What is the general configuration of s-block elements?

Answer 1

The s-block elements consist of the alkali metals and alkaline earth metals, including hydrogen and helium. Most reactions of s-block elements involve the loss of an electron to form a singly positively charged ion.

Answer 2

GENERAL TRENDS IN ATOMIC AND PHYSICAL

PROPERTIES OF S - BLOCK ELEMENTS

Generally all the elements of s- block show a striking resemblance in their physical and chemical properties. This is because of their identical configuration (ns1) in their valence shell.

The trends that are observed in the variation of different physical

properties are explained below:

1. ATOMIC AND IONIC RADII:

The atoms of alkali metals have the largest size in their respective periods. The atomic radii increase on moving down the group among the alkali metals.

Reason: On moving down the group, there is a progressive addition

of new energy shells. Although the nuclear charge also increases down the group but the effect of addition of new shells is a more predominant and hence atomic radius of alkali metals increase on going the group from top to bottom.

Alkali metals change into positively charged ions by losing their lone valence electron. The monovalent ions (M+) are smaller than the size of the parent atom. Within the group, the ionic radii increase with increase in atomic number.

PHYSICAL PROPERTIES OF GROUP1 ELEMENTS (ALKALI METALS)

(a) Metallic radius (coordination number 8); (b) Values are for

coordination number 6 (c) Pauling scale; (d) 293K; (e) 298K

2. HYDRATION ENTHALPIES:

The alkali metals are extremely hydrated in aqueous solutions. Smaller the size of the ion greater will be the extent of hydration. The order of hydration enthalpies for alkali metals is as

Lithium has high degree of hydration that is why lithium salts exist

as hydrated saltse.g.LiCl.2H2O

3. IONIZATION ENTHALPY:

The first ionization enthalpies of alkali metals are quite low as compared to the elements of the other group elements belonging to the same period. But within the group, the ionization enthalpies of alkali metals decrease down the group.

Reason: The size of atoms of the alkali metals is the largest in their

respective periods, therefore, the outer most electrons which are far away from the nucleus experience a less force of attraction from the nucleus and hence can be removed easily.

The decrease in the ionization enthalpy on moving down the group is due to increase in the size of the atoms of alkali metals and increase in the magnitude of screening effect which is due to increase in the number of intervening electrons.

Due to low ionization enthalpy of the alkali metals, these have strong tendency to lose their valence electrons and are able to have strong electropositive or metallic character. As this tendency increases down the group, hence electropositive character increases.Caesium is the most electropositive element.

4. Oxidation State:

The alkali metals have strong tendency to lose electrons and change into unipositive ions. Thus, alkali metals show+1oxidation state in their compounds.

Reason:Since all the alkali metals have low ionization enthalpies

and by losing solitary valence electron, these metals acquire noble gas configuration. Therefore these elements have strong tendency to form M+ ions in their compounds. The magnitude of second ionization enthalpies is very high and is not available under the conditions of forming chemical bonds. Hence alkali metals don't form M2+ions.

5. Metallic character:

Elements of this group are typical metals. These are soft in nature and can be cut with a knife. The metallic character increases down the group.

Reason: The metallic character of an element depends on the

tendency to loose the valence electron. Due to low ionization enthalpy, these elements have a strong tendency to loose electron and hence are typical metals. The decrease in ionization enthalpy is due to the increase in size of the atoms down the group, which increases the metallic character.

Reason:Since all the alkali metals have low ionization enthalpies

and by losing solitary valence electron, these metals acquire noble gas configuration. Therefore these elements have strong tendency to form M+ ions in their compounds. The magnitude of second ionization enthalpies is very high and is not available under the conditions of forming chemical bonds. Hence alkali metals don't form M2+ions.

5. Metallic character:

Elements of this group are typical metals. These are soft in nature and can be cut with a knife. The metallic character increases down the group.

Reason: The metallic character of an element depends on the

tendency to loose the valence electron. Due to low ionization enthalpy, these elements have a strong tendency to loose electron and hence are typical metals. The decrease in ionization enthalpy is due to the increase in size of the atoms down the group, which increases the metallic character.

Since Li+ ion is the smallest in size, therefore, the large amount of energy released in step III (enthalpy of hydration) compensates for the higher ionization enthalpy needed in step II and hence explains the low value of reduction potential of lithium and its strongest reducing character.

7. Flame coloration:

When alkali metals or their salts are heated in the flame of a Bunsen

burner, they impart characteristic colors to the flame as given below;

Reason: When salts of these elements absorb energy especially

chlorides because of their volatile nature, the electrons in their valence shells get excited to higher energy states. When these electrons jump back to their ground states they emit energy in the form of radiations, which fall in the visible region, imparting a characteristic coloration to the flame.

9. Density:

The alkali metals have low density .it increases down the group Lithium is the lightest metal having a density of 0.534g/cm3.It can't be stored in kerosene oil because it floats on the surface. It is kept wrapped in paraffin wax. Potassium is lighter than sodium and stored in kerosene.

Reason:The low density of alkali metals is due the large size and

weak metallic bond. But when we go down in a group the atomic size

and Atomic Mass increases

But the corresponding increase in atomic mass is not neutralized by the increase in atomic volume. Thus ratio, mass/ volume, i.e. density gradually increases.

10. Melting point and Boiling point:

On moving down in a group, melting point decreases.

Alkali metals have generally low melting points.

Reason:The decrease in melting point is due to the increase in the

atomic size and decrease in the strength of metallic points. The low

melting point of alkali metals is attributed to their large atomic size,

due to which the binding energies of their atoms in the crystal lattice

are low.

11. Soft Metals:

All the alkali metals are generally soft and can be cut with the help of

Knife.

Reason:The softness of the alkali metals is due to weak metallic

bonding in them because of large size of the atoms. Further if we move down the group metallic bonding weakens and hence softness increases. Potassium is softer than sodium.

Chemical properties and general characteristics of alkali metal

compounds

Alkali metals are highly reactive elements. The cause for their high

reactivity is

a) Low value of first ionization enthalpy

b) Large size

c) Low heat of atomization

As we know ionization enthalpy decreases down the group from lithium to caesium. Therefore the reactivity of alkali metals increases. The compounds formed by alkali metals are colourless and diamagnetic. But they can also form coloured compounds on combining with colored anions like MnO4- , MnO42- or Cr2O72-. All alkali metals have body centered cubic lattice with coordination number of 8.

Answer 3

ns1 or ns2