Name: periodic trends in atomic size
Uploaded: 2020-09-03T13:38:28.000Z
Duration: 3 min 8 s
Channel: JoVE

K12 Standards

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North Carolina K-12 Science Standard Course of Study

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Chemistry

Matter and its Interactions

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Atomic Radii and Effective Nuclear Charge

La dimensione di un atomo è dettata dagli elettroni o dai loro orbitali. Tuttavia, gli orbitali non descrivono uno spazio limitato, ma piuttosto la probabilità statistica di dove è possibile trovare un elettrone. Allora come viene definita la dimensione atomica, e cosa la influenza?

Un raggio atomico può essere descritto in due modi. Il raggio atomico non legante, o raggio di Van der Waals di un atomo, è la metà della distanza fra nuclei adiacenti nel solido atomico. Al contrario, un raggio atomico di legame, o raggio covalente, è distinto tra metalli e non metalli.

Nei metalli, il raggio è descritto per gli atomi nella loro struttura cristallina come metà della distanza fra i centri di due atomi vicini. Nelle molecole biatomiche non metalliche, il raggio è descritto come metà della distanza tra i centri degli atomi legati. La tavola periodica raffigura le variazioni dei raggi covalenti che sono spesso chiamati raggi atomici, e sono influenzati da due fattori:il numero dei principali livelli energetici degli elettroni di valenza e la carica nucleare effettiva.

L'andamento dei raggi atomici per gli elementi del gruppo principale lungo le colonne è illustrato qui. Spostandosi verso il basso in un gruppo, il numero quantico principale, n, aumenta di uno per ogni elemento. Pertanto, man mano che gli elettroni esterni si allontanano dal nucleo, il raggio atomico aumenta attraverso il gruppo.

Per esempio, spostandosi verso il basso nel gruppo 1, il raggio atomico aumenta da litio a cesio. Questa tendenza è dimostrata dall'intera tavola periodica. Inoltre, la trama rivela che il raggio atomico è massimo per ogni metallo alcalino e scende al minimo per ogni gas nobile in tutto il periodo.

La diminuzione dei raggi atomici in un periodo può essere spiegata dalla carica nucleare effettiva. Ricordiamo il concetto di carica nucleare effettiva:in qualsiasi atomo multielettronico, gli elettroni del guscio interno proteggono parzialmente gli elettroni del guscio esterno dall'attrazione del nucleo. Pertanto, la carica nucleare effettiva, ovvero la carica percepita da un elettrone esterno, è inferiore alla carica nucleare effettiva.

Gli elettroni nello stesso guscio di valenza non si schermano l'un l'altro in modo molto efficace. Lungo il periodo, la carica nucleare aumenta mentre il numero di elettroni del guscio interno rimane costante. Pertanto, man mano che la carica nucleare effettiva aumenta costantemente, la schermatura degli elettroni esterni diminuisce e questo porta a una diminuzione dei raggi atomici.

I raggi della maggior parte degli elementi di transizione, tuttavia, rimangono più o meno costanti su ciascuna riga. Questo perché il numero di elettroni nel livello di energia principale più esterno è quasi costante.

raggi atomici e carica nucleare effettiva

Gli elementi nei gruppi della tavola periodica mostrano un comportamento chimico simile. Questa somiglianza si verifica perché i membri di un gruppo hanno lo stesso numero e la stessa distribuzione degli elettroni nei loro gusci di valenza.

Percorrendo un periodo da sinistra a destra, con ogni elemento successivo si aggiunge un protone al nucleo e un elettrone al guscio di valenza. Scendendo lungo gli elementi di un gruppo, il numero di elettroni nel guscio di valenza rimane costante, ma il numero quantico principale aumenta ogni volta di uno. La comprensione della struttura elettronica degli elementi ci permette di esaminare alcune delle proprietà che governano il loro comportamento chimico. Queste proprietà variano periodicamente al variare della struttura elettronica degli elementi.

Variazione del raggio atomico

Il quadro della meccanica quantistica rende difficile stabilire una dimensione definita di un atomo. Tuttavia, esistono diversi modi pratici per definire il raggio degli atomi e, quindi, per determinare le loro dimensioni relative che danno valori più o meno simili.

Il raggio atomico nei metalli è la metà della distanza tra i centri di due atomi vicini. È la metà della distanza tra i centri degli atomi legati per gli elementi che esistono come molecole biatomiche.

Muovendosi lungo un periodo da sinistra a destra, generalmente, ogni elemento ha un raggio atomico minore dell'elemento che lo precede. Ciò potrebbe sembrare controintuitivo perché implica che gli atomi con più elettroni abbiano un raggio atomico più piccolo. Ciò può essere spiegato sulla base del concetto di carica nucleare efficace. In qualsiasi atomo multielettronico, gli elettroni del guscio interno schermano parzialmente gli elettroni del guscio esterno dall'attrazione del nucleo. Pertanto, la carica nucleare effettiva, la carica percepita da un elettrone, è inferiore alla carica nucleare effettiva (Z) e può essere stimata come segue:

Zeff = Z – σ

dove Zeff è la carica nucleare effettiva, Z è la carica nucleare effettiva e σ è la costante di schermatura, dove la costante di schermatura è maggiore di zero ma minore di Z.

Ogni volta che ci spostiamo da un elemento all'altro in un periodo, Z aumenta di uno, ma la schermatura aumenta solo leggermente. Pertanto, Zeff aumenta quando ci si sposta da sinistra a destra in un periodo. L'attrazione più forte (maggiore carica nucleare effettiva) sperimentata dagli elettroni sul lato destro della tavola periodica li avvicina al nucleo, rendendo i raggi atomici più piccoli.

Gli elettroni del nucleo proteggono efficacemente gli elettroni del livello principale più esterno dalla carica nucleare, ma gli elettroni più esterni non si schermano efficacemente l’un l’altro dalla carica nucleare. Maggiore è la carica nucleare effettiva, maggiore è la presa del nucleo sugli elettroni esterni e minore è il raggio atomico.

Tuttavia, i raggi di alcuni elementi di transizione rimangono pressoché costanti su ciascuna riga. Questo perché il numero di elettroni nel livello di energia principale più esterno è quasi costante e sperimentano una carica nucleare effettiva pressoché costante.

All'interno di ciascun periodo, l'andamento del raggio atomico diminuisce all'aumentare di Z; All'interno di ciascun gruppo, la tendenza è che il raggio atomico aumenta all'aumentare di Z.

Analizzando un gruppo, il numero quantico principale, n, aumenta di uno per ciascun elemento. Pertanto, gli elettroni vengono aggiunti in una regione dello spazio sempre più distante dal nucleo. Di conseguenza, la dimensione dell'atomo (e il suo raggio atomico) deve aumentare man mano che aumenta la distanza degli elettroni più esterni dal nucleo. Questa tendenza è illustrata per i raggi atomici degli alogeni nella tabella seguente.

<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;">Atomic Radii of the Halogen Group Elements</td>
		</tr>
		<tr>
			<td style="color: black; border: 1px solid black; text-align: center;">Atom</td>
			<td style="color: black; border: 1px solid black; text-align: center;">Atomic radius (pm)</td>
			<td style="color: black; border: 1px solid black; text-align: center;">Nuclear charge, 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>



Questo testo è adattato da <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>

Raggi atomici e carica nucleare effettiva

Gli elementi nei gruppi della tavola periodica mostrano un comportamento chimico simile. Questa somiglianza si verifica perché i membri di un gruppo hanno lo stesso numero ...

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Periodic Properties of the Elements

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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...

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...

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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.

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Periodic Trends in Atomic Size

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