Question

(5) Discuss the reason for variation of the following properties along the period and along a group : (i) atomic radius (ii) electronegativity

(6) Mention some of the important differences of Li from remaining alkali metals. How can you justify these differences ?

Answer 1

5) First, let's understand the factors affecting atomic size:

1) The number of shells the atomic size increases with an increase in the number of electronic shells. atomic radius is proportional to number of the electronics in the shell.

2) Nuclear charge: Atomic radius decrease this with an increase in nuclear charge. If the nuclear charge is more, then the nucleus attract the electrons towards it and atomic size decreases. In other words, the atomic radius is inversely proportional to the nucleya charge.

3) Screening effect or shielding effect: In an atom having many electrons, the electrons in the inner shell tend to prevent the attractive influence of the nucleus from reaching the outermost electrons. Thus, the act as a screen or shield between the nuclear attraction and outermost or valence electron. this effect of inner electrons on the outermost electrons is known as screening effect with an increase in screening effect, radius increases.

Now let's see the variation of the atomic radius with the period: Across the period the nuclear charge increases as the atomic number increases. The number of shells in the atom of all the elements in a period remains the same. Therefore, the nuclear attraction of the outermost electron increases due to which the atomic radius decreases.

In a period: The atomic size is largest for alkali metals, decreases gradually and it becomes smallest for the halogen elements. At the end of every period, inert gases have comparatively larger Van der Waals radius due to the completed outer most shell within all the electrons pairs which causes greater repulsion. Variation of the atomic radius with the atomic number across the second period from Lithium to fluorine atomic radius decreases. nuclear attraction on the outermost electrons increases and the atomic radius decreases. In s and p-block elements, along a period with the increase in atomic number, the electron density of inner shell remains unchanged because the electrons are added to outermost shell. Therefore, the screening effect on inner electrons remains the same and the nuclear charge increases. help the atomic radius decreases significantly as the atomic number increases along the period. therefore , the s and p block element of a period show difference in their properties as the properties also depend on the atomic radius. In case of d-block elements as the atomic number increases in a period the electrons are added to the penultimate shell. It leads to a steady increase in the electron cloud density of the inner shell and screening effect of inner electrons increases. The nuclear charge progressively increases with increase in atomic number the increase in the effective nuclear charge on the outer electron decreases. As a result , atomic radius decreases to a lesser extent then what has been observed among s and p block elements. In the case of f-block elements with the increase in atomic number, the addition of electrons takes place in pre penultimate shell. this increases the electron density in pre penultimate shell resulting into increase in screening effect by the large number of electrons. Thus the increase in effective nuclear charge on the outer electron decreases more than that has been observed in d-block elements.

In the groups: The nuclear charge increases as the atomic number of elements increases in a group. Simultaneously the number of shells in their atoms also increases. Hence in a group, the net result in an increase in atomic radius with the increase in atomic number.

Electronegativity:

Variation in a period: In a period electronegativity increases from left to right for example Lithium to fluorine. In a period as atomic number increases from left to right atomic radius decreases and the nuclear charge increases from left to right. Hence, a tendency to attract a shared pair of electrons increases from left to right in a period.

In a group: In a group electronegativity decreases down the group. in a group with an increase in atomic number atomic radius increases and due to which tendency to attract a shared pair of electrons decreases and hence electronegativity decreases down the group.

6) Anomalous properties of lithium among the alkali metals: Lithium has the smallest size cation and highest polarization power that is charged to radius ratio. it is due to this reason that Lithium is different from the rest of its family member. covalent character of lithium compound is higher which is responsible for their solubility in organic solvents.   Further, Lithium shows a diagonal relationship with magnesium.

1. Lithium is hard metal its melting point and boiling point higher than other alkali metals.

2. Lithium is least reactive among all alkali metals and it is the strongest reducing agent.

3. Lithium hydride is stablest of all alkali metal hydrides

4. Lithium Hydroxide is a weak base while Hydroxide of other alkali metals behave as a strong base

5. Lithium nitride when heated gives monoxide other alkali gives corresponding nitride

6. lithium carbonate is wearing is soluble in water while corresponding other salt other alkali metals are soluble in water

7. Lithium hydrogen carbonate cannot be obtained in a solid form where as other alkali metal hydrogen carbonates can be form in solid state

8. Lithium combines with Ammonia to give Lithium imide (Li2NH), whereas other alkali metals form an amide (NaNH2).

9. lithium carbonate decompose to carbon dioxide and Lithium oxide where other alkali metal carbonates do not.