What is the physical properties of alkali metals?

What is the physical properties of alkali metals?

Aug 26, 2022, 16:45 IST

Atomic ​and physical properties:

1. Physical state (Appearance): All alkali metals are silvery white metals with metallic lustre.

2.Softness: These are soft, malleable and ductile metals which can be cut with knife. They posses metallic lustre when freshly cut due to oscillation of electrons.

3. Atomic and ionic r adii: The alkali metals have the largest atomic as well as ionic radii in their respective period because each period in the periodic table starts with an alkali metals (except 1 ^st period). Atomic radii as well as ionic radii increases as we move down the group from Li to Cs due to addition of new shell at each step and increases in shielding effect. But increasing nuclear charge tend to decrease the atomic as well as ionic radii.

Density: These are quite light metals having low densities. Lithium is the lightest known metal. On moving down the group, density increases from Li to Cs.

4. Melting and Boiling points: The melting and boiling points of alkali metals are quite low and decrease down the group.

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Reason: Alaklimetals have only one electron in their valence shell. So, in these metals, the inter atomic forces are weak. This is why, the melting and boiling points of alkali metals are low. In addition to this weaking of metallic bond is also responsible for low mps and bps.

On moving down the group melting and boiling points of alkali metals are decreased.

Reason: As the size of atom increases, the repulsion of the non-bonding electrons also increases. This increased repulsion between the non-bonding electrons causes lowering of the mps & bps down the group. Francium is liquid at room temperature.

5. Ionization energy: Alkalimetals have relatively lowest first ionization energies amongst the elements in their respective periods.

Reason: The s – electron in valence shell of alkali metals is far away from the nucleus. So, the outer most s-electron is weakly held by the nucleus. Therefore, to remove it very little amount of energy is required. This is why ionization energies of alkali metals are low.

Table–2 First ionization energies of alkali metals

On going down the group, ionization energy decreases.

Reason: The distance of valence electron from the nucleus increases as we go down the group. As a result, the ionization energy decreases from Li to Cs.

6. Electropositive character (metallic character): Alkalimetals are strongly electropositive or metallic in nature and this character increases from Li to Cs.

Reason: It is because of low ionization energies of alkali metals.

Cesium (Cs) is the most highly electropositive or metallic element due to its low I.E.

7. Electrode potentials: Alkali metals are highly electropositive and tend to undergo oxidation, whenever placed in water liberating hydrogen gas.

Table – 3 The standard electrode potentials for M ⁺ / M half cell

Negative values of the electrode potential (E ⁰ ) for alkali metals indicate that all these metals have strong tendency to undergo oxidation.

Thus, Li ⁺ / Li electrode has most negative electron potential. Therefore, Li has the highest tendency to lose electrone. As a result, lithium (Li) is the strongest reducing agent.

8. Reducing character: All the alkalimetals are good reducing agent due to their low ionization, energies. Their reducing character, follows the order Na < Cs < Rb < K < Li (in aqueous solution)

Among the alkali metals ‘Li’ has the highest negative electrode potential (E ⁰ = - 3.05V), which depends upon its (Li) heat of vaporization, ionization energy and its heat of hydration. Hence, Lithium is the strongest reducing agent.

9. Electronegativity: Alkali metals have low electronegativity values and decrease down the group.

Reason: Because of large size and low nuclear charge, the alkalimetals are not able to attract electrons towards them. As a result, these elements show very low electronegativity, since, the atomic size increases from top to bottom in a group, hence, the electronegativity decreases from Li to Cs.

10. Oxidation states: All alkali metals exhibit only + 1 oxidation state in their compounds.

Reason: Because alkali metals have only one electron in their valence shell and therefore, can lose this valence electron (ns ¹ ) to form unipositive cations. These cations have stable gas configuration in the valence shell i.e., ns ² np ⁶ .

So, all alkali metals in their compounds exhibit + 1 oxidation state.

Since, alkali metal ions (M ⁺ ) have noble gas configuration (ns ² np ⁶ ) with no unpaired electrons, they are diamagnetic in nature (while alkali metals are paramagnetic) and colourless.

However, all the compounds of alkali metals are colourless. But their permanganates and dichromate’s are coloured because their anions, MnO ₄ ^- & Cr ₂ O ₇ ^- are coloured.

11. Hydration of ions (cations): All the alkali metal salts are ionic (except lithium) and soluble in water, due to the fact that the alkali metal cations (ions) exhibit strong tendencies toward hydration. i.e.,

M ⁺ (g) + H ₂ O (excess) ® M ⁺ (aq)

Thus, the reaction is exothermic. Since, the hydration tendency depends upon charge to radius ratio (i.e., q/r), of cation, hence it decreases down the group from Li ⁺ to Cs ⁺ . Thus, smaller the size of cation, greater is its hydration energy.

Relative ionic radii is Li ⁺ < Na ⁺ < K ⁺ < Rb ⁺ < Cs ⁺

Thus, relative degree of hydration is

13. Flame colouration: All alkalimetals and their salts impart characteristic colour to a non-luminous flames.

Table 4: Flame colouration of Alkalimetals

Reason: When an alkali metal or its salt is placed in a flame its electrons get excited to the higher energy levels due to the absorption of energy. The excited state is an unstable state. So, the electrons tend to come back to the ground state, the light of particular colour is emitted out. It depends upon the difference of energy between ground and excited states of the electrons in the atom or ion. On moving down the group, the ionization energy goes on decreasing and hence, the frequency (energy) of emitted light increases from Li to Cs.

13. Photoelectric effect: Due to low ionization energy and work function, alkali metals specially K and Cs show photoelectric effect and hence are used in photoelectric cells.

14. Electrical conductivity: All the alkali metals are good conductor of heat and electricity due to the preserve of loosely held valence electrons which are free to move through out the crystal structure. On going down the group, electrical conductivity increases from Li ⁺ to Cs ⁺ , i.e.,

Li ⁺ < Na ⁺ < k ⁺ < Rb < Cs ⁺

What Are the physical properties of alkali metals pdf