alkali metals

Meaning: The alkali metals are Group 1 elements other than hydrogen: Li, Na, K, Rb, Cs and Fr. Each has one ns¹ valence electron that is easily lost, so each forms a +1 cation with a noble-gas configuration. Going down the group, atomic radius increases and the valence electron is farther from the nucleus and more shielded, so first ionization energy and electronegativity decrease and reactivity increases. The second ionization energy is very large because the next electron comes from a noble-gas core.

Equation: 2M + 2H₂O → 2MOH + H₂, where M is an alkali metal, MOH is its strongly basic hydroxide, and H₂ is hydrogen gas; amounts are in mol.

On the MCAT: Questions ask you to rank Li through Cs by radius, ionization energy or reactivity, to predict that these elements form M⁺ ions rather than M²⁺ in ordinary compounds, or to compare them with the alkaline earth metals (Group 2), which have higher ionization energies and form 2+ ions.

Common confusion: Hydrogen sits in Group 1 but is a nonmetal, not an alkali metal. Reactivity rises down Group 1 but falls down Group 17; metals react by losing electrons, which gets easier as ionization energy drops.