Name: electron affinity trends on the periodic table
Uploaded: 2020-09-03T13:38:28.000Z
Duration: 3 min 7 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.2.2

Electron Affinity

Quando um elétron é adicionado a um átomo gasoso, observa-se uma alteração na energia chamada afinidade de elétrons. A afinidade com o elétron mede a facilidade de ganhar um elétron por um átomo. Por exemplo, a afinidade dos elétrons com o cloro é de 348, 6 kilojoules por mol.O sinal negativo indica que se trata de uma alteração exotérmica. O argônio, contudo, tem uma afinidade positiva com os elétrons, indicando que a formação de um ânion de argônio requer que a energia seja fornecida. Em geral, quanto maior for a atração entre um átomo e um elétron adicionado, mais negativa é a afinidade dos elétrons.As afinidades dos elétrons, semelhantes às energias de ionização, mostram as tendências na tabela periódica. Descendo ao grupo 1, o tamanho atómico aumenta à medida que os elétrons ocupam um número quantum principal mais elevado. Os elétrons que chegam, portanto, experienciam menos atração nuclear levando a menos afinidades de elétrons negativos.No entanto, existem exceções. Nos halógenos, o cloro tem um valor de afinidade mais negativo com os elétrons do que o flúor, mas porque, o flúor é o menor átomo dos halógenos e um elétron que chega experiencia uma repulsão significativa dos elétrons já presentes. No entanto, no ânion de cloreto, o novo elétron é acrescentado à terceira camada, ocupando mais espaço.Isto reduz as repulsões dos elétrons, tornando-o mais atrativo para a obtenção de um elétron. Geralmente, movendo-se ao longo de um período, as afinidades dos elétrons tornam-se mais negativas. Os halogéneos têm as afinidades dos elétrons mais negativas, uma vez que o elétron que entra ajuda a alcançar as configurações do gás nobre.Em comparação, os gases nobres têm uma camada completamente cheia. O elétron que chega tem de ser acomodado no nível superior de energia principal, que é energeticamente desfavorável. Assim, as afinidades dos elétrons para estes elementos são positivas.O grupo 2 apresenta exceções. A configuração dos elétrons indica que o elétron que chega precisa de entrar numa subcamada de elevada energia. Assim, os valores de afinidade dos elétrons ou são positivos ou menos exotérmicos.Curiosamente, o grupo 15 tem menos afinidades negativas de elétrons do que o grupo 14. Comparemos o fósforo e o silício. Ao contrário do silício, o fósforo tem uma camada meio preenchida de p e o elétron que chega tem de ser emparelhado com um elétron já a residir no orbital p.Isto aumentaria as repulsões dos elétrons e é, portanto, um processo desfavorável energeticamente que se reflete também na afinidade menos negativa do elétron relativamente ao silício.

afinidade eletrónica

A afinidade eletrónica (EA) é a mudança de energia para adicionar um eletrão a um átomo gasoso para formar um anião (ião negativo).

<img alt="Eq1" src="https://www.jove.com/files/core/CDNSource/lessons/1050/1050_Equations-01.png" style="height: 40px;" />

Este processo pode ser endotérmico ou exotérmico, dependendo do elemento. Muitos destes elementos têm valores negativos de EA, o que significa que a energia é libertada quando o átomo gasoso aceita um eletrão. No entanto, para alguns elementos, é necessária energia para que o átomo se torne negativamente carregado, e o valor da sua EA é positivo. Assim como com a energia de ionização, valores de EA subsequentes estão associados à formação de iões com mais carga. A segunda EA é a energia associada à adição de um eletrão a um anião para formar um ião 2–, e assim por diante.

Como se pode prever, torna-se mais fácil adicionar um eletrão por uma série de átomos à medida que a carga nuclear efetiva dos átomos aumenta. À medida que vamos da esquerda para a direita ao longo de um período, as EAs tendem a tornar-se mais negativas. As excepções encontradas entre os elementos do grupo 2 (2A), grupo 15 (5A) e grupo 18 (8A) podem ser entendidas com base na estrutura eletrónica desses grupos. Os gases nobres, grupo 18 (8A), têm uma camada completamente cheia, e o eletrão que entra deve ser adicionado a um nível n mais alto, o que é mais difícil de fazer. O grupo 2 (2A) tem uma subcamada ns cheia, pelo que o eletrão adicionado a seguir entra na np com maior energia, e então, novamente, o valor de EA observado não é como a tendência prediria. Por fim, o grupo 15 (5A) tem uma subcamada np semi-cheia, e o eletrão seguinte tem de ser emparelhado com um eletrão np existente. Em todos estes casos, a estabilidade relativa inicial da configuração eletrónica perturba a tendência na EA.

Pode-se esperar que o átomo no topo de cada grupo tenha a EA mais negativa; os seus primeiros potenciais de ionização sugerem que esses átomos têm as maiores cargas nucleares efetivas. No entanto, à medida que nos deslocamos para baixo, vemos que o segundo elemento do grupo tem, na maioria das vezes, a EA mais negativa. Isto pode ser atribuído ao pequeno tamanho da camada n = 2 e às grandes repulsões eletrão-eletrão resultantes. Por exemplo, o cloro, com um valor de EA de –348 kJ/mol, tem o valor mais elevado de qualquer elemento na tabela periódica. A EA do flúor é de –322 kJ/mol. Quando adicionamos um eletrão a um átomo de flúor para formar um anião fluoreto (F–), adicionamos um eletrão à camada n = 2. O eletrão é atraído para o núcleo, mas há também uma repulsão significativa dos outros eletrões já presentes nesta pequena camada de valência. O átomo de cloro tem a mesma configuração eletrónica na camada de valência, mas uma vez que o eletrão que entra vai para a camada n = 3, ele ocupa uma região consideravelmente maior do espaço e as repulsões de eletrão-eletrão são reduzidas. O eletrão que entra não experiencia tanta repulsão, e o átomo de cloro aceita um eletrão adicional mais prontamente, resultando em uma EA mais negativa.

Este texto é adaptado de <a href="https://openstax.org/books/chemistry-2e/pages/6-5-periodic-variations-in-element-properties">OpenStax Chemistry 2e, Section 6.5: Periodic Variations in Element Properties.</a>

Afinidade Eletrónica

A afinidade eletrônica (EA) é a mudança de energia para adicionar um elétron a um átomo gasoso para formar um ânion (íon negativo).

Este processo pode ser ...

Education

JoVE Core

Química

Periodic Properties of the Elements

Propriedades Periódicas dos Elementos

KEY TERMS AND CONCEPTS

TERMOS-CHAVE E CONCEITOS

electron affinity trends on the periodic table

Electron affinity is the energy change that happens when a gaseous atom gains an electron and becomes a negative ion, or anion. For many elements, this process releases energy, so the electron affinity value is negative. For some elements, energy must be added instead, so the value is positive.

Electron affinity also applies when more than one electron is added. The second electron affinity is the energy change for adding an electron to an anion to make a 2− ion. Later electron affinities follow the same idea for ions with even more charge.

Across a period, electron affinity usually becomes more negative from left to right. This pattern fits the increase in effective nuclear charge, which makes it easier for atoms to attract an added electron. Some groups do not follow the trend well, though. Group 2 has a filled ns subshell, group 15 has a half-filled np subshell, and group 18 has a full shell, so the next electron must enter a less favorable state.

These exceptions are linked to electron arrangement and stability. Noble gases must place the new electron in a higher n level. Group 2 elements add the electron to a higher-energy np orbital. Group 15 elements must pair the new electron with one already in the np subshell. In each case, the stable starting configuration changes the expected electron affinity.

Down a group, the most negative electron affinity is often found in the second element rather than the top one. The small size of the n = 2 shell can create stronger electron-electron repulsions. That is why chlorine has a more negative electron affinity than fluorine. Chlorine’s value is −348 kJ/mol, while fluorine’s is −322 kJ/mol. When fluorine gains an electron, the added electron enters the small n = 2 shell. In chlorine, the electron goes into the larger n = 3 shell, so repulsion is lower and the atom accepts the electron more readily.

Electron Affinity Trends on the Periodic Table

electron-affinity-trends-on-the-periodic-table

periodic classification of the elements

How Valence Electrons Shape the Periodic Table

The periodic table organizes elements by increasing atomic number, and it also shows why elements in the same group behave alike. As the electron configurations are placed into the table, a repeating pattern appears in the outer shells of atoms. These outer electrons are called valence electrons, and they play the biggest role in chemical reactions. They are the highest-energy electrons in an atom, so they are easier to lose or share than core electrons. Valence electrons also help determine...

ionization energy

Ionization Energy Trends in the Periodic Table

Ionization energy is the energy needed to remove an electron from a gaseous atom or ion in its ground state. The first ionization energy, or IE1, removes the most loosely bound electron and forms a 1+ cation. The second ionization energy, IE2, removes the next electron, and each later ionization energy removes another electron from the atom or ion. Ionization always requires energy, so these processes are endothermic. That means ionization energy values are always positive. In general, larger...

atomic radii and effective nuclear charge

atomic-radii-and-effective-nuclear-charge

Periodic Trends in Atomic Size

Periodic trends in atomic size help explain why atoms get larger or smaller across the periodic table. Elements in the same group show similar chemical behavior because they have the same number and arrangement of valence electrons. Across a period, each new element adds one proton to the nucleus and one electron to the valence shell. Down a group, the number of valence electrons stays the same, but the principal quantum number increases by one. Atomic radius is not easy to define with perfect...

ionic radii

How Ionic Charge Changes Ion Size

Ionic radius describes the size of an ion. It changes when an atom gains or loses electrons. Cations are positive ions with fewer electrons than the parent atom, and they are smaller than the atoms they come from. Aluminum shows this pattern clearly. A neutral aluminum atom has a covalent radius of 118 pm, while Al3+ has an ionic radius of 68 pm. When electrons are removed from the outer valence shell, the remaining core electrons feel a stronger effective nuclear charge, or Zeff. That pull...

electron affinity

Electron affinity is the energy change that happens when a gaseous atom gains an electron and becomes a negative ion, or anion. For many elements, this process releases energy, so the electron affinity value is negative. For some elements, energy must be added instead, so the value is positive. Electron affinity also applies when more than one electron is added. The second electron affinity is the energy change for adding an electron to an anion to make a 2− ion. Later electron affinities...

alkali metals

Alkali Metals: Reactivity Trends

Alkali metals are Group 1 elements with one valence electron. They are soft, shiny metallic solids that are malleable, ductile, and good conductors of heat and electricity. Their melting points are unusually low for metals and drop as you move down the group. Density generally increases down the group, except for potassium. The trend in atomic size helps explain their behavior. Moving from lithium to cesium, each element has a higher principal quantum number, so the outer electron is farther...

halogens

Halogens: Trends and Reactions

Halogens are Group 17 nonmetals that show clear trends in structure and reactivity. At room temperature, fluorine and chlorine are gases, bromine is a liquid, and iodine is a solid. Astatine is highly unstable and radioactive, so most of its properties are still unknown because it has a very short half-life. Tennessine is a synthetic element that is also predicted to belong to this group. These elements do not usually exist as single atoms. Instead, they form diatomic molecules, which means...

noble gases

Noble Gases: Sources, Uses, and Reactions

Noble gases are the group 18 elements helium, neon, argon, krypton, xenon, and radon. They were called “noble” because they were once thought to be nonreactive. That idea changed in 1962, when Dr. Neil Bartlett at the University of British Columbia showed that this assumption was not fully correct. These gases occur in very small amounts in the atmosphere. Some natural gas contains 1–2% helium by mass. Helium is separated from natural gas by removing the condensable components, which leaves...

key terms and concepts

Learn how electron affinity changes across the periodic table, why some groups are exceptions, and why chlorine has a more negative value than fluorine.

Electron Affinity Trends on the Periodic Table | JoVE K12

chemistry

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Physical Science

PS.PSc.1.6

Pennsylvania State Academic Standards

Chemical Reactions

3.2.9-12.C

Structure and Properties of Matter

3.2.9-12.A

Louisiana Student Standards for Science

High School

Matter And Its Interactions

HS-PS1-1

HS-PS1-2

ohios-learning-standards-and-model-curriculum

Ohio’s Learning Standards and Model Curriculum

Study Of Matter

Periodic trends of the elements

Structure And Properties Of Matter

Periodic Table

Tennessee Academic Standards

Matter and Its Interactions

CHEM1.PS1.12

Virginia  Standards of Learning

HIgh School

CH.2

CH.2.c

PS.2

PS.2.b

PS.4

PS.4.a

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Arizona Science Standards

HS.P1

HS.P1U1.1

HS+C.P1

HS+C.P1U1.1

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CHEM.5

CHEM.5.A

CHEM.5.C

CHEM.7

CHEM.7.A

Alabama Course of Study Science

9-12.C.1.1

9-12.C.1.1.e

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Motion and Stability: Forces and Interactions

9-12.C.2.2.f

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Next Generation Science Standards

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IPC.7.B

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New York State Science Learning Standards

HS. Structure and Properties of Matter

HS-PS1-1.

HS. Chemical Reactions

HS-PS1-2.

Georgia Standards of Excellence

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Periodic Table Trends

SC1.

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Periodic Table Trends: Atomic Radii, Ionization, and Electronegativity

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The Atomic Theory of Matter

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Radioactive Decay and Radiometric Dating

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Controlled nuclear fission reactions are used to generate electricity. Any nuclear reactor that produces power via the fission of uranium or plutonium by ...

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PS.Chm.1.2

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The Aufbau Principle and Hund's Rule

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Use models to explain how electrons are distributed in atoms.

PS.Chm.1.3

Periodic Classification of the Elements

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The Periodic Table

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Use the Periodic Table as a model to predict the relative properties of elements based on the pattern of valence electrons in the outermost energy levels of atoms.

PS.Chm.2.1

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Ionic Radii

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Ionization Energy

The amount of energy required to remove the most loosely bound electron from a gaseous atom in its ground state is called its first ionization energy ...

The electron affinity (EA) is the energy change for adding an electron to a gaseous atom to form an anion (negative ion).



This process can be either ...

Construct an explanation to infer the atomic size, reactivity, electronegativity, and ionization energy of an element based on its position in the Periodic Table.

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Bond Polarity

The absolute value of the difference in electronegativity (&Delta;EN) of two bonded atoms provides a rough measure of the polarity to be ...

Analyze and interpret data to explain the mechanisms and properties of the two main types of intramolecular (ionic and covalent) bonds.

PS.Chm.3.1

Analyze the structure of atoms and isotopes.

Electron Affinity Trends on the Periodic Table

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