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

As ligações iónicas são formadas pela transferência de elétrons dos metais para os não metálicos. Em comparação com as ligações covalentes onde os elétrons são partilhados, nas ligações iónicas os metais tendem a perder e os não metálicos tendem a a aceitar elétrons mas porque, a configuração de eltrões mais estável de um átomo é um octeto completo. Atingir este estado em conjunto com a redução das energias potenciais é uma grande força motriz para a formação de ligações.Mas como prever quando é que os elétrons são transferidos e não partilhados, para tal, verifiquemos as energias de ionização e a afinidade do elétron dos átomos. Na tabela periódica, metais do bloco s têm baixas energias de ionização. Isto facilita a perda de elétrons para alcançar um octeto enquanto se formam cátions.Pelo contrário, os não-metálicos do bloco p, exceto para os gases nobres, têm afinidades elevadas com os elétrons e aceitam prontamente os elétrons para formarem ânions. Os cátions e ânions com carga contrária experienciam fortes interações eletrostáticas, atraindo-se um ao outro para formarem ligações iónicss. Os compostos iónicos formam frequentemente estruturas de cristais duros e bem definidos com pontos de fusão elevados, devido à forte atração eletrostática entre os íons.Consideremos o fluoreto de lítio, um composto iónico feito de lítio e flúor. Quando o lítio perde um elétron para se tornar um cátion de lítio com um dueto, o elétron é absorvido pelo flúor para formar o ânion com um octeto. De acordo com a lei de Coulomb, estas duas partículas carregadas são atraídas uma para a outra para formarem fluoreto de lítio, um composto neutro.O modelo de Lewis pode ser utilizado para descrever e prever a fórmula química dos compostos iónicos. O símbolo de Lewis mostra cargas no canto superior direito e retrata os ânions e cátions com o símbolo escrito entre parênteses. Consideremos o cloreto de estrôncio, um sal vulgarmente utilizado para obter uma cor vermelha brilhante nos fogos de artifício.O símbolo de Lewis para o estrôncio tem dois elétrons não emparelhados e o cloro tem um elétron não emparelhado. O estrôncio deve perder dois elétrons, enquanto o cloro deve ganhar um elétron para alcançar um octeto. Portanto, um cátion de estrôncio liga-se a dois ânions de cloreto, na proporção de 1:2.

ligações iónicas e transferência de eletrões

Iões são átomos ou moléculas que possuem uma carga elétrica. Um catião (um ião positivo) forma-se quando um átomo neutro perde um ou mais eletrões da sua camada de valência, e um anião (um ião negativo) forma-se quando um átomo neutro ganha um ou mais eletrões na sua camada de valência. Os compostos constituídos por iões são chamados compostos iónicos (ou sais) e os iões que os constituem são mantidos juntos por ligações iónicas: forças eletrostáticas de atração entre catiões e aniões opostamente carregados. 

<h4>Propriedades de Compostos Iónicos</h4>

As propriedades dos compostos iónicos trouxeram alguma luz relativamente à natureza das ligações iónicas.

<ul>
<li>Os sólidos iónicos apresentam uma estrutura cristalina e tendem a ser rígidos e quebradiços; eles também tendem a ter pontos de fusão e ebulição elevados, o que sugere que ligações iónicas são muito fortes. </li>
<li>Os sólidos iónicos são também condutores pobres de eletricidade pela mesma razão — a força das ligações iónicas impede que os iões se movam livremente no estado sólido. </li>
<li>No entanto, a maioria dos sólidos iónicos dissolve-se rapidamente em água. Uma vez dissolvidos ou derretidos, os compostos iónicos são excelentes condutores de eletricidade e calor, pois os iões podem mover-se livremente.</li>
</ul>

<h4>A Formação de Compostos Iónicos</h4>

Muitos elementos metálicos têm potenciais de ionização relativamente baixos e perdem eletrões facilmente. Esses elementos ficam à esquerda em um período ou perto da parte inferior de um grupo na tabela periódica. Átomos de não-metais têm afinidades eletrónicas relativamente altas e, assim, rapidamente ganham eletrões perdidos por átomos de metal, enchendo assim as suas camadas de valência. Elementos não metálicos são encontrados no canto superior direito da tabela periódica.

Como todas as substâncias devem ser neutras em termos elétricos, o número total de cargas positivas dos catiões de um composto iónico deve ser igual ao número total de cargas negativas dos seus aniões. A fórmula de um composto iónico representa a relação mais simples dos números de iões necessários para obter números idênticos de cargas positivas e negativas. 

<h4>Os Compostos Iónicos Formam Estruturas Tridimensionais Regularmente Arranjadas</h4>

É importante notar, no entanto, que a fórmula de um composto iónico não representa o arranjo físico dos seus iões. É incorrecto referir-se à “molécula” de cloreto de sódio (NaCl), porque não existe uma única ligação iónica, per se, entre qualquer par específico de iões de sódio e cloreto. As forças atrativas entre iões são isotrópicas — iguais em todas as direções — o que significa que qualquer ião em particular é igualmente atraído por todos os iões de carga oposta próximos. Isto resulta em iões que se organizam em uma estrutura de rede tridimensional bem ligada. O cloreto de sódio, por exemplo, consiste em um arranjo regular de números iguais de catiões Na+ e aniões Cl–. A forte atração eletrostática entre iões de Na+ e Cl– mantém-os bem juntos no NaCl sólido. São necessários 769 kJ de energia para dissociar um mole de NaCl sólido em iões gasoso Na+ e Cl– separados. 

<h4>Estruturas Eletrónicas de Catiões</h4>

Ao formar um catião, um átomo do elemento do grupo principal tende a perder todos os seus eletrões de valência, assumindo assim a estrutura eletrónica do gás nobre que o precede na tabela periódica. 

<ul>
<li>Para os grupos 1 (metais alcalinos) e 2 (metais alcalinoterrosos), os números dos grupos são iguais aos números de eletrões na camada de valência e, consequentemente, às cargas dos catiões formados a partir dos átomos destes elementos quando todos os eletrões camada de valência são removidos. </li>
<li>Por exemplo, o cálcio é um elemento do grupo 2 cujos átomos neutros têm 20 eletrões e uma configuração de eletrões em estado fundamental de 1s22s22p63s23p64s2. Quando um átomo de Ca perde ambos os seus eletrões de valência, o resultado é um catião com 18 eletrões, uma carga 2+, e uma configuração eletrónica de 1s22s22p63s23p6. O ião Ca2+ é, portanto, isoeletrónico com o gás nobre Ar.</li>
<li>Para os grupos 13–17, os números dos grupos excedem o número de eletrões de valência em 10 (contando com a possibilidade de subcamadas d cheias em átomos de elementos no quarto período e superiores). Assim, a carga de um catião formado pela perda de todos os eletrões de valência é igual ao número do grupo menos 10. Por exemplo, o alumínio (no grupo 13) forma iões 3+ (Al3+).</li>
</ul>

<h4>Excepções</h4>

<ul>
<li>As excepções ao comportamento esperado envolvem elementos mais abaixo nos grupos. </li>
<li>Além dos iões Tl3+, Sn4+, Pb4+, e Bi5+ esperados, uma perda parcial dos eletrões da camada na valência destes átomos também pode levar à formação de iões Tl+, Sn2+, Pb2+, e Bi3+. A formação desses catiões 1+, 2+, e 3+ é atribuída ao efeito de par inerte, que reflete a energia relativamente baixa do par de eletrões s de valência para átomos dos elementos pesados dos grupos 13, 14, e 15. </li>
<li>O mercúrio (grupo 12) também apresenta um comportamento inesperado: forma um ião diatómico, Hg22+ (um ião formado por dois átomos de mercúrio, com uma ligação Hg-Hg), além do esperado ião monatómico Hg2+ (formado apenas por um átomo de mercúrio).</li>
<li>Os elementos metálicos de transição e transição interna comportam-se de forma diferente dos elementos do grupo principal. A maioria dos catiões de metais de transição tem carga 2+ ou 3+ que resultam da perda de eletrões s mais exteriores primeiro, por vezes seguida da perda de um ou dois eletrões d da camada exterior seguinte.</li>
<li>Embora as orbitais d dos elementos de transição sejam—de acordo com o princípio de Aufbau—as últimas a serem preenchidas ao criar configurações eletrónicas, os eletrões mais exteriores são os primeiros a serem perdidos quando estes átomos ionizam. Quando os metais de transição interna formam iões, eles normalmente têm carga 3+, resultante da perda dos seus eletrões s mais distantes e um eletrão d ou f.</li>
</ul>

<h4>Estruturas Eletrónicas de Aniões</h4>

A maioria dos aniões monatómicos forma-se quando um átomo não metálico neutro ganha eletrões suficientes para encher completamente as suas orbitais s e p exteriores, alcançando assim a configuração eletrónica do gás nobre seguinte. Assim, é simples determinar a carga de um ião assim negativo: A carga é igual ao número de eletrões que devem ser obtidos para encher as orbitais s e p do átomo que lhe deu origem. O oxigénio, por exemplo, tem a configuração eletrónica 1s22s22p4, enquanto que o anião de oxigénio tem a configuração eletrónica do gás nobre néon (Ne), 1s22s22p6. Os dois eletrões adicionais necessários para preencher as orbitais de valência fornecem ao ião óxido a carga de 2– (O2–).

Este texto é adaptado de <a href="https://openstax.org/books/chemistry-2e/pages/7-1-ionic-bonding">Openstax, Chemistry 2e, Section 7.3: Ionic Bonding.</a>

Ligações Iónicas e Transferência de Eletrões

Íons são átomos ou moléculas que carregam uma carga elétrica. Um cátion (um íon positivo) se forma quando um átomo neutro perde um ou mais elétrons de sua valência ...

Education

JoVE Core

Química

Chemical Bonding: Basic Concepts

Ligações Químicas: Conceitos Básicos

KEY TERMS AND CONCEPTS

TERMOS-CHAVE E CONCEITOS

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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lewis symbols and the octet rule

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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...

electronegativity

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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...

trends in lattice energy: ion size and charge

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lewis structures of molecular compounds and polyatomic ions

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resonance

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key terms and concepts

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Ionic Compounds and Crystal Lattices | JoVE K12

chemistry

JoVE Core - Química dá vida a conceitos gerais de química usando vídeos de animação fáceis de entender. Além disso, nos vídeos Cientistas em ação, demonstramos experimentos de pesquisa clássicos e originais realizados atualmente por cientistas em laboratórios em todo o mundo.

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

ohios-learning-standards-and-model-curriculum

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

CHEM1.PS1.13

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

Physics

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

new-jersey-student-learning-standards-for-science

New Jersey Student Learning Standards for Science

HS-PS1-2

Alabama Course of Study Science

9-12.C.2.2.b

Florida Department of Education

SC.912.P.8

SC.912.P.8.6

Texas Essential Knowledge and Skills

Chemistry and Physics

IPC.7

IPC.7.A

new-york-state-science-learning-standards

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.

bonding-types-from-nonpolar-covalent-to-ionic

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