Name: alkali metals: reactivity trends
Uploaded: 2020-09-03T13:38:28.000Z
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Alkali Metals

I metalli alcalini sono elementi del gruppo 1, fra cui litio, sodio, potassio, rubidio, cesio e francio. Il francioè radioattivo e non ha isotopi stabili. Tutti i metalli alcalini sono insolitamente morbidi e condividono proprietà metalliche, come l'elevata conduttività elettrica e termica.

Scendendo nel gruppo, la massa, la densità e i raggi atomici aumentano, mentre i punti di fusione e le energie di ionizzazione diminuiscono. La configurazione elettronica di questi elementi implica che il guscio di valenza abbia un solo elettrone. Le loro basse energie di ionizzazione indicano che i metalli alcalini perdano facilmente elettroni per diventare cationi con una configurazione di gas nobile.

Questo spiega anche perché i metalli alcalini sono eccellenti agenti riducenti, con il litio meno reattivo, e il cesio il più reattivo. I metalli alcalini reagiscono vigorosamente con i non metalli, come gli alogeni, per formare sali. La reazione del sodio metallico con il cloro gassoso è altamente esotermica e rilascia scintille e calore.

L'intensità della reazione aumenta scendendo lungo il gruppo con il diminuire delle energie di ionizzazione dei metalli alcalini. I metalli alcalini reagiscono con veemenza con l'acqua per generare gas idrogeno e una soluzione di idrossido di metallo alcalino come l'idrossido di potassio, che è una base spesso utilizzata nella produzione di sapone. La reazione diventa più violenta per i metalli alcalini più pesanti date le loro energie di ionizzazione inferiori.

Il calore prodotto durante la reazione può persino infuocare il gas idrogeno rilasciato, provocando un incendio o un'esplosione. I metalli alcalini possono anche reagire con l'idrogeno per formare idruri di metalli alcalini, spesso usati come basi forti. In questa reazione, l'atomo di idrogeno guadagna un elettrone ed è presente come ione idruro.

Per via della loro elevata reattività, i metalli alcalini si ossidano facilmente se esposti all'aria e sono quindi immagazzinati in oli minerali. Quando i sali di metalli alcalini vengono posti vicino ad una fiamma e ridotti ad atomi di metallo gassoso, emettono colori caratteristici. Il calore eccita l'elettrone di valenza a un livello di energia più elevato.

Quando gli elettroni eccitati ritornano allo stato fondamentale, l'energia viene emessa sotto forma di radiazione nella regione visibile. Lo spettro di emissione di ogni elemento è unico e utilizzato per identificare qualitativamente l'elemento.

metalli alcalini

Gli elementi del gruppo 1 sono metallici solidi morbidi e lucenti. Sono malleabili, duttili e buoni conduttori di calore ed elettricit. I punti di fusione dei metalli alcalini sono insolitamente bassi per i metalli e diminuiscono scendendo nel gruppo, mentre la densit aumenta scendendo nel gruppo ad eccezione del potassio (Tabella 1).

Table 1: Properties of the alkali metals

<table style="margin-left: auto; margin-right: auto;">
	<tbody style="border: 1px solid black;">
		<tr>
			<td style="color: black; border: 1px solid black; text-align: center;">Element</td>
			<td style="color: black; border: 1px solid black; text-align: center;">Electron Configuration</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;">IE1 (kJ/mol)</td>
			<td style="color: black; border: 1px solid black; text-align: center;">Melting Point (°C)</td>
			<td style="color: black; border: 1px solid black; text-align: center;">Density at 25 °C (g/cm3)</td>
		</tr>
		<tr>
			<td style="color: black; border: 1px solid black; text-align: center;">Li</td>
			<td style="color: black; border: 1px solid black; text-align: center;">[He] 2s1</td>
			<td style="color: black; border: 1px solid black; text-align: center;">152</td>
			<td style="color: black; border: 1px solid black; text-align: center;">520</td>
			<td style="color: black; border: 1px solid black; text-align: center;">181</td>
			<td style="color: black; border: 1px solid black; text-align: center;">0.53</td>
		</tr>
		<tr>
			<td style="color: black; border: 1px solid black; text-align: center;">Na</td>
			<td style="color: black; border: 1px solid black; text-align: center;">[Ne] 3s1</td>
			<td style="color: black; border: 1px solid black; text-align: center;">186</td>
			<td style="color: black; border: 1px solid black; text-align: center;">496</td>
			<td style="color: black; border: 1px solid black; text-align: center;">98</td>
			<td style="color: black; border: 1px solid black; text-align: center;">0.97</td>
		</tr>
		<tr>
			<td style="color: black; border: 1px solid black; text-align: center;">K</td>
			<td style="color: black; border: 1px solid black; text-align: center;">[Ar] 4s1</td>
			<td style="color: black; border: 1px solid black; text-align: center;">227</td>
			<td style="color: black; border: 1px solid black; text-align: center;">420</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;">0.86</td>
		</tr>
		<tr>
			<td style="color: black; border: 1px solid black; text-align: center;">Rb</td>
			<td style="color: black; border: 1px solid black; text-align: center;">[Kr] 5s1</td>
			<td style="color: black; border: 1px solid black; text-align: center;">248</td>
			<td style="color: black; border: 1px solid black; text-align: center;">400</td>
			<td style="color: black; border: 1px solid black; text-align: center;">39</td>
			<td style="color: black; border: 1px solid black; text-align: center;">1.53</td>
		</tr>
		<tr>
			<td style="color: black; border: 1px solid black; text-align: center;">Cs</td>
			<td style="color: black; border: 1px solid black; text-align: center;">[Xe] 6s1</td>
			<td style="color: black; border: 1px solid black; text-align: center;">265</td>
			<td style="color: black; border: 1px solid black; text-align: center;">380</td>
			<td style="color: black; border: 1px solid black; text-align: center;">29</td>
			<td style="color: black; border: 1px solid black; text-align: center;">1.93</td>
		</tr>
	</tbody>
</table>

Scendendo lungo un gruppo, il numero quantico principale, n, aumenta di uno per ciascun elemento. Pertanto, gli elettroni esterni si allontanano dal nucleo e il raggio atomico aumenta dal litio al cesio.

La configurazione elettronica di questi elementi mostra che il guscio di valenza ha un solo elettrone (Tabella 1). La perdita di questo elettrone produce un catione con la configurazione del gas nobile. Pertanto, i metalli alcalini hanno una grande tendenza a cedere questo elettrone e hanno energie di prima ionizzazione basse, che diminuiscono lungo il gruppo. La loro capacit di perdere facilmente un elettrone li rende altamente reattivi e ottimi agenti riducenti.

I metalli alcalini reagiscono vigorosamente con i non metalli, come gli alogeni. La reazione di sodio e cloro per produrre cloruro di sodio esotermica. I metalli alcalini reagiscono violentemente anche con l'acqua e vengono prodotti idrogeno gassoso e un idrossido di metallo alcalino. Il calore prodotto durante la reazione pu accendere l'idrogeno gassoso rilasciato, provocando un'esplosione. Entrambe queste reazioni diventano pi esotermiche per i metalli alcalini pi pesanti a causa della loro minore energia di ionizzazione.

L'esposizione all'aria, poich reagiscono con l'ossigeno per formare ossidi, riduce la lucentezza metallica dei metalli alcalini. Il litio reagisce con l'ossigeno per dare ossido di litio, mentre altri metalli alcalini come il sodio formano ossidi e perossidi. Lo ione perossido ha un singolo legame covalente ossigeno-ossigeno ed un potente accettore di ioni idrogeno, rendendo i perossidi dei metalli alcalini basi forti. Anche i metalli alcalini come potassio, rubidio e cesio producono superossidi. I superossidi sono caratterizzati dalla presenza di O2−. Il superossido di potassio un solido giallo che si decompone a 560C.

Questo testo 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>

Metalli alcalini

Gli elementi del gruppo 1 sono solidi metallici morbidi e lucidi. Sono malleabili, duttili e buoni conduttori di calore ed elettricità. I punti di fusione del ...

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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 from the nucleus and the atomic radius increases. Their electron configurations show a single electron in the valence shell. When that electron is lost, the atom forms a cation with a noble gas configuration.

Because they give up that outer electron so easily, alkali metals have low first ionization energies and are highly reactive. Their first ionization energies decrease down the group, which makes the heavier elements even more reactive. They are also strong reducing agents. In reactions with nonmetals such as halogens, they react vigorously. A common example is sodium reacting with chlorine to form sodium chloride, and the reaction releases heat.

Alkali metals also react violently with water. These reactions produce hydrogen gas and an alkali metal hydroxide. The heat can ignite the hydrogen gas, which may lead to an explosion. The reactions become more exothermic for the heavier alkali metals because their ionization energies are lower.

Exposure to air causes alkali metals to react with oxygen and lose their metallic luster. Lithium forms lithium oxide. Other alkali metals such as sodium can form oxides and peroxides. A peroxide contains a single oxygen-oxygen covalent bond and acts as a strong base because it strongly accepts hydrogen ions. Potassium, rubidium, and cesium can also form superoxides, which contain O2- . Potassium superoxide is a yellow solid that decomposes at 560 °C.

Alkali Metals: Reactivity Trends

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alkali metals

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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Explore how alkali metals change down Group 1, from atomic size and ionization energy to reactions with water, oxygen, and halogens.

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Alkali Metals: Reactivity Trends

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