- The elements, lithium, sodium, potassium, rubidium, cesium and francium belong to Group-1 of the periodic table.
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| ALKALI METALS :THE ELEMENTS OF GROUP -1 |
- They are highly reactive and readily decompose water even at room temperature to form metal hydroxides which are strong alkalies.Therefore, These elements are known as alkali metals.
- The last element, francium, is radioactive.
Electronic configuration :
- Li (3) : 2 ,1
- Na (11) : 2, 8 ,1
- K (19) : 2, 8, 8, 1
- Rb (37) : 2, 8,18, 8, 1
- Cs (55) : 2, 8, 18, 18, 8, 1
- Fr (87) : 2, 8, 18, 32, 18, 8, 1
Physical properties :
Density:
- They have low densities.
- Density gradually increases in moving down from lithium towards cesium.But Potassium is lighter than sodium.
- Lithium, sodium and potassium are even lighter than water. The reason is that these metals have much larger molar volumes and hence much larger atomic radii than most other metals.
Molar Volumes.
- Molar volume increase in moving down the group from Li to Cs.
Melting Points :
- All the alkali metals are soft and have low melting points. This is due to the fact that they have only metal atom. Hence the energy binding the atoms in the crystal lattice of the metal is relatively low.
- The melting points invariably decrease in moving down from lithium towards cesium. Lithium, the first element, melts at 1800°C while cesium, the last element, melts at 285°C.
- Thus, the melting point decreases with increase in the size of the atom of the element.
Boiling Points:
- The boiling points also decrease in the same order as the melting points. The boiling point of lithium is 1326°C while that of cesium is 690°C.
Atomic and Ionic Radii:
- Atomic radius increases on moving down the group from lithium to cesium.
- This is on account of the presence of an extra shell of electrons as we move down from one element to the other within the group.
- An alkali metal changes into a positive ion by the loss of the valance electron. This leads to the elimination of the shell itself. At the same time, the number of protons of nucleus becomes greater than the number of electrons and, therefore, the electrons are attracted strongly towards the nucleus. As a result of both these effects, a positive ion is smaller than the corresponding atom.
- These ions are diamagnetic and colourless because of they have no unpaired electrons.
Heat of atomization:
- Heat of atomization measures the strength of metal-metal bond in the lattice of an element.
- Heat of atomization is maximum in the case of lithium and is much lower in the case of the next element which is sodium.
- The fall in the case of the subsequent elements is relatively small.
- Thus, metal-metal bond strength is maximum in the lithium.
Ionization Energies:
- The electron in the outer shell of these elements can be taken out relatively easily. So the ionization energies of these elements are relatively 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, ionization energy decreases on moving down from lithium towards cesium.
Electropositive Character:
- On account of their low ionization energies, these metals have great tendency to lose their valence electrons and thus change into positive ions.
M → M+ + e-1
- These elements are, therefore, said to have strong electropositive character. Since ionization energy decreases on moving down the group, the electropositive character increases on moving down the group from lithium towards cesium.
- These elements are so highly electropositive that they emit electrons even when exposed to light (photoelectric effect). This property is responsible for their use in photoelectric cells. Cesium and potassium are used, in particular, for this purpose.
Formation of Univalent Positive Ions:
- on account of relatively low ionization energies, these elements have a strong tendency to change into M+ ions, Hence, they form mostly ionic compounds.
- The alkali metals are thus univalent and form ionic compounds. However, in certain cases, they do form covalently bonded diatomic molecules like Li2 (Li --Li), Na2 (Na--Na) and Cs2 (Cs --Cs).
Hydration of Ions:
- The alkali metal ions are extensively hydrated. The smaller the size of the ion, the greater is the degree of hydration.
- Thus, Lithium, which is smallest in size and has the highest charge/size ratio among the alkali metal ions, gets much more hydrated (i.e., holds more water molecules in its hydration sphere).
- The degree of hydration decreases moving down the group form Li to Cs.
Electronegativities:
- Since electropositive character increases on the group, the electronegativity decreases in the same order.
- When these elements react with other elements having high electronegativities (e.g, the halogens), the compounds formed are ionic in character.
Characteristic Flame Coloration.
- All the alkali metals give characteristic colours in bunsen flame.
- The reason is that when an alkali metal or any of its compounds is heated in a Bunsen flame, the electrons get excited to higher energy levels.
- When these electrons return to their original (ground) level, the excitation energy which had been absorbed by them, is released in the form of light in the visible region of the spectrum.
- when the electron returns to the ground state, energy released is lowest in Li and increases in the order : Li - Cs.
- As a result of this, the frequency of the light emitted in the Bunsen flame will be minimum in the case of lithium. It increases in the order Li - Cs.
- Thus, the colour of the flame is crimson red in the case of Li, golden yellow in the case of Na, violet in the case of K and almost the same in the case of Rb and Cs.
Lattice Energies:
- Salts of alkali metals consist of cations and anions only and are, therefore, called ionic solids.
- The lattice energy of an ionic solid is defined as the energy released to form one mole of ionic crystal from atoms.
