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 alkali metals are unusually low for metals and decrease going down the group, while the density increases going down the group with the exception of potassium (Table 1).

Table 1: Properties of the alkali metals

Element	Electron Configuration	Atomic Radius (pm)	IE₁(kJ/mol)	Melting Point (°C)	Density at 25 °C (g/cm³)
Li	[He] 2s¹	152	520	181	0.53
Na	[Ne] 3s¹	186	496	98	0.97
K	[Ar] 4s¹	227	420	64	0.86
Rb	[Kr] 5s¹	248	400	39	1.53
Cs	[Xe] 6s¹	265	380	29	1.93

Moving down a group, the principal quantum number, n, increases by one for each element. Thus, the outer electrons get farther from the nucleus, and the atomic radius increases from lithium to cesium.

The electron configuration of these elements shows that the valence shell has only one electron (Table 1). The loss of this electron produces a cation with the noble gas configuration. Thus, alkali metals have a great tendency to give away this electron and have low first ionization energies, which decrease down the group. Their ability to lose an electron readily makes them highly reactive and excellent reducing agents.

The alkali metals react vigorously with nonmetals, such as halogens. The reaction of sodium and chlorine to produce sodium chloride is exothermic. The alkali metals also react violently with water, also, and hydrogen gas and an alkali metal hydroxide are produced. The heat produced during the reaction can ignite the hydrogen gas released, leading to an explosion. Both of these reactions become more exothermic for the heavier alkali metals due to their lower ionization energies.

Exposure to air, as they react with oxygen to form oxides, reduces the metallic luster of the alkali metals. Lithium reacts with oxygen to give lithium oxide, while other alkali metals like sodium form oxides and peroxides. The peroxide ion has a single oxygen-oxygen covalent bond and is a powerful hydrogen ion acceptor, making the peroxides of the alkali metals strong bases. The alkali metals such as potassium, rubidium, and cesium also produce superoxides. Superoxides are characterized by the presence of O₂⁻. Potassium superoxide is a yellow solid that decomposes at 560 °C.

This text is adapted from Openstax, Chemistry 2e, Section 6.5: Periodic Variations in Element Properties.