Name: alkali metals: reactivity trends
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Alkali Metals

Les métaux alcalins sont des éléments du groupe 1, y compris le lithium, le sodium, le potassium, le rubidium, le césium et le francium. Francium est radioactif et n'a pas d'isotopes stables. Tous les métaux alcalins sont exceptionnellement doux et partagent des propriétés métalliques, telles qu'une conductivité électrique et thermique élevée.En descendant le groupe, la masse, la densité et les rayons atomiques augmentent, tandis que les points de fusion et les énergies d'ionisation diminuent. La configuration électronique de ces éléments implique que la coquille de valence n'a qu'un seul électron. Leurs faibles énergies d'ionisation indiquent que les métaux alcalins perdent facilement des électrons pour devenir des cations avec une configuration de gaz noble.Cela explique également pourquoi les métaux alcalins sont d'excellents agents réducteurs, le lithium étant le moins réactif et le césium le plus. Les métaux alcalins réagissent vigoureusement avec les non-métaux, tels que les halogènes, pour former des sels. La réaction du métal de sodium avec le gaz de chlore est fortement exothermique et libère des étincelles et de la chaleur.L'intensité de la réaction augmente dans le groupe à mesure que les énergies d'ionisation des métaux alcalins diminuent. Les métaux alcalins réagissent avec véhémence avec l'eau pour générer de l'hydrogène gazeux et une solution d'hydroxyde de métal alcalin comme l'hydroxyde de potassium, qui est une base souvent utilisée dans la fabrication du savon. La réaction devient plus violente pour les métaux alcalins plus lourds étant donné leurs énergies d'ionisation plus faibles.La chaleur produite au cours de la réaction peut même enflammer l'hydrogène gazeux libéré conduisant à un incendie ou à une explosion. Métaux alcalins peuvent également réagir avec l'hydrogène pour former des hydrures de métaux alcalins, souvent utilisés comme bases fortes. Dans cette réaction, l'atome d'hydrogène gagne un électron et se présente comme l'ion hydrure.En raison de leur réactivité élevée, les métaux alcalins s'oxydent facilement lors de l'exposition à l'air et sont donc stockés dans des huiles minérales. Lorsque les sels de métaux alcalins sont placés dans une flamme et réduits à leurs atomes de métal gazeux, ils émettent des couleurs caractéristiques. La chaleur excite l'électron de valence à un niveau d'énergie plus élevé.Lorsque les électrons excités reviennent à l'état fondamental, l'énergie est émise sous forme de rayonnement dans la région visible. Le spectre d'émission de chaque élément est unique et utilisé pour identifier l'élément qualitativement.

métaux alcalins

Les éléments du groupe 1 sont des solides métalliques mous et brillants. Ils sont malléables, ductiles et de bons conducteurs de chaleur et d'électricité. Les points de fusion des métaux alcalins sont exceptionnellement bas pour des métaux et ils diminuent lorsqu'on descend dans le groupe. La densité diminue également dans la même direction, à l'exception du potassium (tableau 1). 
 
 Tableau 1 : propriétés des métaux alcalins 
 
 <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;">Élément </td>
 <td style="color: black; border: 1px solid black; text-align: center;">Configuration électronique </td>
 <td style="color: black; border: 1px solid black; text-align: center;">Rayon atomique (pm) </td>
 <td style="color: black; border: 1px solid black; text-align: center;">Ei1 (kJ/mol) </td>
 <td style="color: black; border: 1px solid black; text-align: center;">Point de fusion (°C) </td>
 <td style="color: black; border: 1px solid black; text-align: center;">Densité à 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>
 
 En descendant d'un groupe, le nombre quantique principal, n, augmente de un pour chaque élément. Ainsi, les électrons externes s'éloignent du noyau et le rayon atomique augmente du lithium au césium. 
 
 La configuration électronique de ces éléments montre que la couche de valence n'a qu'un seul électron (tableau 1). La perte de cet électron produit un cation avec la configuration d'un gaz noble. Ainsi, les métaux alcalins ont une grande tendance à céder cet électron et ils ont des énergies de première ionisation faibles, qui diminuent en allant vers le bas du groupe. Leur capacité à perdre un électron facilement fait qu'ils sont hautement réactifs et d'excellents agents réducteurs. 
 
 Les métaux alcalins réagissent vigoureusement avec les non-métaux, comme les halogènes. La réaction du sodium et du chlore pour produire du chlorure de sodium est exothermique. Les métaux alcalins réagissent aussi violemment avec l'eau, et de l'hydrogène et un hydroxyde de métal alcalin sont produits. La chaleur produite pendant la réaction peut enflammer l'hydrogène gazeux libéré, ce qui entraîne une explosion. Ces deux réactions deviennent plus exothermiques pour les métaux alcalins plus lourds en raison de leurs énergies d'ionisation plus faibles. 
 
 L'exposition à l'air, lorsqu'ils réagissent avec l'oxygène pour former des oxydes, réduit l'éclat métallique des métaux alcalins. Le lithium réagit avec l'oxygène pour donner de l'oxyde de lithium, tandis que d'autres métaux alcalins comme le sodium forment des oxydes et  des peroxydes. L'ion peroxyde possède une seule liaison covalente oxygène-oxygène et c'est un puissant accepteur d'ions hydrogène, ce qui fait que les peroxydes des métaux alcalins sont des bases fortes. Les métaux alcalins tels que le potassium, le rubidium et le césium produisent également des superoxydes. Les superoxydes sont caractérisés par la présence d'O2−. Le superoxyde de potassium est un solide jaune qui se décompose à 560 °C.  
 
 Ce texte est adapté de <a href="https://openstax.org/books/chemistry-2e/pages/6-5-periodic-variations-in-element-properties">Openstax, Chimie 2e, Section 6.5 : Variations périodiques dans les propriétés des élément.</a>

Métaux alcalins

Les éléments du groupe 1 sont des solides métalliques mous et brillants. Ils sont malléables, ductiles et bons conducteurs de chaleur et d’électricité. Les points de fusion de la ...

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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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This process can be either ...

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Alkali Metals: Reactivity Trends

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