Alkali metals , s block group IA elements

Here, we learn chemistry of alkali metals(group 1 elements) of s block. Occurrence. reactions, physical properties of group 1 elements are discussed in this tutorial. Usually, group 1 elements of s block are very reactive and exist as compounds. Their only valence is one.

Elements of alkali metals

  • H
  • Li
  • Na
  • K
  • Rb
  • Cs
  • Fr


In room temperature condition... Hydrogen exists as H2 gas and rest of others are solid.


Electron Configuration of alkali metals

Those elements have only one electron in their last shell
commonly, we can show it as 1s2 2s22p6.... ns1

Example:
Na - 1s2 2s22p6 3s1
K - 1s2 2s22p6 3s23p6 4s1


Reactions of alkali metals

alkali metals are very reactive due to existence of only one electron in the last shell.
Lot of compounds of these elements are soluble in water.
Most solids of alkali metal compounds take white colour.


Water and alkali metals reactions

Li, Na, K, Rb, Cs reacts with water and H2(g)is emitted and relevant hydroxide of metal is formed.

Reaction of K with water is a highly flammable.

Water and alkali metals reactions


Acids and alkali metals reaction

Alkali metals react with strong acids (HCl , HNO3 , H2SO4) and H2 will be omitted, relevant salt will be produced.

Acids and alkali metals reaction


Hydrogen and alkali metals reaction

Metal hydride (H-) is given.

Hydrogen and alkali metals reaction

Those ionic hydrides have H-. These ionic hydrides react with compounds which can supply H+ ions. Therefore these hydrides behave as strong reducing agents. So they alkali metal hydrides react with water, alcohols, ammonia and alkyne to eliminate H2 gas.

ionic hydrides strong reducing agents


Reactions of air and alkali metals

Liquid Li reacts(burning) with O2(g) and make Li2O.

Na , K reacts with Oxygen and make relevant Oxide of metal.

alkali metals and oxygen reaction

When Na, K, Rb, Cs is heated in a hot air stream, peroxide are formed.

sodium peroxide forming

Also K,Rb can make super oxides

Rb superoxides


Reaction of water and alkali metal oxides

Above oxides can react with water and make hydroxides.

Reaction of water and alkali metal oxides


Halogen and alkali metals

group IA elements react with halogen and form halides.

Halogen and alkali metals


Nitrogen and alkali metals

Only Li reacts with nitrogen and forms Li3N.

lithium nitride

Li3N reacts with water and and makes NH3.

lithium nitride and water reaction


Sulfur and alkali metals

Na reacts S and give sulfide when heat is supplied.

sodium and sulfur

Those sulfides can react with dilute acids. And H2S gas is released.

sodium sulfide and dilute HCl


NH3 and alkali metals

Na Reacts with NH3 and H2 gas is emitted giving NaNH2.

Na + NH3


Reaction with Phosphorus and alkali metals

Na Reacts with P when heat is available.

sodium and phosphorus


Reaction with hydrogen Sulfide and alkali metals

Na reacts with H2S to give sulfide and H2 gas.

sodium and hydrogen sulfide


Alkali metal halides

The alkali metal halides are high melting colourless crystalline solids.
Ex: NaCl , KBr
These alkali metal halides are prepared by the reaction of appropriate oxide, hydroxide or carbonate with aqueous hydrohalic acid (HF, HCl, HBr, HI)

Alkali metal halides preparing

Stability of alkali metal halides

Fluorides of alkali metals are most stable while iodides of alkali metals are least stable.


Melting and boiling points of alkali metal halides

Melting and boiling points of alkali metal halides decreases with the increase in atomic mass of the halides as:
F- > Cl- > Br- > I-
Ex: Melting point of NaCl is higher than NaBr

For given halide ion, melting and boiling points of LiX is always less than NaX.


Solubility of alkali metal halides

All alkali metal halide compounds are soluble in water exception of LiF. LiF is insoluble in water due to its high lattice energy because of small cation and small anion size. Other halide ions of lithium are soluble in ethanol, acetone, ethyl acetate. LiCl is soluble in pyridine. CsI has also low solubility due to smaller hydration energy of its two ions.


Salts of oxo acids

Alkali metals are electropositive metals. Therefore they form salts with oxo acids.


What are the oxo acids?

Oxo acids are those in which the acidic proton which is on a hydroxyl group with an oxo group attached to the same acid. Carbonic acid(H2CO3), sulfuric acid(H2SO4), phosphoric acid(H3PO4) are some examples.


The alkali metals form salts with all oxo acids. These alkali metal oxo salts are generally soluble in water and thermally stable. But Li2CO3 is not stable to heat and decomposes readily. LiHCO3 does not exist as a solid.


carbonates and bicarbonates of alkali metals

Carbonates of alkali metals are stable to heat (except Li). The stability of those carbonates are increased down the group.

LiHCO3 does not exist as a solid. Li2CO3 is not stable to heat as other alkali metal carbonates

Bicarbonates of alkali metals (except Li) decomposes into the carbonates, carbon dioxide and water.

decomposition of bicarbonates of alkali metals

Colors of super oxides of alkali metals

KO2 - Orange

RbO2 - Brown

KO2 - Dark Orange


Why LiCl is soluble in organic solvents?

Li+ has very high polarizing power. Therefore LiCl is covalant in nature. Being covalent in nature, it is soluble in organic solvents.


What are the factors of anomalous behavior of lithium?

  • Small size of atom and ion
  • high ionization enthalpy
  • absence of d- orbitals in its valence shell

What are the precipitates formed by alkali metals?

Many compounds of alkali metals are soluble in water. But few of alkali metal compounds are insoluble in water specially some compounds of lithium. Lithium carbonate, lithium fluoride, lithium carbonate, lithium oxalate are sparingly soluble in water. Solubility of sodium bicarbonate is also less than potassium bicarbonate.


Biological importance of sodium and potassium

  • Sodium and potassium are most common cations in biological systems.
  • Though Na+ and K+ ions are quite similar chemically but surprisingly they differ greatly in their biological activities.
  • Their distribution in biological systems is also different. A typical man weighing 70Kg contains about 90g of sodium and 170g of potassium as compared with only 5g of iron and 0.06g of copper.
  • The most salient feature is that K+ ions are present in higher concentrations inside the cells than Na+ ions while their relative concentrations are reverse outside the cells.
  • In other words, K+ ions are concentrated inside the cells while Na+ ions are concentrated outside the cells.
  • Na+ ions are primarily found in blood plasma and in the interstitial fluid which surrounds the cells.
  • On the other hand, K+ ions are most abundant cations within cell fluids. There is very considerable variation in the concentration of Na+ and K+ ions found on the opposite sides of cell membranes. For example, in blood plasma, Na+ ions are present to the extent of 143m .

Written by : Heshan Nipuna
Last Edition : 09/10/2018

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