S Block Elements

In S block the electron enters in the s-orbital.As the s block element can accommodateonly two electrons so s block elements have two groups ,group 1 and 2 .

Group 1 elements consists of  lithium, sodium, potassium, rubidium, caesium and  francium and collectively known as the alkali metals.

Group 2 element consist of  beryllium, magnesium,calcium, strontium, barium and  radium. Except Beryllium they are known as alkaline

The general electronic configuration is ns^1-2

Alkali Metal  :-

Group 1 element have one electron in their valence shell,they are reactive and soft so they do not occur in their native state.As their oxides and hydroxides form strong alkalies like NAOH,KOH etc these are also know as alkali metals,lithium is known as bridge element.

 General characterstics of Alkali metals:-

1)Electronic configuration  :-
Element =Li ,atomic number=3 and electronic configuration=[He]2s¹

Element =Na ,atomic number=11 and electronic configuration=[Na]3s¹

Element =K ,atomic number=19 and electronic configuration=[K]4s¹

Element =Rb ,atomic number=37 and electronic configuration=[Rb]5s¹

Element =Cs ,atomic number=55 and electronic configuration=[Cs]6s¹

Element =Fr ,atomic number=87 and electronic configuration=[Fr]7s¹

2)Atomic Radii :-

The atomic radii of alkali metals are biggest in their respective periods and it increases as we move down the group and all these form lattice with cordination number =4

3)Ionic Radii :-
Ionic radii increases down the group due to addition of a new energy subshell and ionic radii of alkali metals are much smaller than their corresponding metals due to lesser number of shells and contraction of increased nuclear charge.

4)Density :-It increases on moving down the group.Li is the lightest known metal.The density of potassium is lesser than sodium due to abnormal increase in size on moving down the group from Na to K

5)Melting and Boiling Point :-

melting and boiling point dcreases down the group due to weak mettalic bond as we go down,Fr is liquid at room temperature

6)Softness :-

These are soft ,malleable,ductile and having lusture,it can also be cut with knife.

7)Atomic Volume :-

Atimic volume is highest of alkali metal in their respective period and it increases down the group.

8)Ionisation Enthalpy :-

The first ionisation enthalpy is low for alakali metal ,but second ionisation enthalpy is very high as by losig one electron it aquires noble gas configuration.so removal of second electron is very difficult.

9)Electropositive character :-
Due to low ionisation enthalpy alakali metals are very electropositive or mettalic in nature it increases down the group due to increase in enthalpy.

10)Oxidation state :-

Alkali metals show oidation state +1 as they form noble gas configuration by losing one electron.

11)Hydration of ions :-

It decrease down the group,smaller is the size of cation greater is its hydration enthalpy

12)Flame coloration :-

All the alkali metals give flame coloration as they have one valence electrons in the outermost shell so it easily get excited to higher energy level and when it returns to lower energy level it releases visible light of characteristic wavelength which provides color to flame all the metals provide different colors that are:-

Li- crimson red
Na- yellow
K- violet
Rb - red violet
Cs - Blue
13)Photoelectric effect :-

As ionisation enthalpy is low for alakli metals so it ejects electron when exposed to light easily specially "cs"
so all exhibit photoelctric effect.

14)Electrical Conductivity :-

Due to the presence of loseley held electron ,all the alakli metals are good conductor of electricity.electrical conductivity increases from top to bottom.

15)Reducing character :- As the ionization energy is low for alkali metals so it has good reducing power and the order is given below:-

Na<K<Rb<Cs<Li

Chemical properties of alakli metals :-

 1)Action of air :- They form oxides,hydroxides and carbonates on exposure to moist air and their surface gets tarnished.
Due to their reactivity they are kept under kerosene oil or paraffin wax ,as lithium has lowest density so its floats on the surface of inert liquids like kerosene oil so it is wrapped in paraffin wax.CCl₄ is used to extinguish fire occurred by alkali metals.

 4Na(s) + o₂(g)----> 2Na₂o(s)
Na₂o(g) + H₂ O(l) -----> 2NaOH(s) 2NaOh(s) + CO₂(g) -----> Na₂Co₃(s)
Na₂o(g) + Co₂(l) -----> 2NaOH(s) 2NaOh(s) + CO₂(g) -----> Na₂Co₃(s)

 2)Action of oxygen :-

 All the alkali metals react with oxygen to form different types of oxides some form oxides and some form super oxide e.g lithium forms lithium oxide(Li₂),sodium forms sodium peroxide(Na₂O₂), while rest form super oxides which is KO₂,RbO₂,CsO₂ The stability oxides and super oxides increases as the size of alkali metal increases. Ko₂ (Potassium super oxide) is used as a source of oxygen in submarines,space shuttles and in emergency breathing apparatus such as oxygen masks.

3)Action of water or compounds containing acidic hydrogen :-

2M +H₂O ----->2MOH + H₂O (Where M = Li, Na,K,Rb and Cs )

reactivity order with water is given below it increase from top to bottom due to increase in electropositive character :-

Li<Na<K<Rb<Cs

LiOH is used to remove carbon dioxide from exhaled air in confined quarters like submarines and space vehicles.


4)Action of Hydrogen :-

The reactivity with hydrogen decreases down the group and the order is given below:-

Li>Na>K>Rb>Cs

2M + H₂O -----> 2MH (Where M = Li,Na,K,Rb and Cs)

5)Reaction with Halogens :-

Alkali metals combines fast with halogens to form ionic halides with the exception of some lithium halides.The order of reactivity given below:-

Li<Na<K<Rb<Cs

2M + XH₂O ----> 2M⁺XH^- (where M = Li,Na,Ketc and X = F,Cl,Br,I

Bigger the anion larger will be polarisability therefore covalent character will be in order given below :-
LiI>LiBr>LiCl>LiF

3)Action of water or compounds containing acidic hydrogen :-

2M + 2H₂O ----->2MOH + H₂ (Where M = Li, Na,K,Rb and Cs )

reactivity order with water is given below it increase from top to bottom due to increase in electropositive character :-

Li<Na<K<Rb<Cs

LiOH is used to remove carbon dioxide from exhaled air in confined quarters like submarines and space vehicles.


4)Action of Hydrogen :-

The reactivity with hydrogen decreases down the group and the order is given below:-

Li>Na>K>Rb>Cs

2M + H₂O -----> 2MH (Where M = Li,Na,K,Rb and Cs)

5)Reaction with Halogens :-

Alkali metals combines fast with halogens to form ionic halides with the exception of some lithium halides.The order of reactivity given below:-

Li<Na<K<Rb<Cs

2M + X₂ ----> 2M⁺X^-(where M = Li,Na,Ketc and X = F,Cl,Br,I

Bigger the anion larger will be polarisability therefore covalent character will be in order given below :-
LiI>LiBr>LiCl>LiF

6) Solubility in liquid ammonia :- All alkali metals forms a blue solution on reaction with ammonia ,the blue color is due to ammoniated metal cations and ammoniated electrons in the solution.
The blue color is due to the excitation of ammoniated electron to higher energy levels and the solution is highly conducting and para magnetic due to the presence of ammoniated cation and anion

 M + (x + y)NH₃ -----> [M(NH₃)_x]⁺ (ammoniated cation) + [e(NH₃)_y)]^-    (ammoniated electron)