una mole di NaCl solido in ioni Na^+ e Cl– gassosi separati.

Strutture elettroniche dei cationi

Un atomo dell'elemento del gruppo principale, formando un catione, tende a perdere tutti i suoi elettroni di valenza, assumendo cos la struttura elettronica del gas nobile che lo precede nella tavola periodica.

<ol style="list-style-type: disc;">
	<li>Per i gruppi 1 (i metalli alcalini) e 2 (i metalli alcalino-terrosi), i numeri di gruppo sono uguali al numero degli elettroni del guscio di valenza e, di conseguenza, alle cariche dei cationi formati dagli atomi di questi elementi quando tutti gli elettroni del guscio di valenza vengono rimossi.</li>
	<li>Ad esempio, il calcio un elemento del gruppo 2 i cui atomi neutri hanno 20 elettroni e una configurazione elettronica dello stato fondamentale di 1s22s22p63s23p64s2. Quando un atomo di Ca perde entrambi i suoi elettroni di valenza, il risultato un catione con 18 elettroni, una carica 2+ e una configurazione elettronica di 1s22s22p63s23p6. Lo ione Ca^2^+ quindi isoelettronico con il gas nobile Ar.</li>
	<li>Per i gruppi 13-17, i numeri dei gruppi superano di 10 il numero degli elettroni di valenza (tenendo conto della possibilit di subshell d completi negli atomi di elementi nel quarto periodo e in quelli maggiori). Pertanto, la carica di un catione formato dalla perdita di tutti gli elettroni di valenza uguale al numero del gruppo meno 10. Ad esempio, l'alluminio (nel gruppo 13) forma ioni 3+ (Al^3^+).</li>
</ol>

Eccezioni

<ol style="list-style-type: disc;">
	<li>Le eccezioni al comportamento previsto coinvolgono gli elementi verso il fondo dei gruppi. Oltre agli ioni previsti Tl^3+, Sn^4^+, Pb^4^+ e Bi^5^+, una perdita parziale degli elettroni del guscio di valenza di questi atomi pu anche portare alla formazione di ioni Tl^+, Sn^2^+, Pb^2^+ e Bi^3^+. La formazione di questi cationi 1+, 2+ e 3+ attribuita all'effetto di coppia inerte, che riflette l'energia relativamente bassa della coppia di elettroni s di valenza per gli atomi degli elementi pesanti dei gruppi 13, 14 e 15.</li>
	<li>Anche il mercurio (gruppo 12) mostra un comportamento inaspettato: forma uno ione biatomico, Hg_2^2^+ (uno ione formato da due atomi di mercurio, con un legame Hg-Hg), oltre all'atteso ione monoatomico Hg^2^+ (formato da un solo atomo di mercurio ).</li>
	<li>Gli elementi metallici di transizione e di transizione interna si comportano diversamente rispetto agli elementi del gruppo principale. La maggior parte dei cationi dei metalli di transizione hanno cariche 2+ o 3+ che risultano prima dalla perdita del loro elettrone s pi esterno, talvolta seguita dalla perdita di uno o due elettroni d dal guscio immediatamente pi esterno.</li>
	<li>Sebbene gli orbitali d degli elementi di transizione siano, secondo il principio di Aufbau, gli ultimi a riempirsi durante la formazione delle configurazioni elettroniche, gli elettroni pi esterni sono i primi ad essere persi quando questi atomi si ionizzano. Quando i metalli di transizione interni formano ioni, solitamente hanno una carica 3+, risultante dalla perdita dei loro elettroni pi esterni s e di un elettrone d o f.</li>
</ol>

Strutture elettroniche degli anioni

La maggior parte degli anioni monoatomici si forma quando un atomo non metallico neutro acquisisce abbastanza elettroni per riempire completamente i suoi orbitali s e p esterni, raggiungendo cos la configurazione elettronica del successivo gas nobile. Pertanto, semplice determinare la carica di uno ione negativo di questo tipo: la carica uguale al numero di elettroni che devono essere acquistati per riempire gli orbitali s e p dell'atomo genitore. L'ossigeno, ad esempio, ha la configurazione elettronica 1s22s22p4, mentre l'anione ossigeno ha la configurazione elettronica del gas nobile neon (Ne), 1s22s22p6. I due elettroni aggiuntivi necessari per riempire gli orbitali di valenza conferiscono allo ione ossido la carica 2– (O2–).

Questo testo adattato da <a href="https://openstax.org/books/chemistry-2e/pages/7-1-ionic-bonding">Openstax, Chemistry 2e, Section 7.3: Ionic Bonding.</a>

Legame ionico e trasferimento di elettroni

Gli ioni sono atomi o molecole che portano una carica elettrica. Un catione (uno ione positivo) si forma quando un atomo neutro perde uno o più elettroni dalla sua valenza ...

Education

Didattica

JoVE Core

Chimica

Chemical Bonding: Basic Concepts

Legami chimici: concetti fondamentali

KEY TERMS AND CONCEPTS

TERMINI E CONCETTI CHIAVE

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 melting and boiling points. They are also poor conductors of electricity in the solid state because the ions cannot move freely. Most ionic solids dissolve readily in water. When an ionic compound is melted or dissolved, it conducts electricity and heat well because the ions can move about freely.

Ionic compounds form most easily when metals lose electrons and nonmetals gain them. Many metals have low ionization potentials, so they lose electrons easily. These elements are found on the left side of the periodic table or near the bottom of a group. Nonmetals have high electron affinities, so they readily gain the electrons lost by metals. They are found in the upper-right corner of the periodic table.

Ionic compounds must remain electrically neutral. That means the total positive charge from the cations must equal the total negative charge from the anions. The formula of an ionic compound gives the simplest ratio of ions needed to balance charge. The formula does not show the physical layout of the ions. In solid sodium chloride, for example, the ions form a tightly bound three-dimensional lattice. The attraction between Na+ and Cl- ions is strong, and it takes 769 kJ of energy to separate one mole of solid NaCl into gaseous ions.

Main-group cations often form by losing all valence electrons and taking on the electron structure of the noble gas before them. For groups 1 and 2, the group number matches the number of valence electrons and also the charge of the ion formed. Calcium is a group 2 element. A neutral calcium atom has 20 electrons, and Ca2+ has 18 electrons with the same electron configuration as argon. For groups 13 through 17, the cation charge equals the group number minus 10. Aluminum, in group 13, forms Al3+.

Some heavy main-group elements do not follow the expected pattern exactly. Thallium, tin, lead, and bismuth can also form lower-charge ions such as Tl+, Sn2+, Pb2+, and Bi3+. This is explained by the inert pair effect, which reflects the relatively low energy of the valence s-electron pair in these heavy atoms. Mercury is another exception. It can form the diatomic ion Hg2^2+, which contains a Hg-Hg bond, as well as the monatomic ion Hg2+.

Transition metals and inner transition metals form ions in a different way. Most transition metal cations have 2+ or 3+ charges. These usually come from losing the outermost s electron or electrons first, and sometimes one or two d electrons after that. Even though d orbitals fill later according to the Aufbau principle, the outermost s electrons are lost first when transition metals ionize. Inner transition metals usually form 3+ ions by losing their outermost s electrons and one d or f electron.

Most monatomic anions form when a nonmetal atom gains enough electrons to fill its outer s and p orbitals. The charge on the ion equals the number of electrons needed to reach the next noble-gas electron configuration. Oxygen is a clear example. A neutral oxygen atom has the configuration 1s2 2s2 2p4. The oxide ion, O2-, has the neon configuration, 1s2 2s2 2p6, after gaining two electrons.

Ionic Compounds and Crystal Lattices

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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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...

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ionic bonding and electron transfer

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...

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trends-in-lattice-energy-ion-size-and-charge

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...

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lewis structures of molecular compounds and polyatomic ions

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Drawing Lewis Structures with Valence Electron Counts

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Ionic Compounds and Crystal Lattices | JoVE K12

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Physical Science

PS.PSc.2.1

Pennsylvania State Academic Standards

Structure and Properties of Matter

3.2.9-12.B

3.2.9-12.N

Louisiana Student Standards for Science

High School

Matter And Its Interactions

HS-PS1-3

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Ohio’s Learning Standards and Model Curriculum

Study Of Matter

Bonding and compounds

Structure And Properties Of Matter

Chemical bonding

Physical Geology

Minerals

Chemical bonding (ionic, covalent, metallic)

Tennessee Academic Standards

Matter and Its Interactions

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CHEM1.PS1.14

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Motion and Stability: Forces and Interactions

CHEM1.PS2.1

Virginia  Standards of Learning

HIgh School

CH.3

CH.3.d

PS.3

PS.3.c

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PH.9

PH.9.e

Minnesota Academic Standards in Science

1.2.1 9L.4.2.2.1

Arizona Science Standards

HS.P1

HS.P1U1.2

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New Jersey Student Learning Standards for Science

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Alabama Course of Study Science

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SC.912.P.8.6

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Chemistry and Physics

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IPC.7.A

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New York State Science Learning Standards

HS. Structure and Properties of Matter

HS-PS1-3.

HS. Chemical Reactions

HS-PS1-2.

Georgia Standards of Excellence

SPS2.

Chemical Bonds

SC2.

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