História das ligações quimicas
Autor: Matheus Salles | Publicado: 2012-12-05T06:13:00-08:00





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Os átomos raramente
podem ser encontrados isoladamente. As ligações

químicas unem os átomos,
porem nem todos os átomos conseguem formar ligações.

Dois átomos de um gás
nobre exercem entre si uma atração mutua tão fraca que

não conseguem formar
uma molécula. Por outro lado, a maioria dos átomos forma

ligações fortes com átomos
da própria espécie e com outros tipos de átomos.

Historicamente, a
propriedade dos átomos de formar ligações foi descrita como sendo

a sua Valencia. Este
conceito e pouco utilizado atualmente. Hoje o termo e usado como adjetivo como,
por exemplo, elétron de Valencia ou camada de Valencia.

Quando o conceito de Valencia
foi introduzido não se tinha o conhecimento de

elétrons, prótons e nêutrons.
O descobrimento do elétron, em 1897, possibilitou o

desenvolvimento das
teorias de Valencia e das ligações químicas. Embora o conceito de Valencia já
tivesse sido introduzido em 1857 pelo químico Friedrich August Kekule Von Stradonitz,
o conceito de ligações químicas não havia sido proposto ainda.

De acordo com Kekule,
a Valencia era um numero que representava o poder de

combinação de um
elemento e obedecia a regras simples. Em 1901, o químico Gilbert Newton Lewis
tentou explicar a tabela periódica em

termos de distribuição
eletrônica, porem, o conhecimento mais detalhado da

distribuição dos elétrons
nos átomos só estaria disponível anos mais tarde, com o

desenvolvimento da mecânica
quântica.

Lewis propôs, em
1916, uma forma de representação em termos de diagramas

estruturais onde os elétrons
aparecem como pontos. Um pouco antes dessa data, Ernest. Rutherford havia
mostrado que o numero total de elétrons em um átomo neutro era igual ao seu
numero de ordem sequencial, ou numero atômico, na tabela periódica. A teoria de
Lewis e frequentemente chamada de teoria do octeto, por causa do agrupamento cúbico
de oito elétrons. Por exemplo, o flúor encontra-se no grupo VIIA da tabela periódica,
e precisa receber um elétron para completar oito.

Isso e conseguido através
de uma ligação. O carbono esta no grupo IVA e

precisa de quatro elétrons para completar
oito; forma assim quatro ligações.