S - BLOCK ELEMENTS Introduction -
The elements if IA group i.e. alkali metals and IIA group i.e. alkaline earth metals, are known as s-block elements. The s-block elements can have outer electronic configuration ns1 or ns2, where, n = number of outer electrons. Electronic configuration -
♦
Group IA :Outer electronic configuration of Group IA is ns1. Due to the identical outer electronic
configuration, they show similar properties. At.No. Symbol Name 3 Li Lithium
Electronic configuration 1s2 2s1 1s2 2s2 2p6 3s1
11
Na
Sodium
19
K
Potassium 1s2 2s2 2p6 3s2 3p6 4s1
37
Rb
Rubidium
1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6 5s1
55
Cs
Caesium
1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6 5s2 4d10 5p6 6s1
87
Fr
Francium
1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6 5s2 4d10 5p6 6s2 4f14 5d10 6p6 7s1
♦
Group IIA :Outer electronic configuration of Group IIA is ns2. Due to the identical outer
electronic configuration, they show similar properties. At.No. Symbol Name 4 Be Beryllium
Electronic configuration 1s2 2s2
12
Mg
Magnesium 1s2 2s2 2p6 3s2
20
Ca
Calcium
1s2 2s2 2p6 3s2 3p6 4s2
38
Sr
Strontium
1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6 5s2
56
Ba
Barium
1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6 5s2 4d10 5p6 6s2
88
Ra
Radium
1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6 5s2 4d10 5p6 6s2 4f14 5d10 6p6 7s2
Comparative study of groups -
In s-block elements, only francium and radium are radioactive. They disintegrate into other elements and therefore do not occur in nature in large amount.
Family Relationship 1
Size -
i)
Alkali metals - Alkali metals are largest in the corresponding period. The ions are smaller than the atoms. Atomic and ionic radii increase from Li to Fr.
Alkaline earth metals - Their atoms and ions are
larger, but smaller than corresponding alkali metals. Atomic radii and ionic radii, increases from Be to Ra. ii)
Ionization potential (Ionization energy) Alkali metals - In alkali metals, ionization potential is lowest. As size increases, ionization potential decreases from top to bottom, from Li to Fr. Alkaline earth metals - In alkaline earth metals, ionization potential is lower and it decreases from top to bottom, from Be to Ra. Their ionization potential is higher than the ionization potential of corresponding alkali metals.
iii)
Electronegativity Alkali metals - Alkali metals show very low electronegativity i.e. they have lowest tendency to attract electrons. Alkaline earth metals - They also have low electronegativity but not lower than the alkali metals.
iv)
Electropositivity Alkali metals - Alkali metals are highly electropositive and their electropositive character increases from Li to Cs. Alkaline earth metals - They are also electropositive, but their electropositivity is lower than the corresponding alkali metals. Their electropositive character increases from Be to Ra.
v)
Reactivity - Both alkali metals and alkaline earth metals are very reactive, due to low ionization potential and high electropositive character.
vi)
Melting point and boiling point Alkali metals - Alkali metals have low melting and boiling points. Due to less number of valence electrons, the binding is weak and so they have low melting and boiling points. Their M.P. and B.P. decreases from Li to Fr. Alkaline earth metals - They have higher melting and boiling points than those of corresponding alkali metals.
vii)
Flame colour - The s- block elements and their salts, give colour to the flame, if introduced in the flame. In Be and Mg, electrons are bound strongly due to smaller atomic size and therefore do not give colour to the flame. 2
Alkali Metals Li - Crimson red
Alkaline earth metals Be - do not give colour
Na - Golden yellow
Mg - do not give colour
K - Lilac
Ca - Brick red
Rb - Violet
Sr - Crimson red
Cs - Violet
Ba - Apple green
Chemical Properties -
Alkali Metals - They have low ionization potential and therefore high chemical reactivity. i.
Action of air - On exposure to air, alkali metals react readily with oxygen to form their oxides. So they are stored in kerosene. 4Li
+
O2
2Li2O Sodium gives peroxide Na 2O2
while Rb and Cs give superoxides. ii.
Action of water – The alkali metals react readily with water forming their hydroxides MOH with evolution of hydrogen gas. 4Li
+
H2 O
2LiOH
+ H2
The reactivity with water increases from Li to Cs. iii.
Action with hydrogen - The alkali metals react with hydrogen to form their hydrides MH. 2Na
iv.
+
H2
2NaH
Action with halogen – Alkali metals react with halogen to form halides MX. These halides are ionic compounds. Sodium reacts with halogens to form halides like NaCl, NaBr, NaI, etc. While potassium forms KCl, KBr, KI. 2Na
+
Cl2 3
2NaCl
Action with mercury – Alkali metals combine readily with mercury
v.
to form amalgam. e.g. Na-Hg (sodium amalgam) Complexes of alkali metals – Alkali metals are smaller in size,
vi.
they have lower charges on their cations, so they do not form stable complexes with simple ligands. However, they can form complexes with bidentate ligands. Anomalous properties of Lithium – Lithium has highest
vii.
ionization potential as compared to other alkali metals. Its atomic and ionic sizes (Li & Li+) are also is smallest, so Li+ have greatest polarizing power, as compared to other elements of IA group. Due to this Lithium shows slightly different properties than other IA gr. elements. Alkaline earth Metals – Alkaline earth metals are less reactive than alkali metals. Action of air - On exposure to air, alkaline earth
i.
metals slowly oxidize to form their oxides. 2Ca
+
O2
CaO
The reactivity increases from Be to Ra. Action of water – The alkaline earth metals react
ii.
slowly with water to form their hydroxides M(OH)2 with evolution of hydrogen gas. Ca
+
2H2O
Ca(OH)2 + H2 Action with hydrogen – Except Beryllium, all
iii.
other alkaline earth metals combine directly with hydrogen to form their hydrides MH 2. BeH2 is covalent, while other hydrides are electrovalent. Mg +
H2
MgH2
Action with halogen – Alkaline earth metals react
iv.
with halogen at higher temperatures to form halides MX2. BeH2 is covalent, while other halides are electrovalent. Mg
+
Cl2
MgCl2 Action of acids
v.
– Alkaline earth metals react
readily with acids to form their halides along with liberation of hydrogen gas. Mg
+
2HCl
MgCl2 + H2
4
Complexes of alkaline earth metals – Alkaline
vi.
earth metals form stable complexes with oxygen and nitrogen donor ligands. The tendency to form complexes decreases from Be to Ba. Thus, Be shows greatest tendency to form complexes while, Ba forms less number of complex compounds. Anomalous properties of Beryllium – Beryllium
vii.
shows slightly different properties than other alkaline earth metals, due to its smaller size and high electronegativity.
Halides -
Alkali Metal halides Alkali metals form MX type compounds with halogens. These compounds are called as alkali metal halides. e.g. LiCl, LiF, NaCl, KCl, KI, KF,etc. Preparation : Alkali metals combine with halogens to from halides, along with the evolution of heat. e.g.
2Li + Cl2
2LiCl
Properties : Alkali metal halides are ionic compounds.
1.
They are good conductors and completely soluble in water. 2.
They form crystal lattices with FCC, BCC
type structures. 3.
They are thermally stable.
4.
They have high melting and boiling points.
5.
The ionic character of these halides increases
from Li-halides to Cs-halides. Applications : 1. LiF solutions are used in the enamel industries. 2. LiCl is used in air–conditioning plants. 3. KBr is used in the photography.
Alkaline Earth Metal halides Alkaline earth metals form MX2 type compounds with halogens. These compounds are called as alkaline earth metal halides. e.g. BeCl2, MgCl2, CaCl2, MgF2,etc. 5
Preparation : Alkaline earth metals combine with halogens to from halides. e.g.
Mg + Cl2
MgCl2
Properties : 1.
Alkaline earth metal halides are covalent and polymeric compounds.
2.
They are good conductors in molten state.
3.
They are soluble in the organic solvents.
4.
They form polymeric structure.
Hydroxides -
Alkali Metal hydroxides Alkali metals form hydroxides MOH. Preparation : Alkali metal hydroxides are prepared by the action of water on metals or metal oxides. e.g.
2Na + H2O
2NaOH + H2
Na2O + H2O
2NaOH
Properties : 1. Alkali metal hydroxides are ionic compounds. They are completely soluble in water. They are basic in nature. Their basic nature increases from
2.
LiOH to CsOH. 3.
In molten state these hydroxides are good conductors.
4.
On sublimation they form dimmers.
5.
The ionization increases from LiOH to CsOH, as size of cation increases.
Applications : 1.
They are used as bases in number of chemical reactions. LiOH is used in the production of
2.
photographic developer. NaOH absorbs CO2 to form Na2CO3, sodium
3.
carbonate. 4.
Alcoholic solution of KOH is an important reagent in organic chemistry.
6
Alkaline Earth Metal hydroxides Alkaline earth metals form hydroxides M(OH)2 Preparation : Alkaline earth metal hydroxides are prepared by the action of water on metals or metal oxides. e.g.
2Ca + 2H2O
2Ca(OH)2 + H2
CaO + H2O
Ca(OH)2
Properties : 1. Beryllium hydroxide Be(OH)2 is amphoteric in nature, while other hydroxides are basic in
nature. 2. Be(OH)2 is covalent while other hydroxides are ionic. 3. Be(OH)2 and Mg(OH)2 are insoluble in water. Solubility increases further and Ba(OH)2 is
most water soluble.
Diagonal Relationship –
The first element of a group show similar properties with the second element of the next group. This is known as diagonal relationship. Reason – In the periodic table, while moving from left to right atomic and ionic size decreases and polarizing power increases. While on moving from top to bottom, atomic and ionic size increases and polarizing power decreases. Thus when we consider these properties, the elements which are present diagonally show similar properties. This is called as diagonal relationship. The elements showing diagonal relationship are, Groups
IA
IIA
IIIA
IVA VA
VIA VIIA
Li
Be
B
C
N
O
F
Na
Mg
Al
S
P
S
Cl
Diagonal Relationship between Li and Mg – i)
Electronegativities of Li & Mg are 1.0 and 1.2 respectively.
ii)
Atomic radius of Li & Mg are 134 pm and 136 pm respectively.
iii)
Ionic Radius of Li+ & Mg+2 is 60 and 65 respectively.
iv)
They are hardest elements in their groups.
v)
On heating with air or oxygen, they form monoxides Li2O and Mg2O
vi)
They show comparable enthalpies of vaporization.
vii)
They form ionic nitrides Li3N and Mg3N2
viii)
They decompose water with evolution of hydrogen gas. 7
ix)
Their carbonates, hydroxides and nitrates decompose on heating to form their
oxides. x)
They form less soluble, less basic hydroxides LiOH & Mg(OH)2
xi)
Their halides are soluble in alcohol and undergo hydrolysis in water.
Diagonal Relationship between Be and Al – i)
Ionic Radius of Be+2 and Al+3 is 30 pm and 50 pm resp.
ii)
Charge/radius ratio for both Be & Al is 6.
iii)
Electronegativity of Be & Al is 1.5
iv)
Be & Al has same heat of vaporization.
v)
They form covalent compounds due to their smaller size & high charge density.
vi)
Both Be & Al are resistant to acids.
vii)
Both Be & Al become passive on reaction with nitric acid.
viii)
Both Be & Al form stable complexes & they form dimers. Their dimmers are bridged compounds.
ix)
They dissolve in strong alkali to form soluble complexes beryllates & aluminates.
x)
Their oxides BeO and Al2O3 are amphoteric in nature.
xi)
Be & Al reacts with NaOH & liberate hydrogen gas.
xii)
They form stable complexes.
xiii)
Anhydrous chlorides of Be & Mg dissolve in organic solvents. -----------------------------------
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