Group 1 Elements —The Alkali Metals
The elements are namely, lithium (Li), sodium (Na), potassium (K), Rubidium (Rb), cesium (Cs) and francium (Fr). Because of their high reactivity, they are never found in the freestate. These elements are commonly known as alkali metals because their oxides and hydroxides are very strong alkalis.
Among these six elements, sodium and potassium are the sixth and seventh most abundant element.
All alkali metals have one s-electron in their outermost orbit (valency shell) as shown below
Li = [He] 2s1, Na = [Ne] 3s1, K = [Ar] 4s1, Rb = [Kr] 5s1 Cs = [Xe] 6s1 , Fr = [Rn] 7s1
Physical properties OF ALKALI METALS
1. Physical state: All elements are silvery-white, soft and light metals. Group 1 elements are reactive metals because they have low ionisation energies and have a few valency electrons as compared to available vacant orbitals. These are highly malleable and ductile. When freshly out, they have a bright luster which quickly tarnishes on exposure to air.
2. Atomic radii: The atomic radii increase on moving down the group from Li to Fr.
Atomic Radii (Å) Li = 1.52 Na = 1.86 K = 2.27
Rb = 2.48 Cs = 2.65 Fr = 3.75;
The increase in atomic size on stepping down the group is due to following two factors.
(i) One extra energy shell is being added with each succeeding element, e.g. in Li and Na the total number of energy shells are 2 and 3 respectively.
(ii) With the increase in energy shell the screening effect of the inner filled shells on the valence s-electron also increases; with the result the later is comparatively less attracted by the nucleus and hence the electron cloud tends to expand leading to greater size of the atom. On the contrary, the increase in atomic number e.g. from 3 in Li to 11 in Na, also increases the nuclear charge which tends to decrease the atomic radii by attracting the electron cloud more forcibly, However, the screening effect is so large that it overcomes the contractive effect of the increased nuclear charge and hence we can say that the increase in size from Li to Fr is primarily due to the predominant screening effect (adding of a new energy shell) of inner filled shells on the valence s-electrons,
3. Ionic radii: Remember that a positive ion is always smaller than its atom. Further, higher the positive charge, smaller will be the ionic radius. This can be explained in the following ways.
(A) When the single valence s-electron is lost from the alkali metal, the resulting cation has one energy level less and hence its size will also be less than the parent atom.
(B) With the loss of one electron, the effective nuclear charge increases and thus the remaining electrons are attracted towards nucleus thereby decreasing the size of each individual energy shell.
Further, like atomic radii, the ionic radii increase as we move down the group from Li to Cs primarily due to the addition of a new energy shell with each succeeding element.
4. Density: Their densities are quite low as compared to other lighter than water, lithium is the lightest known metal (density 0.534).
Density of various elements of this family
Density g/cc Li = 0.534 Na = 0.97 K = 0.80 Rb = 1.93 Cs = 1.90
Low density of alkali metals is due to their large size, which cause the atomic nuclei to separate widely in their crystal lattices. Further, the atomic size and atomic mass increase on moving down the group. The densities of alkali metals increase from Li to Cs. However, potassium has lower density than sodium probably due to an unusual decrease in atomic size of potassium due to presence of greater force of attraction due to increased nuclear charge.
5. Melting and boiling points: The melting and boiling points are very low because of weak bonding in the crystal lattice of the metals (weak bonding in the crystal lattice also explains the softness of alkali metals). The weak interatomic bonds (binding energies) are due to their large atomic radii.
M. Pt (ºC) Li = 108.5 Na = 97.8 K = 63.2 Rb = 39.0 Cs = 28.5
6. Ionisation energy: Due to their large size, the outermost s-electron is at a large distance from the nucleus and, therefore, can be removed. Thus their ionisation energies are low. Further, as the atomic radius increases on moving down the group, the outer electron gets farther and farther away from the nucleus and, therefore, ionisation energy decreases on moving down from Li to Cs.
I. E. kJ / mol Li Na K Rb Cs
I 520 496 419 403 376
II 7298 3562 8051 2633 2230
7. Electropositive character (the tendency of the element to lose electron): On account of their low ionisation energies, these metals have a great tendency to lose the ns1 electron and form positive ions.
M → M+ + e–
Thus alkali metals are strongly electropositive (or metallic in nature). Further since ionisation energy decreases from Li to Cs, the electropositive character increases in going down from Li to Cs.
Due to their strong electropositive nature, they emit electrons even when exposed to light (photoelectric effect). Cesium is preferably used in photochemical cell as it has lower ionization energy and is non-radioactive also.
8. Electronegativity: Since these metals are highly electropositive, their electronegativity (i.e. tendency to attract electrons) values are very low. Further since electropositive character increase on moving down the group, the electronegativity decreases in the same order, i.e. from Li to Cs.
9. Oxidation state: The alkali metal atoms show only + 1 oxidation state. Because of their low ionisation energies, they easily lose the outermost s electron to form the unipositive ions. Since the unipositive ions have the stable noble gas configuration (s2 or s2 p6) in the valency shell, the energy required to pull out another electron from the valency shell is very high. Hence the second ionsation energies of alkali metals are very high. Therefore, we can say that alkali metals form univalent ions and form ionic compounds.
10. Conductivity: Due to the presence of loosely held valence electrons which are free to move throughout the metal structure, the alkali metals are good conductors of heat and electricity.
11. Flame colouration: The alkali metals and their salts, when introduced into the flame, give characteristic colour to flame.
| Li | Na | K | Rb | Cs |
| Crimson | Golden yellow | Pale violet | Violet | Violet |
This property of the alkali metals offers a very sensitive and reliable test (flame test) for alkali metals which are difficult to be identified by chemical methods as they form very few insoluble compounds.
The reason for flame colouration is that when an alkali metal or its any salt is introduced in flame the outermost electrons of the alkali atoms absorbs energy and excited to the higher energy levels. When the excited electrons return to their original (ground) level, they release the absorbed (excited) energy as visible light. The alkali metal salts give colours in the reduction zone of the flame. The metal ion temporarily gets reduced and thus there is excitations of this electron. Now for the same excitation energy, the energy level to which the electron in Li will rise is lower than that to which the electron in Na will rise and this, in turn, is lower than the level to which the electron in K will rise and so on.This property of the alkali metals offers a very sensitive and reliable test (flame test) for alkali metals which are difficult to be identified by chemical methods as they form very few insoluble compounds.
These differences are due to differences in their ionisation energies. consequently, when the electron returns to the ground state, energy released will be lowest in Li+ and will increase in the order : Li+ < Na+ < K+ < Rb+ and Cs+. As a result of this, the frequency of the light emitted in the Bunsen flame is minimum in lithium and corresponds to the red region of spectra. In potassium, the frequency of the light emitted corresponds to violet region of spectra.
12. Hydration of ions: All alkali metal salts are ionic (except LiI) and soluble in water, a solvent of high dielectric constant. The solubility in water is due to the fact that the cations get hydrated by water molecules. The degree of hydration depends upon the size of the cation. Smaller the size of a cation, greater is its charge density and hence greater is its tendency to draw electrons from molecules which are thus polarised. Lithium ion, being smaller in size among alkali metal ions, is the most extensively hydrated while Cs+ion, the largest alkali metal ion, is the least hydrated.
CS+ > Rb+ > K+ > Na+ > Li+ (Relative ionic radii)
Li+ > Na+ > K+ > Rb+ > Cs+ (Relative ionic radii in water)
(Relative degree of hydration)
Lithium ion, being heavily hydrated, moves very slowly, under the effect of electric current and is thus the poorest conductor of electricity as compared to other alkali metal ions. Thus electrical conductivity measurements indicated for alkali metal ions :
Cs+ > Rb+ > K+ > Na+ > Li+ (Relative electrical conductivity)
13. Hydration energy: Hydration of ions is an exothermic process. The energy released in the hydration of ions is known as hydration energy. Since the degree of hydration of M+ ions decrease as we go down the group, the hydration energy of alkali metal ions decreases from Li+ to Cs+. But in aqueous solution lithium metal is the best reducing agent.
