Properties and Chemical Reactivity — Core Principles

Core Principles

Alkali metals (Group 1: Li, Na, K, Rb, Cs, Fr) are highly reactive metals characterized by a single valence electron ($ns^1$). This electron is easily lost, resulting in very low first ionization enthalpies and the formation of stable unipositive cations ($M^+$).

They are strongly electropositive and powerful reducing agents. Their reactivity increases down the group due to decreasing ionization enthalpy and increasing atomic size. They are soft, silvery-white metals with low melting points and densities.

A key property is their ability to impart characteristic colors to a flame (Li-crimson, Na-golden yellow, K-lilac). They react vigorously with air (forming oxides, peroxides, or superoxides), water (forming hydroxides and hydrogen), halogens (forming halides), and hydrogen (forming hydrides).

They dissolve in liquid ammonia to produce deep blue solutions of ammoniated electrons. Lithium exhibits anomalous behavior due to its small size and high charge density, and shows a diagonal relationship with Magnesium.

Understanding these trends and exceptions is crucial for NEET.

Important Differences

vs Alkaline Earth Metals (Group 2)

Aspect	This Topic	Alkaline Earth Metals (Group 2)
Valence Electrons	1 ($ns^1$)	2 ($ns^2$)
Ionization Enthalpy	Very low (lowest in period)	Low, but higher than alkali metals (higher $IE_1$, much higher $IE_2$)
Oxidation State	+1	+2
Reactivity	Extremely high (increase down group)	High, but less reactive than alkali metals (increase down group)
Reducing Power	Very strong	Strong, but weaker than alkali metals
Hardness	Very soft	Harder than alkali metals
Density	Low	Higher than alkali metals
Flame Coloration	Distinctive colors (Li, Na, K, Rb, Cs)	Distinctive colors (Ca, Sr, Ba)
Reaction with Oxygen	Form oxides, peroxides, superoxides	Form normal oxides (except Ba forming peroxide)
Nature of Hydroxides	Strongly basic, highly soluble	Less basic, less soluble than alkali metal hydroxides

Alkali metals (Group 1) and alkaline earth metals (Group 2) are both s-block elements, but significant differences arise from their valence electron count. Alkali metals have one valence electron, leading to lower ionization enthalpies, higher reactivity, and a +1 oxidation state. They are softer, less dense, and stronger reducing agents than alkaline earth metals, which have two valence electrons, resulting in a +2 oxidation state. While both groups exhibit characteristic flame colors, their reactivity with oxygen and the basicity/solubility of their hydroxides differ, with alkali metals generally being more reactive and forming stronger bases.