Name: how valence electrons shape the periodic table
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Chemistry

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Periodic Classification of the Elements

Os elétrons que ocupam a camada mais exterior de um átomo são elétrons de valência, enquanto que os elétrons que ocupam os níveis energéticos principais interiores são elétrons de núcleo. Um átomo de sódio com uma configuração de elétrons de 3s1 tem três níveis energéticos principais. Os níveis completos de energia principal interior com 2s22p6 indicam que existem dez elétrons de núcleos seguidos do terceiro nível mais externo que contém o restante, um elétron.Portanto, o sódio tem um elétron de valência. Da mesma forma, o cloro tem dez elétrons de núcleo e sete elétrons de valência. Os elétrons de valência são os mais afastados do núcleo e são mantidos de forma mais solta.Por isso, são os mais fáceis de perder ou partilhar e têm um papel importante na ligação química. Os elementos que têm o mesmo número de elétrons de valência têm propriedades químicas semelhantes, como se pode ver na tabela periódica moderna. Para os elementos do grupo principal, o número do grupo de letras é igual ao número de elétrons de valência e o número da fila é igual ao número quantum principal mais elevado desse elemento.Cada elemento de um grupo tem o mesmo número de elétrons disponível para ligação. Abaixo do grupo, o número quântico principal aumenta em um, enquanto o número de elétrons de valência permanece o mesmo. As duas colunas da extrema-esquerda da tabela periódica constituem o bloco-s.Para estes elementos, o último elétron entra nums orbital s. Os elementos do grupo 1, exceto para o hidrogênio, são chamados de metais alcalinos e são extremamente reativos uma vez que têm apenas um elétron de valência. Os elementos do grupo 2 são os metais alcalinos da terra com dois elétrons na camada da valência.As seis colunas da extrema-direita formam o bloco p. A camada de valência destes elementos ocupou completamente as orbitais s e o último elétron entra na orbital p. Do grupo três A para o grupo oito A, o número de elétrons nos orbitais p aumenta em um.Os gases nobres têm oito elétrons de valência exceto o hélio, que pertence ao bloco s, e só tem dois elétrons. O bloco d é constituído pelas dez colunas colocadas entre os blocos s e p. Estes elementos são denominados de metais de transição.O último elétron entra na orbital d da camada principal número um a menos do que o número da fila. Na quarta fila, o preenchimento de três orbitais, na quinta fila, quatro orbitais d, e assim por diante. Os elementos de transição interna constituem o bloco f e têm o último elétron a entrar numa orbital f.O número quântico principal das orbitais f que se preenchem em cada fila são menos dois do que o número da fila. Na sexta fila, o preenchimento das quatro orbitais f, e, na sétima fila, o preenchimento das cinco orbitais f. Os elementos de transição interna estão dispostos na série lantanídea e na série actinídea.O número de colunas em cada bloco indica quantos elétrons podem ser preenchidos na subcamada do bloco. As duas colunas no bloco s correlacionam-se com a orbital s com dois elétrons, seis colunas no bloco p representam três orbitais p com seis elétrons, enquanto dez colunas no bloco d correspondem a cinco orbitais d com dois elétrons cada. Finalmente, o bloco f é composto por catorze colunas indicando a capacidade máxima de sete orbitais f.

classificação periódica dos elementos

A tabela periódica organiza átomos com base no aumento do número atómico para que elementos com as mesmas propriedades químicas ocorram periodicamente. Quando as configurações eletrónicas são adicionadas à tabela, observa-se uma recorrência periódica de configurações eletrónicas semelhantes nas camadas exteriores desses elementos. Como estão nas camadas exteriores de um átomo, os eletrões de valência desempenham o papel mais importante nas reações químicas. Os eletrões exteriores têm a energia mais elevada dos eletrões em um átomo e são mais facilmente perdidos ou partilhados do que os eletrões centrais. Os eletrões de valência são também o factor determinante em algumas propriedades físicas dos elementos.

As linhas horizontais são conhecidas como períodos. Ao longo de um período, cada elemento consecutivo tem um protão adicional no núcleo e um eletrão adicional na camada de valência. As colunas verticais são grupos. Os elementos de qualquer grupo (ou coluna) têm o mesmo número de eletrões de valência (Figura 1); os metais alcalinos lítio e sódio têm cada um apenas um eletrão de valência, os metais alcalinoterrosos berílio e magnésio têm cada um dois, e os halogénios fluorino e cloro têm cada um sete eletrões de valência.  É a perda, ganho, ou partilha de eletrões de valência que define como os elementos reagem. A semelhança nas propriedades químicas entre elementos do mesmo grupo ocorre porque têm o mesmo número de eletrões de valência. 

É importante lembrar que a tabela periódica foi desenvolvida com base no comportamento químico dos elementos, bem antes de qualquer idéia da sua estrutura atómica estar disponível. Agora, compreende-se o arranjo da tabela periódica; elementos cujos átomos têm o mesmo número de eletrões de valência estão no mesmo grupo. As secções coloridas da Figura 1 mostram as três categorias de elementos classificados pelas orbitais que estão a ser preenchidas.

<img alt="Eq1" src="https://www.jove.com/files/core/CDNSource/lessons/1046/1046Image_1v2.png" /> 
Figura 1: Esta versão da tabela periódica mostra a configuração de cada elemento. Observe que em cada grupo, a configuração é frequentemente semelhante.

Os elementos do grupo principal são por vezes chamados de elementos representativos. Estes são os elementos onde o último eletrão adicionado entra em uma orbital s ou p na camada mais externa, mostrado em azul e vermelho na Figura 1. Esta categoria inclui todos os elementos não-metálicos, bem como os metalóides e muitos metais. Os eletrões de valência para os elementos do grupo principal são aqueles de maior nível n. Por exemplo, o gálio (Ga, número atómico 31) tem a configuração eletrónica [Ar]4s23d104p1, que contém três eletrões de valência (sublinhados). As orbitais d completamente cheias contam como eletrões centrais, não de valência.

As duas colunas da extrema-esquerda compreendem o bloco s e as seis colunas da extrema-direita constituem o bloco p. Os gases nobres, que fazem parte do bloco p, têm todos oito eletrões de valência, excepto o hélio, que tem dois. Estes elementos são altamente estáveis e não reativos.

Elementos de transição ou metais de transição: Este são elementos metálicos nos quais o último eletrão adicionado entra em uma orbital d. Os eletrões de valência (os adicionados após a última configuração de gás nobre) nestes elementos incluem os eletrões d ns e (n – 1). A definição oficial do IUPAC de elementos de transição especifica aqueles com orbitais d parcialmente cheias. Assim, os elementos com orbitais completamente cheias (Zn, Cd, Hg, bem como Cu, Ag, e Au na Figura 1) não são tecnicamente elementos de transição. No entanto, o termo é frequentemente utilizado para referir todo o bloco d (colorido a amarelo na Figura 1).

O bloco d é composto por 10 colunas. O número quântico principal das orbitais d que se enchem a cada linha é igual ao número da linha menos um. Na quarta linha, as orbitais 3d são preenchidas, na quinta linha, as orbitais 4d são preenchidas, e assim por diante.

Elementos de Transição Interna: Eles são mostrados a verde na Figura 1. As camadas de valência dos elementos de transição interna são constituídas pelas subcamadas (n – 2)f, (n – 1)d, e ns. Os elementos de transição interna constituem o bloco f à medida que o último eletrão entra em uma orbital f. O número quântico principal das orbitais f que se enchem em cada linha é igual ao número da linha menos dois. Na sexta linha, as orbitais 4ff

<ol>
<li>A série dos lantanídeos: Lantanídeo (La) até lutécio (Lu)</li>
<li>A série dos actinídeos: Actinídeo (Ac) até laurêncio (Lr)</li>
</ol>

Lantânio e actínio, devido às suas semelhanças com os outros membros das séries, são incluídos e utilizados para dar nome às séries, mesmo sendo metais de transição sem eletrões f.

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.</a>

Classificação Periódica dos Elementos

A tabela periódica organiza os átomos com base no aumento do número atômico para que elementos com as mesmas propriedades químicas se repitam periodicamente. Quando seus ...

Education

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Periodic Properties of the Elements

Propriedades Periódicas dos Elementos

KEY TERMS AND CONCEPTS

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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 some physical properties of elements.

The horizontal rows on the periodic table are periods. Moving across a period, each next element has one more proton in the nucleus and one more electron in the valence shell. The vertical columns are groups. Elements in the same group have the same number of valence electrons, which is why they often show similar chemical behavior. For example, lithium and sodium each have one valence electron, beryllium and magnesium each have two, and fluorine and chlorine each have seven. Because elements react by losing, gaining, or sharing valence electrons, their group placement helps predict how they will behave.

It is important to remember that the periodic table was built from chemical behavior before atomic structure was understood. Today, the pattern makes sense because elements with the same number of valence electrons are placed in the same group. The colored regions of the table also show three categories based on which orbitals are being filled. Main group elements, also called representative elements, are the elements whose last added electron enters an s or p orbital in the outermost shell. This category includes all nonmetals, the metalloids, and many metals. For main group elements, the valence electrons are the electrons with the highest n level. Gallium, for example, has the electron configuration [Ar] 4s2 3d10 4p1, so it has three valence electrons. The filled d orbitals are counted as core electrons, not valence electrons.

The two far-left columns form the s-block, and the six far-right columns form the p-block. The noble gases are part of the p-block. They usually have eight valence electrons, except helium, which has two. This helps explain why noble gases are highly stable and unreactive. Transition elements, or transition metals, are metallic elements whose last added electron enters a d orbital. Their valence electrons include the ns and (n - 1)d electrons added after the last noble gas configuration. By the official IUPAC definition, transition elements have partially filled d orbitals, so Zn, Cd, Hg, and also Cu, Ag, and Au are not technically transition elements, even though the term is often used for the whole d-block.

The d-block has 10 columns. As you move across each row, the d orbitals that fill have a principal quantum number that is one less than the row number. In the fourth row, 3d orbitals fill; in the fifth row, 4d orbitals fill; and so on. Inner transition elements form the f-block, where the last electron added enters an f orbital. Their valence shells include the (n - 2)f, the (n - 1)d, and the ns subshells. The f orbitals that fill across each row have a principal quantum number that is two less than the row number. In the sixth row, 4f orbitals fill, and in the seventh row, 5f orbitals fill. The two inner transition series are the lanthanides, from lanthanum through lutetium, and the actinides, from actinium through lawrencium. Lanthanum and actinium are included in those series because they are similar to the other members, even though they are transition metals with no f electrons.

How Valence Electrons Shape the Periodic Table

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periodic classification of the elements

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

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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 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...

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 valence electrons explain periodic table groups, blocks, and element behavior, including main group, transition, lanthanide, and actinide elements.

How Valence Electrons Shape the Periodic Table | JoVE K12

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Analyze the structure of atoms and isotopes.

How Valence Electrons Shape the Periodic Table

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