s-Block Elements Chapter 10 Class 11 Chemistry NCERT Textbook PDF

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s Block Elements

Chapter 10: s-block Elements

The s-block elements of the Periodic Table are those in which the last electron enters the outermost s-orbital.

As the s-orbital can accommodate only two electrons, two groups (1 & 2) belong to the s-block of the Periodic Table.

Group 1 of the Periodic Table consists of the elements:
lithium, sodium, potassium, rubidium, cesium, and francium. They are collectively known as alkali metals.

These are so-called because they form hydroxides on reaction with water which are strongly alkaline in nature.

The elements of Group 2 include beryllium, magnesium, calcium, strontium, barium, and radium. These elements with the exception of beryllium are commonly known as alkaline earth metals.

These are so-called because their oxides and hydroxides are alkaline in nature and these metal oxides are found in the earth’s crust.

10.1 Group 1 Elements: Alkali Metals

The alkali metals show regular trends in their physical and chemical properties with the increasing atomic number.

The atomic, physical, and chemical properties of alkali metals are discussed below

10.1.1 Electronic Configuration

All the alkali metals have one valence electron,ns1 (Table 10.1) outside the noble gas core.

The loosely held s-electron in the outermost valence shell of these elements makes them the
most electropositive metals.

They readily lose electrons to give monovalent M+ions. Hence they
are never found in the free state in nature.

10.1.2 Atomic and Ionic Radii

The alkali metal atoms have the largest sizes in a particular period of the periodic table.

With the increase in atomic number, the atom becomes larger. The monovalent ions (M+) are smaller then the parent atom.

The atomic and ionic radii of alkali metals increase on moving down the group i.e., they increase in size while going from Li to Cs.

10.1.3 Ionization Enthalpy

The ionization enthalpies of the alkali metals are considerably low and decrease down the group from Li to Cs.

This is because the effect of increasing size outweighs the increasing nuclear charge, and the outermost electron is very well screened from the nuclear charge.

10.1.4 Hydration Enthalpy

The hydration enthalpies of alkali metal ions decrease with an increase in ionic sizes.
Li+ Na+ K+ Rb+ Cs+ Li+ has the maximum degree of hydration and for this reason, lithium salts are mostly hydrated, e.g., LiCl· 2H2O.

10.1.5 Physical Properties

All the alkali metals are silvery-white, soft, and light metals. Because of the large size, these
elements have a low density which increases the group from Li to Cs. However, potassium is
lighter than sodium.

The melting and boiling points of the alkali metals are low indicating weak metallic bonding due to the presence of only a single valence electron in them. The alkali metals and their salts impart characteristic color to an oxidizing flame.

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CategoryChemistry
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NCERT Solutions Class 11 Chemistry Chapter 10 s-block Elements

Q 10.1

What are the common physical and chemical features of alkali metals?

Ans 10.1

Physical properties:

(1) The alkali metal is soft and so we can cut them easily. We can able to cut the sodium metal even by using a knife.

(2) Generally, the alkali metal is lightly colored and mostly they appear as silvery white.

(3) Its atomic size is larger and so its density is low. The density of the alkali metal increases down in the group from Li to Cs except for K which has low density than sodium.

(4)  Alkali metal is weak in its metallic bonding and so they are having low boiling and melting points.

(5) The salts present in alkali metals exposes color to flames because the heat of the flame excites electron which is located on the outer orbital to a higher energy level. In this excited state of electron getting reversed back to the ground level, the emission of excess energy in the form of radiation falls in the visible region.

(6) Metals like K and Cs lose electrons when they get irradiated with light and also display photoelectric effect.

Chemical properties:

(1) Alkali metal reacts with water and forms oxides and hydroxides. So, the reaction will be more spontaneous while moving down the group.

(2) Alkali metal reacts with water and forms dihydrogen and hydroxides.

General reaction :     2M + 2H20 \rightarrow→ 2M+ + 2OH + H2   

(3) Dihydrogen reacts with alkali metals and forms metal hydrides. The hydrides from this have higher melting points and they are solids that are ionic.

2M + H2  \rightarrow→  2M+ H          

(4) Alkali metals directly react with halogens and form ionic halides except for Li.

2M + CL2 \rightarrow→  2MCI               (M = Li ,K, Rb, Cs)

It has the ability to easily distort the cloud of the electron which is around the –ve halide ion because lithium-ion is smaller in size. Hence, Lithium halide is naturally covalent.

(5) Alkali metals are very stronger reducing agents. This increases as we move down the group except for lithium. Due to its high hydration energy, it results in a strong reducing agent among all alkali metals.

(6) To result in blue colored solution(deep blue) which is naturally conducting, they get dissolved in liquid ammonia.

M +(x + y) NH3   \rightarrow→  [ M ( NH)] +  [ e ( NH3)y ]

Q10.2

Discuss the general characteristics and gradation in properties of alkaline earth metals

Ans 10.2

General characteristics:

(i)  (Noble gas) ns  is the electronic configuration of alkaline earth metal.

(ii) To occupy the nearest inert gas configuration, these metals lose two of their electrons; and so their oxidation state is +2.

(iii)  The ionic radii and atomic radii are smaller than alkali metals. When they moved down towards the group, there is an increase in ionic radii and atomic radii due to a decrease in effective nuclear charge.

(iv) The ionization enthalpy is low because the alkaline earth metals are larger in size. The first ionization enthalpy is higher than metals of group 1.

(v) They appear in lustrous and silvery-white. They are soft as alkali metals.

(vi) Factors that cause alkaline earth metals to contain high boiling point and melting point :

(*)  Atoms of alkali metals are larger than that of alkaline earth metals.

(*)  The strong metallic bonds are formed by two valence electrons.

(vii) Ca- brick red, Sr- crimson red, Ba-apple green results in colors to flames.

Electrons are bounded strongly to get excited in Be and Mg. Therefore, they do not expose any colors to the flame.

The alkali metals are more reactive than alkaline earth metals.

Chemical properties:

(i)  Reaction with air and water:  Due to the formation of an oxide layer on their surface, beryllium and Mg are most inert to water and air.

(a)  Beo and Be3 N2 are formed when powdered Be is burnt in the air.

(b) For the formation of MgO and MgN2, Mg is burnt in the air with dazzling sparkle. Since Mg is more electropositive.

(c) The formation of respective nitrides and oxides is by instant reaction of Sr, Ca, and Ba with air.

(d) Ca, Sr, and  Ba can able to react vigorously even with water that is cold.

(ii)  when they react with halogens, halides are formed at high temperatures.

M +X2   \rightarrow→ MX2 (X = F,CL,Br,I)  

(iii)  Except for Be, all the alkaline earth metals react with hydrogen to form hydrides.

(iv)  alkaline earth metals instantly react with acids to form salts with the liberation of hydrogen gas.

M +2HCl\rightarrow→ MCl2  + H2(X) 

(e) Reducing Nature: Alkaline earth metals are strong reducing agents like alkali metals, but the reducing power is less when compared to alkali metals. In general, the reducing character increases from top to bottom.

(f) Solutions in liquid ammonia: Alkaline earth metals dissolve in liquid ammonia to give deep blue-black solutions like alkali metals.

Q 10.3

Why are alkali metals not found in nature?

Ans 10.3

Sodium, cesium, lithium, francium, potassium, and rubidium all together comprise the alkali metals.

They consist of only one electron on its valence shell, which gets loose easily due to their low ionizing energies. So, alkali metals are not found naturally in their elemental state as they are highly reactive.

s-Block Elements Textbook With Solutions PDF Free Download

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