Chemistry · Ch 10 — The s-Block Elements

Physical Properties

10.1.5

Physical Properties

All the alkali metals are soft enough to cut with a knife, light in mass, and silvery-white when freshly cut. Their low density is a direct consequence of their large atomic size — the atoms pack loosely — and density rises down the group from Li to Cs, though not perfectly monotonically: potassium is actually lighter (less dense) than sodium, despite being the heavier element.

Because each atom contributes only a single valence electron to metallic bonding, the "electron sea" holding these metals together is thin, so their melting and boiling points are unusually low for metals and fall steadily on descending the group.

Table 10.1 — Atomic and Physical Properties of the Alkali Metals

Property	Li	Na	K	Rb	Cs	Fr
Atomic number	3	11	19	37	55	87
Atomic mass / g mol⁻¹	6.94	22.99	39.10	85.47	132.91	(223)
Ionization enthalpy / kJ mol⁻¹	520	496	419	403	376	~375
Hydration enthalpy / kJ mol⁻¹	–506	–406	–330	–310	–276	–
Metallic radius / pm	152	186	227	248	265	–
Ionic radius M⁺ / pm	76	102	138	152	167	(180)
m.p. / K	454	371	336	312	302	–
b.p. / K	1615	1156	1032	961	944	–
Density / g cm⁻³	0.53	0.97	0.86	1.53	1.90	–
E° (M⁺/M) / V	–3.04	–2.714	–2.925	–2.930	–2.927	–

A striking and useful property of these metals (and their salts) is that they colour a flame characteristically — heat from the flame kicks the lone outer electron up to a higher energy level, and when it drops back down the excess energy is released as visible light of a specific wavelength:

Metal	Li	Na	K	Rb	Cs
Flame colour	Crimson red	Yellow	Violet	Red-violet	Blue
λ / nm	670.8	589.2	766.5	780.0	455.5

Table 10.1Atomic and Physical Properties of the Alkali Metals

Atomic number	3	11	19	37	55	87
Atomic mass (g mol^-1)	6.94	22.99	39.10	85.47	132.91	(223)
Electronic configuration	[He] 2s^1	[Ne] 3s^1	[Ar] 4s^1	[Kr] 5s^1	[Xe] 6s^1	[Rn] 7s^1
Ionization enthalpy / kJ mol^-1	520	496	419	403	376	~375
Hydration enthalpy/kJ mol^-1	–506	–406	–330	–310	–276	–
Metallic radius / pm	152	186	227	248	265	–
Ionic radius M^+ / pm	76	102	138	152	167	(180)
m.p. / K	454	371	336	312	302	–
b.p / K	1615	1156	1032	961	944	–
Density / g cm^-3	0.53	0.97	0.86	1.53	1.90	–

Table flameFlame-test colours and characteristic wavelengths of the alkali metals

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