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

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Atomic Radii and Effective Nuclear Charge

De grootte van een atoom wordt bepaald door de elektronen of hun orbitalen. Orbitalen beschrijven echter geen besloten ruimte, maar eerder de statistische waarschijnlijkheid waar een elektron kan worden gevonden. Dus hoe wordt de atomaire grootte gedefinieerd en wat beïnvloedt deze?Een atoomstraal kan op twee manieren worden beschreven. Niet-bindende atoomstraal, of Vanderwaalsstraal van een atoom, is de helft van de afstand tussen aangrenzende kernen in de atomaire vaste stof. Omgekeerd maakt een bindende atoomstraal of covalente straal onderscheid tussen metalen en niet-metalen.Bij metalen wordt de straal beschreven voor atomen in hun kristalstructuur als de helft van de afstand tussen de centra van twee naburige atomen. In niet-metalen, diatomische moleculen, wordt de straal beschreven als de helft van de afstand tussen de centra van gebonden atomen. Het periodiek systeem geeft variaties in covalente stralen weer die vaak atoomstralen worden genoemd, die worden beïnvloed door twee factoren;het aantal belangrijkste energieniveaus van valentie-elektronen en de effectieve nucleaire lading.De trend in atoomstralen voor hoofdgroepelementen langs de kolommen wordt hier weergegeven. Als je een groep naar beneden verplaatst, neemt het belangrijkste kwantumgetal, n, toe met één voor elk element. Dus naarmate de buitenste elektronen verder van de kern komen, neemt de atoomstraal toe naar beneden in de groep.Als bijvoorbeeld in groep 1 naar beneden verplaatst wordt, neemt de atoomstraal toe van lithium naar cesium. Deze trend wordt aangetoond door het hele periodiek systeem. Verder laat de plot zien dat de atoomstraal maximaal is voor elk alkalimetaal en tot een minimum daalt bij elk edelgas over de periode.De afnemende atoomstralen over een periode kunnen worden verklaard door de effectieve nucleaire lading. Denk aan het concept van een effectieve nucleaire lading. In elk multi-elektronenatoom beschermen de elektronen in de binnenste schil gedeeltelijk de elektronen in de buitenste schil tegen de aantrekkingskracht van de kern.De effectieve nucleaire lading, de lading die door een buitenste elektron wordt gevoeld, is dus kleiner dan de feitelijke nucleaire lading. Elektronen in dezelfde valentieschil beschermen elkaar niet erg effectief. Over de periode neemt de nucleaire lading toe terwijl het aantal elektronen in de binnenschil constant blijft.Dus naarmate de effectieve nucleaire lading gestaag toeneemt, wordt de afscherming van buitenste elektronen minder, en dit leidt tot een afname van de atoomstralen. De stralen van de meeste overgangselementen blijven echter ongeveer constant over elke rij. Dit komt doordat het aantal elektronen in het buitenste hoofdenergieniveau bijna constant is.

atoomstralen en effectieve nucleaire lading

De elementen in groepen van het periodiek systeem vertonen vergelijkbaar chemisch gedrag. Deze gelijkenis treedt op omdat de leden van een groep hetzelfde aantal en dezelfde verdeling van elektronen in hun valentieschillen hebben. 

 Over een periode van links naar rechts wordt bij elk opeenvolgend element een proton aan de atoomkern en een elektron aan de valentieschil toegevoegd. Als we de elementen in een groep naar beneden gaan, blijft het aantal elektronen in de valentieschil constant, maar het hoofdkwantumgetal neemt elke keer met één toe. Een goed begrip van de elektronische structuur van de elementen stelt ons in staat enkele eigenschappen te onderzoeken die hun chemisch gedrag bepalen. Deze eigenschappen variëren periodiek naarmate de elektronische structuur van de elementen verandert.  

 <h4>Variatie in atoomstraal</h4> 

 Het kwantummechanische beeld maakt het moeilijk om een definitieve grootte van een atoom vast te stellen. Er zijn echter verschillende praktische manieren om de straal van atomen te definiëren en dus hun relatieve afmetingen te bepalen die ongeveer vergelijkbare waarden opleveren. De atoomstraal in metalen is de helft van de afstand tussen de centra van twee aangrenzende atomen. Het is de helft van de afstand tussen de centra van gebonden atomen voor elementen die bestaan als tweeatomige moleculen. 

 Bewegend over een periode van links naar rechts, heeft elk element over het algemeen een kleinere atomaire straal dan het element dat eraan voorafgaat. Dit lijkt misschien contra-intuïtief omdat het impliceert dat atomen met meer elektronen een kleinere atoomstraal hebben. Dit kan worden verklaard op basis van het concept van een effectieve nucleaire lading. In elk multi-elektronenatoom beschermen de elektronen in de binnenste schil de elektronen in de buitenste schil gedeeltelijk tegen de aantrekkingskracht van de atoomkern. De effectieve nucleaire lading, de lading die een elektron voelt, is dus kleiner dan de werkelijke nucleaire lading (Z) en kan als volgt worden geschat: 

 Zeff = Z – σ 

 waarbij Zeff de effectieve nucleaire lading is, Z de feitelijke nucleaire lading is, en σ de afschermingsconstante is, waarbij de afschermingsconstante groter is dan nul maar kleiner dan Z. 

 Elke keer dat we in een periode van het ene element naar het volgende gaan, neemt Z met één toe, maar de afscherming neemt slechts in geringe mate toe. Zeff neemt dus toe naarmate we over een periode van links naar rechts gaan. De sterkere aantrekkingskracht (hogere effectieve nucleaire lading) die wordt ervaren door elektronen aan de rechterkant van het periodiek systeem trekt ze dichter bij de atoomkern, waardoor de atoomstralen kleiner worden. 

 Kernelektronen beschermen elektronen op het buitenste hoofdniveau efficiënt tegen nucleaire lading, maar de buitenste elektronen beschermen elkaar niet efficiënt tegen de nucleaire lading. Hoe groter de effectieve nucleaire lading, hoe sterker de greep van de atoomkern op buitenste elektronen en hoe kleiner de atoomstraal. 

 De stralen van sommige overgangselementen blijven echter over elke rij ongeveer constant. Dit komt omdat het aantal elektronen in het buitenste hoofdenergieniveau vrijwel constant is, en ze een ruwweg constante effectieve nucleaire lading ervaren. 

 Binnen elke periode neemt de trend in de atomaire straal af naarmate Z toeneemt;  Binnen elke groep is de trend dat de atomaire straal toeneemt naarmate Z toeneemt. 

 Als we een groep naar beneden scannen, wordt het belangrijkste kwantumgetal, n, voor elk element met één verhoogd. De elektronen worden dus toegevoegd aan een gebied in de ruimte dat steeds verder verwijderd is van de atoomkern. Bijgevolg moet de grootte van het atoom (en zijn atoomstraal) toenemen naarmate we de afstand van de buitenste elektronen tot de atoomkern vergroten. Deze trend wordt geïllustreerd voor de atoomstralen van de halogenen in de onderstaande tabel. 

 <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> 
   
 Deze tekst is aangepast van <a href="https://openstax.org/books/chemistry-2e/pages/6-5-periodic-variations-in-element-properties">Openstax Chemistry 2e, Sectie 6.5: Periodieke variaties in elementeigenschappen.</a>

Atoomstralen en effectieve nucleaire lading

The elements in groups of the periodic table exhibit similar chemical behavior. This similarity occurs because the members of a group have the same number ...

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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

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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...

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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...

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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 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 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...

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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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