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The alkali metals—lithium, sodium, potassium, rubidium, cesium, and francium—are soft, highly reactive metals that each possess a single electron in their outermost s‑orbital. Although they belong to Group 1 of the periodic table, their chemical reactivity increases steadily as their atomic number rises.
Three interrelated factors govern how readily an alkali metal donates its valence electron: the positive charge of the nucleus, the shielding effect of inner electrons, and the distance of the outer electron from the nucleus (i.e., the atomic radius). Understanding each element helps explain the observed reactivity trend from lithium to francium.
The nuclear charge equals the atomic number, so the effective positive pull on the valence electron grows from lithium (Z = 3) to francium (Z = 87). If nuclear charge were the sole determinant, higher‑Z metals would be less reactive because the stronger attraction would keep the outer electron more tightly bound.
Inner electrons partially shield the valence electron from the nucleus’s full positive charge. This shielding depends on orbital geometry and the number of core electrons. While shielding reduces the effective nuclear charge felt by the outer electron, it is less influential for alkali metals than the radial distance factor.
The dominant factor is the spatial separation between the nucleus and the outer electron. Electrostatic attraction follows the inverse square law; therefore, when the radius doubles, the attraction drops to one‑quarter. Alkali metals exhibit increasing atomic radii: lithium = 167 pm, sodium = 190 pm, potassium = 243 pm, rubidium = 265 pm, cesium = 298 pm, and francium is even larger. The greater the radius, the less tightly the valence electron is held, making the metal more reactive.
Combining these factors yields the clear reactivity sequence: francium > rubidium > cesium > potassium > sodium > lithium. The trend aligns with the increasing atomic radii, confirming that distance from the nucleus governs alkali metal reactivity more than nuclear charge or shielding alone.
