Name: periodic trends in atomic size
Uploaded: 2020-09-03T13:38:28.000Z
Duration: 3 min 8 s
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Atomic Radii and Effective Nuclear Charge

El tamaño de un átomo está determinado por los electrones. o sus orbitales. Sin embargo, los orbitales no describen un espacio confinado, sino más bien la probabilidad estadística de dónde se puede encontrar un electrón.Entonces, cómo se define el tamaño atómico y qué influye en el mismo? Un radio atómico se puede describir de dos formas. Un radio atómico no enlazante, o radio de van der Waals de un átomo, es la mitad de la distancia entre núcleos adyacentes en el sólido atómico.Por el contrario, un radio atómico de enlace, o radio covalente, distingue entre metales y no metales. En los metales, el radio se describe para los átomos en su estructura cristalina como la mitad de la distancia entre los centros de dos átomos vecinos. En las moléculas diatómicas no metálicas, el radio se describe como la mitad de la distancia entre los centros de átomos enlazados.La tabla periódica muestra variaciones en radiofrecuencias covalentes que a menudo se denominan radios atómicos, que están influenciadas por dos factores;el número de los niveles de energía de los electrones de valencia y la carga nuclear efectiva. La tendencia en radios atómicos para los elementos del grupo principal abajo de las columnas se representa aquí. Bajando un grupo, el número cuántico principal, n, aumenta en uno para cada elemento.Por lo tanto, a medida que los electrones externos se alejan del núcleo, el radio atómico aumenta hacia abajo del grupo. Por ejemplo, moviéndose hacia abajo del grupo 1, el radio atómico aumenta de litio a cesio. Esta tendencia se demuestra en toda la tabla periódica.Además, esto revela que el radio atómico es el máximo para cada metal alcalino y que cae al mínimo con cada gas noble a lo largo del período. Los radios atómicos decrecientes a lo largo de un período pueden explicarse por la carga nuclear efectiva. Recuerde el concepto de carga nuclear efectiva.En cualquier átomo de varios electrones, los electrones de la capa interna protegen parcialmente a los electrones de la capa exterior del la atacción del núcleo. Por lo tanto, en la carga nuclear efectiva, la carga que siente un electrón externo es menor que la carga nuclear real. Los electrones en la misma capa de valencia no se escudan muy eficazmente.A lo largo del período, la carga nuclear aumenta mientras que el número de electrones de la capa interna permanece constante. Por lo tanto, a medida que la carga nuclear efectiva aumenta constantemente, el blindaje de los electrones externos se vuelve menor, y esto conduce a una disminución de los radios atómicos. Sin embargo, los radios de la mayoría de los elementos de transición, se mantienen aproximadamente constantes en cada fila.Esto se debe a que la cantidad de electrones en el nivel de energía principal más externo es casi constante.

radio atómico y carga nuclear efectiva

Los elementos en los grupos de la tabla periódica muestran un comportamiento químico similar. Esta similitud ocurre porque los miembros de un grupo tienen el mismo número y distribución de electrones en sus capas de valencia.
 
 Pasando de izquierda a derecha a través de un período, es agregado un protón al núcleo y un electrón a la capa de valencia con cada elemento sucesivo. Yendo hacia abajo en los elementos de un grupo, el número de electrones en la capa de valencia permanece constante, pero el número cuántico principal aumenta en uno cada vez. La comprensión de la estructura electrónica de los elementos nos permite examinar algunas de las propiedades que rigen su comportamiento químico. Estas propiedades varían periódicamente a medida que cambia la estructura electrónica de los elementos. 
 
 <h4>Variación en el radio atómico</h4>
 
 La imagen de la mecánica cuántica hace difícil establecer un tamaño definido para un átomo. Sin embargo, hay varias maneras prácticas de definir el radio de los átomos y, por lo tanto, determinar sus tamaños relativos que dan valores más o menos similares.
 
 El radio atómico de los metales es la mitad de la distancia entre los centros de dos átomos vecinos. Para los elementos que existen como moléculas diatómicas es la mitad de la distancia entre los centros de los átomos unidos.
 
 Generalmente, al moverse de izquierda a derecha a través de un período, cada elemento tiene un radio atómico más pequeño que el elemento que lo precede. Esto podría parecer contradictorio porque implica que los átomos con más electrones tienen un radio atómico más pequeño. Esto puede explicarse sobre la base del concepto de una carga nuclear efectiva. En cualquier átomo multi-electrón, los electrones de la capa interna protegen parcialmente a los electrones de la capa externa de la tracción del núcleo.  Por lo tanto, la carga nuclear efectiva, la carga sentida por un electrón, es menor que la carga nuclear real (Z) y puede estimarse por lo siguiente: 
 
 Zef =Z – σ              
 
 Donde, Zef es la carga nuclear efectiva,Z es la carga nuclear real, y σ es la constante de apantallamiento, donde la constante de apantallamiento es mayor que cero pero menor que Z.
 
 Cada vez que pasamos de un elemento al siguiente a lo largo de un período, la Z aumenta en uno, pero el apantallamiento aumenta sólo ligeramente. Por lo tanto, la Zef aumenta a medida que nos movemos de izquierda a derecha a través de un período. La tracción más fuerte (mayor carga nuclear efectiva) experimentada por los electrones en el lado derecho de la tabla periódica los acerca al núcleo, haciendo los radios atómicos más pequeños.
 
 Los electrones centrales protegen eficientemente a los electrones en el nivel principal más externo de la carga nuclear, pero los electrones más externos no se protegen eficazmente unos a otros de la carga nuclear. Cuanto mayor sea la carga nuclear efectiva, más fuerte será la sujeción del núcleo a los electrones externos, y más pequeño será el radio atómico.
 
 Sin embargo, los radios de algunos elementos de transición permanecen constantes a lo largo de cada fila. Esto se debe a que el número de electrones en el nivel de energía principal más externo es casi constante, y experimentan una carga nuclear efectiva constante.
 
 Dentro de cada período, la tendencia en el radio atómico disminuye a medida que la Z aumenta;  dentro de cada grupo, la tendencia es que el radio atómico aumente a medida que Z aumenta.
 
 Al explorar un grupo hacia abajo, el número cuántico principal, n, para cada elemento aumenta en uno. Así, los electrones están siendo agregados a una región del espacio que está cada vez más distante del núcleo. Consecuentemente, el tamaño del átomo (y su radio atómico) debe aumentar a medida que aumentamos la distancia entre los electrones más externos y el núcleo. Esta tendencia se ilustra para los radios atómicos de los halógenos en la tabla siguiente.
 
 <table style="margin-left: auto; margin-right: auto;">
 <tbody style="border: 1px solid black;">
 <tr>
 <td colspan="3" style="color: black; border: 1px solid black; text-align: center;">Radio atómico de los elementos del grupo halógeno</td>
 </tr>
 <tr>
 <td style="color: black; border: 1px solid black; text-align: center;">Átomo</td>
 <td style="color: black; border: 1px solid black; text-align: center;">Radio atómico (pm)</td>
 <td style="color: black; border: 1px solid black; text-align: center;">Carga nuclear, Z</td>
 </tr>
 <tr>
 <td style="color: black; border: 1px solid black; text-align: center;">F </td>
 <td style="color: black; border: 1px solid black; text-align: center;">64 </td>
 <td style="color: black; border: 1px solid black; text-align: center;">9+ </td>
 </tr>
 <tr>
 <td style="color: black; border: 1px solid black; text-align: center;">Cl </td>
 <td style="color: black; border: 1px solid black; text-align: center;">99 </td>
 <td style="color: black; border: 1px solid black; text-align: center;">17+ </td>
 </tr>
 <tr>
 <td style="color: black; border: 1px solid black; text-align: center;">Br</td>
 <td style="color: black; border: 1px solid black; text-align: center;">114</td>
 <td style="color: black; border: 1px solid black; text-align: center;">35+</td>
 </tr>
 <tr>
 <td style="color: black; border: 1px solid black; text-align: center;">I</td>
 <td style="color: black; border: 1px solid black; text-align: center;">133</td>
 <td style="color: black; border: 1px solid black; text-align: center;">53+</td>
 </tr>
 <tr>
 <td style="color: black; border: 1px solid black; text-align: center;">At</td>
 <td style="color: black; border: 1px solid black; text-align: center;">148</td>
 <td style="color: black; border: 1px solid black; text-align: center;">85+</td>
 </tr>
 </tbody>
 </table>
 
  
 
 Este texto está adaptado de <a href="https://openstax.org/books/chemistry-2e/pages/6-5-periodic-variations-in-element-properties">Openstax, Química 2e, Sección 6.5: Variaciones periódicas en las propiedades de los elementos.</a>

Radio Atómico y Carga Nuclear Efectiva

Los elementos de los grupos de la tabla periódica exhiben un comportamiento químico similar. Esta similitud se produce porque los miembros de un grupo tienen el mismo número ...

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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 precision because atoms do not have a hard edge. Scientists use practical measurements instead. In metals, atomic radius is one-half the distance between the centers of two neighboring atoms. For elements that form diatomic molecules, it is one-half the distance between the centers of bonded atoms.

Moving from left to right across a period, atomic radius generally decreases. That may seem surprising because atoms on the right have more electrons. The reason is effective nuclear charge, or the charge an electron feels from the nucleus. It can be estimated as Z eff = Z – σ, where Z is the actual nuclear charge and σ is the shielding constant.

As Z increases by one from element to element across a period, shielding rises only a little. This means Z eff also increases from left to right. A stronger pull from the nucleus draws outer electrons closer, so the atomic radius becomes smaller. Core electrons shield outer electrons well, but outermost electrons do not shield one another very effectively. Some transition elements keep nearly the same radius across a row because the number of electrons in the outermost principal energy level changes very little, so the effective nuclear charge stays roughly constant.

The trend within a group is the opposite. Atomic radius increases as Z increases down a group. Each step down adds a principal energy level, so the outermost electrons are farther from the nucleus. That greater distance makes the atom larger. The halogens show this pattern clearly, with atomic radii increasing from fluorine to astatine.

Periodic Trends in Atomic Size

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

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...

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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...

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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...

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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...

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Learn why atomic radius changes across periods and groups, and how effective nuclear charge and shielding shape the size of atoms in the periodic table.

Periodic Trends in Atomic Size | JoVE K12

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Periodic Trends in Atomic Size

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