[Explained] Why is Hydrogen not Considered to be an Alkali Metal?

  In this post we will try to answer why is hydrogen not considered to be an alkali metal.

Hydrogen is the first element in the periodic table and it has the atomic number 1 as it has only one electron in its valence shell.

Since a long time, the correct position of hydrogen in the periodic table has been a matter of debate. And the reason for this is primarily the fact that hydrogen resembles the properties of two groups of the periodic table. Namely, the Group 1(A) commonly known as the Alkali Metals and Group 17 or Group VIIA which is commonly known as the Halogens.

Why is Hydrogen not Considered to be an Alkali Metal?
Water contains Hydrogen


Occurrence


Hydrogen exists as a diatomic gas in nature. In the free state, hydrogen is found in traces in the earth's crust and atmosphere. Volcanic gases contain 0.025% of it, the earth's crust contains 0.98% hydrogen, the earth's atmosphere contains only 0.01% hydrogen and the atmospheres of the Sun and the stars contain 1.1% hydrogen.

Due to very high reactivity, alkali metals do not occur in the free state in nature. They are always found in the combined state, usually in the form of oxides, halides, silicates, carbonates, nitrates, borates, etc.

Lithium is the thirty-fifth most abundant element in the earth's crust and mainly occurs in the form of silicates.

Sodium and potassium are the seventh and eight most abundant elements in Earth's crust.

They also occur in sea water in the form of NaCl and KCl. Sodium is abundantly available in the form of rock salt (NaCl). Other important minerals of sodium are Chile salt peter, borax, mirabilite etc. 

Rubidium and cesium are much less abundant in earth's crust. Francium is radioactive in nature and thus, very rarely found in nature. 


Hydrogen also forms hydrogen bonds with other hydrogen atoms.

Hydrogen shows weak electromagnetism.


Why is Hydrogen not an alkali metal?


The electron configuration of hydrogen and the alkali metals is one of the main reasons why hydrogen is placed in group 1 along with other alkali metals. But is it an alkali metal? Short answer is, no. It isn't. This post will explain you why.

Hydrogen, Lithium, Sodium, Potassium, Rubidium, Cesium all have a single electron in their outermost orbits.

Therefore, all alkali metals including hydrogen have a valency 1. 

Hydrogen unlike other alkali metals can both given and take an electron. What this means is that hydrogen exists in 2 oxidation states, +1 state and the -1 state. Whereas the alkali metals exist only in one oxidation state that is the +1.Alkali metals have a tendency to lose electrons and attain the electron configuration of the closest noble gas.

Metals generally have show low ionization energy whereas when we look at the table below, we find that hydrogen requires a lot more ionization energy to remove the outermost electron. 


We can see in the table that the value of ionization enthalpy of hydrogen is almost double the value of the ionization enthalpy of lithium. 


Hydrogen when compared to alkali metals possesses very high electronegativity. In fact it possesses more than double the electronegativity on the Pauling scale from the rest of the alkali metals.

We can see this in the table below.


Alkali metals form alkaline oxides and hydroxides whereas, hydrogen forms neutral compounds.

You can read all about it in the section below.

Reaction with Oxygen

Alkali metals burn vigorously when heated in oxygen and form different types of oxides depending upon the nature of the metal. Lithium forms monoxide (LI2O), sodium forms a peroxide, while the other alkali metals (Potassium, Rubidium, Cesium) form superoxides having the general formula MO2. Thus,

                                                        4Li + O2    2Li2O

                                                        2Na + O2   Na2O2

                                                         K + O2    KO2

The formation of different types of oxides by different alkali metals can be explained on the basis of their ionic sizes. Lithium ion is the smallest of all alkali metal ions. Due to small size, it has a strong positive field around it. The strong positive field around lithium ion attracts the negative charge so strong that it does not permit the monoxide ion, O2- to combine with another oxygen atom to form peroxide ion, O22-

On the other hand, the weaker positive field around larger sodium ion permits the O2- ion to combine with another oxygen to form O22- ion. This is why lithium forms only monoxide, whereas sodium forms mainly peroxide. The larger K+,Rb+ and Cs+ possess still weaker positive fields around them and allow peroxide ions to further combine with oxygen to form superoxide ions, O2-. This is why potassium, rubidium and cesium form mainly superoxides.

Thus it is generally said that a small cation can stabilize a small anion, whereas a large cation can stabilize a large anion.




Why is Hydrogen not Considered to be an Alkali Metal?
Avocado Contains Potassium


Nature of Oxides

The normal oxides of alkali metals (monoxides) are basic oxides. They are highly soluble in water and form highly alkaline solutions due to formation of hydroxides.

   M2O + H2⟶ 2MOH

The peroxides and superoxides are oxidizing agents. They react with water to form a hydrogen peroxide or oxygen or both.

   Na2O2 + H2 2NaOH + H2O2

  2KO2 + 2H2O   2KOH +  H2O2 + O2

Reaction with Di-hydrogen

Alkali metals react with H2 at about 673K (Li at 1073 K) to form saline hydrides having high melting points.

    2M  +  H2     2M+ H-

Reaction with Water

 Alkali metals react readily and violently with water to form hydroxides with the liberation of hydrogen gas.

    2M  +  H2⟶ 2MOH + H2O

Lithium reacts with water somewhat slowly but the reaction of alkali metals with water is so vigorous that the hydrogen liberated catches fire immediately. This is why alkali metals can not be kept under water.

The reactivity of alkali metals with water increases on going down the group in going from Li to Cs.

Nature of Hydroxides

The alkali metal hydroxides are white crystalline solids, highly soluble in water. They are highly basic in nature and form the strongest bases known.

Basic Strength

The basic strength of hydroxides increases on moving down the group form LiOH to CsOH.

       LiOH< NaOH< KOH< RbOH< CsOH

This may be attributed to an increase in the electropositive character of alkali metals on moving down the group.

      2LiOH + Heat  Li2O + H2

 Stability

Alkali metal hydroxides except LiOH are thermally stable. LiOH decomposes on heating.



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