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

Alkalimetalen zijn elementen uit groep 1, waaronder lithium, natrium, kalium, rubidium, cesium en francium. Francium is radioactief en heeft geen stabiele isotopen. Alle alkalimetalen zijn ongewoon zacht en delen metallische eigenschappen, zoals een hoge elektrische en thermische geleidbaarheid.Naar beneden gaand in de groep, nemen de massa, de dichtheid en de atoomstralen toe, terwijl de smeltpunten en ionisatie-energieën afnemen. De elektronenconfiguratie van deze elementen impliceert dat de valentieschil slechts één elektron heeft. Hun lage ionisatie-energie geeft aan dat alkalimetalen gemakkelijk elektronen verliezen om kationen te worden met een edelgasconfiguratie.Dit verklaart ook waarom alkalimetalen uitstekende reductiemiddelen zijn, waarbij lithium het minst reactief is en cesium het meest. Alkalimetalen reageren heftig met niet-metalen, zoals halogenen, om zouten te vormen. De reactie van natriummetaal met chloorgas is sterk exotherm en geeft vonken en warmte vrij.De intensiteit van de reactie neemt verder omlaag in de groep toe naarmate de ionisatie-energieën van de alkalimetalen afnemen. Alkalimetalen reageren heftig met water om waterstofgas en een oplossing van alkalimetaalhydroxide zoals kaliumhydroxide te genereren, een base die vaak wordt gebruikt bij het maken van zeep. De reactie wordt heftiger voor de zwaardere alkalimetalen gezien hun lagere ionisatie-energieën.De warmte die tijdens de reactie wordt geproduceerd, kan zelfs het vrijkomende waterstofgas doen ontbranden, wat kan leiden tot brand of een explosie. Alkalimetalen kunnen ook reageren met waterstof om alkalimetaalhydriden te vormen, die vaak als sterke basen worden gebruikt. In deze reactie krijgt het waterstofatoom een elektron en is het aanwezig als het hydride-ion.Vanwege hun hoge reactiviteit oxideren alkalimetalen gemakkelijk bij blootstelling aan lucht en worden daarom opgeslagen in minerale oliën. Wanneer alkalimetaalzouten in een vlam worden geplaatst en worden gereduceerd tot hun gasvormige metaalatomen, geven ze karakteristieke kleuren af. De warmte wekt het valentie-elektron op tot een hoger energieniveau.Wanneer de aangeslagen elektronen terugkeren naar de grondtoestand, wordt energie uitgezonden in de vorm van straling in het zichtbare gebied. Het emissiespectrum van elk element is uniek en wordt gebruikt om het element kwalitatief te identificeren.

alkalimetalen

Groep 1-elementen zijn zachte en glanzende metalen vaste stoffen. Ze zijn kneedbaar, ductiel en goede geleiders van warmte en elektriciteit. De smeltpunten van de alkalimetalen zijn ongebruikelijk laag voor metalen en nemen af naarmate de groep lager gaat, terwijl de dichtheid toeneemt naarmate de groep lager gaat, met uitzondering van kalium (Tabel 1). Tabel 1: Eigenschappen van de alkalimetalen <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> Als we een groep naar beneden gaan, wordt het belangrijkste kwantumgetal, n, voor elk element met één verhoogd. De buitenste elektronen raken dus verder van de atoomkern verwijderd en de atoomstraal neemt toe van lithium naar cesium. De elektronenconfiguratie van deze elementen laat zien dat de valentieschil slechts één elektron heeft (Tabel 1). Door het verlies van dit elektron ontstaat een kation met de edelgasconfiguratie. Alkalimetalen hebben dus een grote neiging om dit elektron weg te geven en hebben lage eerste ionisatie-energieën, die in de groep afnemen. Hun vermogen om gemakkelijk een elektron te verliezen, maakt ze zeer reactief en uitstekende reductiemiddelen. De alkalimetalen reageren krachtig met niet-metalen, zoals halogenen. De reactie van natrium en chloor om natriumchloride te produceren is exotherm. De alkalimetalen reageren ook heftig met water en er worden waterstofgas en een alkalimetaalhydroxide geproduceerd. De tijdens de reactie geproduceerde warmte kan het vrijkomende waterstofgas doen ontbranden, wat tot een explosie kan leiden. Beide reacties worden exotherm voor de zwaardere alkalimetalen vanwege hun lagere ionisatie-energieën. Blootstelling aan lucht, omdat ze reageren met zuurstof om oxiden te vormen, vermindert de metaalglans van de alkalimetalen. Lithium reageert met zuurstof om lithiumoxide te geven, terwijl andere alkalimetalen zoals natrium oxiden en peroxiden vormen. Het peroxide-ion heeft een enkele covalente zuurstof-zuurstofbinding en is een krachtige waterstofionenacceptor, waardoor de peroxiden van de alkalimetalen sterke basen worden. De alkalimetalen zoals kalium, rubidium en cesium produceren ook superoxiden. Superoxides worden gekenmerkt door de aanwezigheid van O2−. Kaliumsuperoxide is een gele vaste stof die ontleedt bij 560 °C.  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 de eigenschappen van elementen.</a>

Alkalimetalen

Group 1 elements are soft and shiny metallic solids. They are malleable, ductile, and good conductors of heat and electricity. The melting points of the ...

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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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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...

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Ionization Energy Trends in the Periodic Table

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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 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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Electron Affinity

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Halogens

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Noble Gases


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Covalent Bonding and Lewis Structures

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Nomenclature of Alkanes

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Alkali Metals: Reactivity Trends

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