Métaux alcalins

Alkali metals have low ionization energies because their valence shell contains only one electron, which is relatively easy to remove. This single electron is located far from the nucleus in a higher principal quantum number, experiencing significant shielding from inner electrons. The weak attraction between the valence electron and nucleus makes alkali metals excellent reducing agents, readily losing electrons to achieve a noble gas configuration.

Atomic radii increase significantly moving down group 1 from lithium to cesium. As the principal quantum number increases by one for each element, outer electrons occupy orbitals farther from the nucleus. Although nuclear charge also increases, the effect of additional electron shells dominates, causing the atomic radius to grow progressively larger despite increasing nuclear attraction.

Alkali metals react more violently with water down the group because their ionization energies decrease, making it easier to lose electrons. Heavier alkali metals like potassium and cesium release more energy during the reaction, generating sufficient heat to ignite the hydrogen gas produced. This exothermic reaction can lead to explosions, with reactivity increasing dramatically from lithium to cesium.

Alkali metals react vigorously with halogens to form salts in highly exothermic reactions. For example, sodium reacts with chlorine gas to produce sodium chloride, releasing sparks and heat. The reaction intensity increases down the group as ionization energies decrease, making heavier alkali metals more reactive with halogens and producing more energetic reactions.

Alkali metals are stored in mineral oil because they oxidize rapidly when exposed to air, losing their metallic luster. The mineral oil prevents contact with oxygen and moisture, protecting the reactive metals from degradation. Lithium forms lithium oxide, while heavier alkali metals form oxides, peroxides, and superoxides, all of which compromise the metal's properties and reactivity.

A flame test identifies alkali metals by observing characteristic colors emitted when metal atoms are heated. Heat excites the valence electron to higher energy levels; when electrons return to the ground state, they emit visible radiation. Each alkali metal produces a unique emission spectrum, allowing qualitative identification. This distinctive color emission is a direct result of the single valence electron's energy transitions.

Alkali metals are soft, shiny metallic solids that are malleable and ductile, making them easy to shape. They are excellent conductors of heat and electricity due to their mobile valence electrons. Unusually for metals, alkali metals have low melting points that decrease moving down the group, while their density generally increases, except for potassium, which is lighter than sodium.