Name: shielding and orbital penetration
Uploaded: 2020-09-03T13:38:28.000Z
Duration: 3 min 21 s
Channel: JoVE

K12 Standards

Texas Essential Knowledge and Skills

High School

Chemistry and Physics

IPC.7

IPC.7.E

The Energies of Atomic Orbitals

As orbitais atómicas têm energias diferentes, tal como racionalizado pelas interações Coulomb, o efeito de proteção e penetração orbital. A lei de Coulomb indica que a força atrativa ou repulsiva entre duas partículas carregadas tem uma relação inverso-quadrada com a distância entre elas. Os tamanhos orbitais atómicos aumentam com o número de camadas, e os elétrons são repelidos do espaço ocupado por orbitais de camada inferior.Assim, a lei de Coulomb sugere que como o número de camadas aumenta, os elétrons experimentam menos atração relativamente ao núcleo, o que corresponde a energias orbitais mais elevadas. Para além disso, os elétrons que se encontram aproximadamente à mesma distância de ou mais perto do núcleo têm um efeito de proteção que reduz ainda mais a atração para com o núcleo. Quanto maior for o escudo, menor será a atração sentida relativamente ao núcleo.Esta é uma das razões para as diferenças nas energias orbitais dentro das camadas de elétrons. Por exemplo, os elétrons 3s e 3p de forma significativamente elétrons de proteção 3d. A carga nuclear efetiva sentida por um elétron é calculada subtraindo a constante de proteção S, que depende do número de elétrons de proteção e as subcamadas que ocupam, a partir do número atómico.Por exemplo, os dois elétrons de 1s em lítio, que tem um número atómico de três, filtram os seus 2s de elétrons. A constante de proteção para esse elétron é determinada a partir de regras semiempíricas para ser 1, 7. Assim, a carga nuclear efetiva sentida pelo elétron dos 2s é de 1, 3.As formas das orbitais também ditam a sua energia. Se os elétrons num orbital exterior podem ir para as áreas ocupadas por elétrons internos para estarem perto do núcleo, eles estarão aí muito menos protegidos. Assim, a energia desse orbital exterior é menor.Isto pode ser visualizado com uma função de distribuição radial que descreve a probabilidade de encontrar um elétron a uma dada distância do núcleo. Os lotes da função de distribuição radial para as subcamadas 1s, 2s e 2p e 2p revelam que os elétrons 2s têm uma probabilidade modesta de estar perto do núcleo, enquanto os elétrons de 2p permanecem na sua maioria no exterior ou na borda exterior da região dos 1s. Diz-se, portanto, que os orbitais 2s ter uma maior capacidade de penetração.Na terceira camada, os elétrons 3s penetram mais e os elétrons 3d penetram menos. Geralmente, a energia orbital atómica aumenta com o número de camadas e, ao nível da subcamada de s a f. No entanto, o efeito de penetração torna-se tão significativo na quarta e quinta camadas que as orbitais 4s e 5s têm frequentemente energias relativas inferiores às orbitais 3d e 4d, respectivamente.

as energias das orbitais atómicas

Em um átomo, os eletrões com carga negativa são atraídos para o núcleo com carga positiva. Em um átomo com múltiplos eletrões, são também observadas repulsões electrão-electrão. As forças atrativas e repulsivas dependem da distância entre as partículas, bem como do sinal e magnitude das cargas nas partículas individuais. Quando as cargas nas partículas são opostas, elas atraem-se. Se ambas as partículas tiverem a mesma carga, elas repelem-se uma à outra.

À medida que a magnitude das cargas aumenta, a magnitude da força aumenta. No entanto, quando a separação de cargas é maior, as forças diminuem. Assim, a força de atração entre um eletrão e o seu núcleo é diretamente proporcional à distância entre eles. Se o eletrão estiver mais próximo do núcleo, ele está ligado mais fortemente ao núcleo; portanto, os eletrões nas diferentes camadas (a distâncias diferentes) têm energias diferentes.

Para átomos com múltiplos níveis de energia, os eletrões interiores protegem parcialmente os eletrões exteriores da atração do núcleo, devido a repulsões de eletrão-eletrão. Os eletrões no núcleo protegem os eletrões nas camadas exteriores, enquanto que os eletrões na mesma camada de valência não bloqueiam a atração nuclear experienciada uns pelos outros de forma tão eficiente. Isto pode ser explicado com o conceito de carga nuclear efetiva, Zeff. Esta é a força exercida sobre um eletrão específico pelo núcleo, tendo em conta quaisquer repulsões de eletrão-eletrão. Para o hidrogénio, existe apenas um eletrão, e por isso a carga nuclear (Z) e a carga nuclear efetiva (Zeff) são iguais. Para todos os outros átomos, os eletrões interiores protegem parcialmente os eletrões exteriores da atração do núcleo e, por conseguinte:

<img alt="Eq1" src="https://www.jove.com/files/core/CDNSource/lessons/985/985_Equations-01.png" style="height: 40px;" />

A penetração orbital descreve a capacidade de um eletrão estar mais próximo do núcleo. Os eletrões na orbital s podem aproximar-se do núcleo e ter uma capacidade mais penetrante. A densidade de probabilidade para uma orbital s esférica não é zero no núcleo.  Diferentes subcamadas têm diferentes orientações espaciais. Devido à orbital em forma de haltere, o eletrão p penetra muito menos. A sua função de onda tem um nó que passa pelo núcleo, onde a probabilidade de encontrar o eletrão é zero. Assim, um eletrão de orbital s está ligado mais fortemente ao núcleo e tem energia mais baixa do que o eletrão p. Um eletrão d tem uma penetração ainda mais baixa e uma energia mais elevada do que um eletrão de orbital p. 
Para várias camadas e subcamadas, a tendência de poder de penetração de um eletrão pode ser representada da seguinte forma

<img alt="Eq2" src="https://www.jove.com/files/core/CDNSource/lessons/985/985_Equations-02.png" style="height: 40px;" />

O efeito de proteção e penetração é grande, e um eletrão 4s pode ter energia mais baixa do que um eletrão 3d.

Este texto é adaptado de <a href="https://openstax.org/books/chemistry-2e/pages/6-4-electronic-structure-of-atoms-electron-configurations">Openstax, Chemistry 2e, Section 6.4: Electronic Structure of Atoms (Electron Configurations).</a>

As Energias das Orbitais Atómicas

Em um átomo, os elétrons carregados negativamente são atraídos para o núcleo carregado positivamente. Em um átomo de vários elétrons, as repulsões elétron-elétron são ...

Education

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Chemistry

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Electronic Structure of Atoms

Estrutura Eletrônica dos Átomos

KEY TERMS AND CONCEPTS

TERMOS-CHAVE E CONCEITOS

shielding and orbital penetration

Shielding and orbital penetration help explain why electrons in an atom have different energies. Electrons are attracted to the positive nucleus, but they also repel one another in atoms with more than one electron. The strength of these forces depends on the charge and the distance between the particles. Opposite charges attract, like charges repel, and the force gets weaker as the particles move farther apart.

An electron that stays closer to the nucleus is held more tightly. That means electrons in different shells can have different energies because they sit at different distances from the nucleus. In atoms with several energy levels, inner electrons partly block the nucleus from outer electrons. This blocking is called shielding, and it happens because of electron-electron repulsions.

The pull felt by one electron after shielding is the effective nuclear charge, or Zeff. It describes the attraction from the nucleus after repulsions are taken into account. For hydrogen, which has only one electron, Z and Zeff are the same. In atoms with more than one electron, core electrons shield outer electrons, while electrons in the same valence shell do not block the nuclear pull very well.

Orbital penetration describes how close an electron can get to the nucleus. s orbitals penetrate best because their spherical probability density is non-zero at the nucleus. p orbitals penetrate less because their dumbbell shape has a node through the nucleus, where the chance of finding an electron is zero. d orbitals penetrate even less than p orbitals.

Because of shielding and penetration, s electrons are usually bound more tightly and have lower energy than p electrons. d electrons have higher energy than p electrons. These effects can be strong enough that a 4s electron may have lower energy than a 3d electron.

Shielding and Orbital Penetration

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the energies of atomic orbitals

Shielding and orbital penetration help explain why electrons in an atom have different energies. Electrons are attracted to the positive nucleus, but they also repel one another in atoms with more than one electron. The strength of these forces depends on the charge and the distance between the particles. Opposite charges attract, like charges repel, and the force gets weaker as the particles move farther apart. An electron that stays closer to the nucleus is held more tightly. That means...

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Valence electrons and noble gas shorthand help show how electrons are arranged in multielectron atoms. Sodium, the alkali metal with atomic number 11, has one more electron than neon. That extra electron goes into the lowest-energy subshell available, the 3s orbital, so sodium has the configuration 1s2 2s2 2p6 3s1. Electrons in the outermost shell, where n is highest, are called valence electrons. Electrons in the inner shells are called core electrons. Because core electrons often match a...

key terms and concepts

Learn how shielding, effective nuclear charge, and orbital penetration affect electron energy levels, and why s, p, and d orbitals differ in energy.

Shielding and Orbital Penetration | JoVE K12

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Shielding and Orbital Penetration

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