[Explained] Why are alkali metals more reactive than alkaline earth metals?

 Alkali metals are the elements which are found in the first group of the periodic table. Alkaline earth metals are the elements that are found in the second group of the periodic table.

Alkali metals are characterized by having a single s-electron whereas alkaline earth metals have two s-electrons which gives alkaline earth metals extra stability. The s subshell can only accommodate two electrons therefore, the elements are stable due to electron configuration. 

Possession of single s-electron is the main reason why alkali metals are more reactive than alkaline earth metals. The other reasons include large atomic size, high metallic character and low values of ionization energy and electronegativity.

The group 1 or Group 1 A of the periodic table consists if six elements including hydrogen. These elements are 

  • Hydrogen
  • Lithium 
  • Sodium 
  • Potassium 
  • Rubidium 
  • Cesium 
  • Francium 

They are collectively known as alkali metals.

 Why do Alkali Metals have different oxides?

Why do Alkali Metals have different reactivities?

Why do Alkali Metals form Crystalline compounds?

Why do Alkali Metals form Ionic Compounds?

The table below shows the symbols, atomic numbers and electron configurations of alkali metals. 


Element Symbol Atomic Number Electron Configuration Brief Representation of Electron Configuration
Lithium Li 3 1s2 2s1 [He] 2s1
Sodium Na 11 1s2 2s2 2p6 3s1 [Ne] 3s1
Potassium K 19 1s2 2s2 2p6 3s2 3p6 3d10 4s1 [Ar] 4s1
Rubidium Rb 37 1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 5s1 [Kr] 5s1
Cesium Cs 55 1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 4d10 5s2 5p6 6s1 [Xe] 6s1
Francium Fr 87 1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 4d10 4f14 5s2 5p6 5d10 6s2 6p6 7s1 [Rn] 7s1

We can see in the table that the alkali metals have only a single outermost electron in the s subshell.

The group 2 or Group II A of the periodic table consists of six elements. These are 

  • Beryllium
  • Magnesium
  • Calcium 
  • Strontium
  • Barium
  • Radium.

 The last element radium is radioactive in nature. These elements are collectively known as alkaline earth metals. At first the name alkaline earth metals was given to calcium, strontium and barium because the oxides of these elements were alkaline and existed in the earth's crust. 

Element Symbol Atomic Number Electron Configuration Brief Representation of Electron Configuration
Beryllium Be 4 1s2 2s2 [He] 2s2
Magnesium Mg 12 1s2 2s2 2p6 3s2 [Ne] 3s2
Calcium Ca 20 1s2 2s2 2p6 3s2 3p6 3d10 4s2 [Ar] 4s2
Strontium Sr 38 1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 5s2 [Kr] 5s2
Barium Ba 56 1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 4d10 5s2 5p6 6s2 [Xe] 6s2
Radium Ra 88 1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 4d10 4f14 5s2 5p6 5d10 6s2 6p6 7s2 [Rn] 7s2


The alkaline earth metals have 2 outermost electrons. The completely filled s subshell gives alkaline earth metals extra stability when compared to alkali metals.

Let us now discuss in detail, the reasons why alkali metals are more reactive than alkaline earth metals.


Physical State

Alkali Metals show typical silvery white metallic luster when freshly cut. The metallic luster fades rapidly due to oxidation by atmospheric air. They are soft, malleable and ductile. They are so soft that they can be cut with a knife. Lithium is the hardest of all the elements present in the alkali group.

Atomic and Ionic Radii

The atomic and ionic radii of alkali metals show the following characteristics:

The ionic radii of alkali metals ions are smaller than the atomic radii of the corresponding atoms.  

For example the ionic radius of Na+ ion is 102 pm whereas the atomic radius of Na atom is 186 pm.

Alkali metals possess only one electron in their valence shell. During the formation of cation, the valence s electron is lost. The cation thus formed has one electrons shell less than the parent atom. The removal of an electron shell decreases the size. 

      Na           ⟶     Na+ + e-

     1s22s22p63s1              1s22s22p6         

Moreover, the removal of an electron from the valence shell increases the effective nuclear charge experienced by the remaining electrons. Thus, the remaining electrons are pulled closer to the nucleus resulting in a further decrease in the size of the ion. 

The combined effect of the decrease in the number of the electron shells and an increase in the effective nuclear charge is responsible for the smaller size of alkali metal cations as compared to those of the corresponding alkali metal atoms. 

The atomic and ionic radii of alkali metals are the largest in their respective periods.

Each alkali metal atom is the first element of its period. As one moves from the left to right in a period, the differentiating electrons are added in the same electron shell and the nuclear charge increases with increase in the atomic number. Thus, in going from left to right in a period, the number of shells remains the  same but nuclear charge increases with each succeeding element. 

Thus, the electrons in the valence shell experience a greater pull towards the nucleus. This results in the successive decrease of the atomic and ionic radii with increase in the atomic number. This is why the atomic and ionic radii of alkali metals are the largest in their respective periods. 

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

As one moves from Li to Cs in group 1, a new electron shell is added at each element and the nuclear increases in the atomic number. The addition of an electron shell at each element tends to increase the size of the atom but the increase in the nuclear charge has a tendency to decrease the size of the atom or ion. Thus, the two factors oppose each other. 

The increase in the number of shells increases the screening effect of the inner electrons on the valence s-electron. This results in the expansion of the electron cloud. As the screening effect is quite  large, it over weighs the contractive effect of the nuclear charges with increase in the atomic number. The net result is an increase in the atomic and ionic radii of the alkali metals in going from Li to Cs.

Why are alkali metals more reactive than alkaline earth metals?
Table Salt contains Sodium as a constituent


Density

Alkali metals possess quite low densities as compared to other metals. Li, Na, and K are lighter than water. The density of the alkali metals increases from Li to Cs.

Although alkali metals possess close packed structures. You might be wondering why alkali metals have low density? It is due to the large size of atoms that their densities are low. On moving down the group, the atomic mass as well as the size of the atoms increase. 

The increase in atomic mass overweighs the effect of the increase in the size of the atoms. Therefore, densities of alkali metals increase in going from Li to Cs. However, potassium is an exception and is lighter than sodium.

Melting and Boiling Points

The alkali metals possess low melting and boiling points. The melting and boiling points decrease on moving down the group. The answer to why the melting and boiling points of alkali metals are low is given below.

The alkali metals possess only one electron in their valence shells. Therefore, the inter atomic forces responsible for the binding of atoms in the lattice which come into existence due to formation of metallic bonding are weak. 

Consequently, alkali metals possess low melting and boiling points. On moving down the group, the size of the atoms increases without any increase in the number of valence electrons. This further decreases the inter atomic forces. Therefore, the melting and boiling points further decreases in moving from Li to Cs.

Ionization enthalpy or ionization energy

The ionization energy values follows the following trends:

Alkali metals possess very low values of ionization energy. The ionization energy of an alkali metal atom is lowest in the period.

The alkali metal atoms possess electronic configuration of the type [Noble Gas] ns1.

The noble gas core shields the valence s-electron from the nucleus. Therefore in alkali metals the valence electron is loosely held by the nucleus and can be removed easily by supplying a small amount of energy. This is why alkali metals possess quite low ionization energies.

The ionization enthalpy of alkali metals decrease progressively in going from Li to Cs.

In going from Li to Cs. the distance of the valence electron from the nucleus increases progressively due to the addition of a new shell at each succeeding element. The increase in the number of shells causes an increase in the screening effect which consequently decreases the effective nuclear charge experienced by the valence electron. 

This facilitates an easier removal of the valence electron. This is why the ionization energies of alkali metals decrease on moving down the group. 

The second ionization energies of the alkali metals are very high

When and electron is removed from an alkali metal atom, the cation formed has a stable noble gas configuration. For example. 

Li+ = 1s2

Na+= 1s2 2s2 2p6

The noble gas configuration is a very stable configuration. The removal of an electron from such as configuration is very difficult and requires a large amount of energy. This why the second ionization energies of alkali metals are very high.

Alkali Metal ionElectron Configuration
Li+1s2
Na+1s2 2s2 2p6
K+1s2 2s2 2p6 3s2 3p6



Why are alkali metals more reactive than alkaline earth metals?
Bananas contain a lot of Potassium

 

Electronegativity

Alkali metals possess low values of electronegativity. In general, the electronegativity of alkali metals decrease in going down the group. 

Due to large size and low nuclear charge, the alkali metal atoms are unable to attract electrons towards them. This is why they possess low values of electronegativity. In going from Li to Cs, the size of atom increases further. Consequently, the electronegativity of alkali metals decreases on going down the group. 

Electropositive Character (also known as Metallic character)

 The alkali metals are strongly electropositve. Each alkali metal atom is the most electropositive atom in its period. Due to strong electropositive character alkali metals exhibit strong metallic character. The electropositive character of alkali metal metals increases in going from Li and Cs.

The tendency of an atom to form positive ions by losing its valence electron s determine its electropositive character. Since alkali metals possess very low values of ionization energies, they have strong tendency to lose their valence electrons. This is why they show strong electropositive or metallic character. 

As the ionization energies of alkali metals decrease progressively in going from Li to Cs, their tendency of losing valence electrons also increases progressively. Consequently, the electropositive character increases on moving down the group.

Oxidation State

An alkali metals exhibit only +1 oxidation state in their compounds. They do not show variable oxidation states as shown by several other elements of their periods.

The alkali metal atoms possess only one electron ns1 in their valence shells and can lose it readily due to low ionization energies. On losing the valence electron they form a monopositive cation and thus exhibit +1 oxidation state.

    M ⟶  M+ + e-

The monopositive cation formed has the configuration of a nearest noble gas. As the noble gas configuration is a very stable configuration, the cation formed does not allow the further removal of electrons easily. This is why alkali metal atoms do not exist in higher oxidation states and exhibit only =+1 state.

The alkali metal cations (M+) have no unpaired electrons. Therefore, they are colorless and diamagnetic in nature.

Hydration Energy

The alkali metal cations have a strong tendency to get hydrated.

    M+ (g) + aq (excess water) ⟶  M+ (aq)

The process of hydration is exothermic and the energy involved is called hydration energy. The hydration energy of alkali metal cations decreases in going from Li+  to Cs+.

The hydration energy depends upon the charge-radius ratio(q/r). Since the radius of alkali metal cations increases in going from Li+ to Cs+, the hydration energy decreases in going down the group. 

Hydration energy is a measure of the tendency of an ion to undergo hydration. This is why the tendency of alkali metal cations to undergo hydration in going from Li+  to Cs+. Li+ ions are most heavily hydrated in aqueous solutions.

Flame Coloration

Alkali metals and their salts show characteristic colors when heated in a non-luminous flame. The color imparted to the flame darkens on moving down the group.


ElementColor
LiCrimson
NaGolden Yellow
KPale Violet
RbViolet
CsViolet

When an alkali metal or its salt is heated in a flame, its electrons get excited to higher energy levels due to absorption of energy. The excited states are short lived. When the excited electrons return back to their normal states, the energy is emitted. The emitted energy corresponds to the visible region and therefore a characteristic color is imparted to the flame.

Why are Alkali Metals strong reducing agents?

Why do Alkali Metals impart colour to the flame?

Why is Hydrogen not considered to be an Alkali Metal?

Why are Alkali Metals so reactive?

Physical and Chemical Properties of Alkali Metals

Photoelectric Effect

Alkali metals emit electrons when irradiated with with light. The phenomenon of the emission of electrons from the from the surface of a metal on irradiating the metal surface with electromagnetic radiation is called photoelectric effect and the electrons thus ejected are called photo electrons.

As mentioned above, alkali metals possess low ionization energies. If the energy of the light falling on the surface of the metal is greater than or equal to the work function (the minimum energy required to overcome the attractive forces responsible for binding electrons to the metal), the electrons present on the metal surface get ejected in the form of photoelectrons. 

Due to low ionization energies, alkali metals particularly potassium and cesium have low values of work function and emit photoelectrons easily when exposed to light suitable frequency.

Nature of the Compounds

Alkali metals form ionic compounds. The formation of ionic compounds by alkali metals may be attributed to their electropositive nature. Due to low ionization energies, alkali metals readily form cations by losing their valence electrons. Consequently, they form ionic bonds with non metals.

Why are alkali metals more reactive than alkaline earth metals?
Cesium in a vial



Lattice Energy of Compounds

The salts of alkali metals possess high values of lattice energy. The lattice energy values decrease in going down the group.


SaltLattice Energy(kJ mol-1)
LiCl-840
NaCl-776
KCl-701
RbCl-682
CsCl-630

The change in enthalpy involved in condensing required number of gaseous positive and negative ions to form one mole of lattice of an ionic compound is termed as the lattice energy of the compound.

The compounds of alkali metals (salts) consists of cations and anions and thus are ionic in nature. They are held together by strong electrostatic forces of attraction. This is why a large amount of energy is releases during the condensation of ions to form the lattice. Consequently, the lattice energies of alkali metal salts are quite high. 

Since the lattice energy of a salt is inversely proportional to the sum of ionic radii, the values of lattice energy decreases on moving from Li to Cs.


Now let us look into Alkaline Earth Metals,

Physical State

Alkaline Earth metals are grayish, white, malleable and ductile metals and possess a metallic luster. They are harder than alkali metals. The hardness decreases on moving down the group.

 Atomic and Ionic Radii

The atomic and ionic radii of alkaline earth metals are fairly large but smaller than those of the corresponding alkali metals. The atomic and ionic radii increase in going from Be to Ra.

The alkaline earth metals have a higher nuclear charge as compared to the corresponding alkali metals. Therefore, the electrons experience a greater nuclear pull and the electron cloud is pulled towards the nucleus. Consequently, the atomic and ionic radii of alkaline earth metals are smaller than those of alkali metals.

On moving down the group, a new electron shell is added to each element and the nuclear charge also increases. The increase in the size of atom or ion due to addition of a new electron shell over weighs the reduction in size due to increased nuclear charge. This is why the atomic and ionic radii increase in going from Be to Ra.

Density

The densities of alkaline earth metals are much higher than those of alkali metals. Moreover, the densities of these elements do not vary regularly on going down the group. The densities first decrease in going down from Be to Ca and then increase again from Ca to Ra.

The atoms and ions of alkaline earth metals are smaller and heavier than those of alkali metals. Due to smaller size, they pack more efficiently in the lattice. This is why alkaline earth metals have much higher densities as compared to those of alkali metals. The irregular variations in the densities of these elements is probably due to a change in the crystal structure.

Melting and Boiling Points

The alkaline earth metals have much higher melting and boiling points if compared to those of alkali metals. No regular trend is observed in the melting and boiling points on moving down the group.

Due to smaller size, the atoms or ions of alkaline earth metals are packed more closely to their lattices. Moreover the alkaline earth metals have two valence electrons whereas alkali metals have only one. The larger number of valence electrons leads to the formation of stronger metallic bonds. This is why melting and boiling points of alkaline earth metals are higher than those of alkali metals.

Ionization Enthalpy or Ionization Energy

The following trends are observed in alkaline earth metals regarding the Ionization Energy:

The ionization energies of alkaline earth metals are quite low. However, they are higher than those of the corresponding alkali metals.

The low ionization energies of alkaline earth metals may be attributed to the larger size of their atoms as compared to the other succeeding elements of the same period. The smaller nuclear charge and larger atomic size result in a weaker pull of nucleus on the valence electrons. This is why these metals possess fairly low values of ionization energies.

The first ionization energies of alkaline earth metals are higher than those of alkali metals. This is because alkaline earth metal atoms have smaller size and greater nuclear charge as compared to those of alkali metals.

The ionization energies of alkaline earth metal decrease on moving down the group.

When one moves down the group, the size of the atom increases due to the addition of an electron shell at each element. The nuclear charge also increases but this increase is compensated by an increase in the shielding effect of the inner shell electrons. Thus, the effective nuclear charge decreases and valence electrons experience lesser nuclear pull. Consequently values of ionization energy decrease on moving from Be to Ba.

The second ionization energies of alkaline earth metals are much smaller than those of alkali metals.

The unipositive cation of an alkaline earth metal has configuration of the type ns1 which means that the outermost shell still contains one valence electron which can easily be removed. On the other hand, the unipositive cation of an alkali metal possesses a stable noble gas configuration. Since noble gas configuration is a very stable configuration, it requires a very large amount of energy to remove one more electron from it. This is why the second ionization energy values of alkali metals are very high. In contrast, it requires a much lesser amount of energy to remove ns1 electron from the valence shell of the unipositive cation of an alkaline earth metal.


Electronegativity

Alkaline earth metals have low values of electronegativity. The electronegativity decreases on going down the group. Alkaline earth metals are more electronegative than alkali metals.

The low electronegativity of alkaline earth metals is due to their large size and low nuclear charge. Since the size of atom increases on moving down the group, electronegativity decreases in going down the group from Be to Ra. As the atomic atomic radii of alkaline earth metals are smaller than those of alkali metals, alkaline earth metals are more electronegative than alkali metals.

Electropositive Character (Metallic Character)

The alkaline earth metals are highly electropositive and therefore high metallic character. However, they are less electropositive than alkali metals. The electropositive character or metallic character of alkaline earth metals increase in going down the group.

Alkaline earth metals possess low ionization energies and can easily lose both the valence electrons to form dispositive. This is why they exhibit high electropositive character. As the ionization energies of alkaline earth metals are higher than those of alkali metals, their tendency to lose valence electrons is less than that of alkali metals. This is why alkaline earth metals are electropositive than alkali metals.

On moving down the group, the ionization energies decrease due to an increase in atomic radii. Therefore, the tendency of losing valence electrons increases. Hence, electropositive character increases in going from Be to Ra.

Tendency to form dispositive (M2+ ions)

The alkaline earth metals have a strong tendency to form dispositive ions both in the solid state as well as in aqueous solutions.

From the data in the Table given above, it is clear that the second ionization energies of alkaline earth metals are much higher than their first ionization energies. Therefore, on the basis of values of ionization energies, the formation of mono positive (M+) ions should be more than those of dipositive (M2+) ions which means alkaline earth metals should prefer to form +1 ions rather than +2 ions. But that is not the case. The predominant oxidation state of alkaline earth metals is +2 and they always form dipositive ions for example, Mg2+,Ca2+,Ba2+ etc. 

The tendency of alkaline earth metals to form ions in the +2 oxidation state is due to the following reasons:

The dipositive ions of alkaline earth metals have stable nearest noble gas configuration 

 

In solid state

The di-positive cations form stronger lattices as compared to those formed by mono positive cations. This is evident from the fact that the lattice energies of the compounds containing divalent cations is much higher than those of containing monovalent cations. It may be recalled that the lattice energy is the energy releases in the formation of one mole of an ionic crystal from the constituent gaseous positive and negative ions. Therefore, during the formation of an ionic compound containing dipositive alkaline earth metal ions, a large amount of energy is releases. This more than compensates the high second ionization energy of the corresponding alkaline earth metal atom. Thus, the formation and existence of dipositive cations of alkaline earth metals in the solid state is facilitated by the higher values of lattice energies.

In aqueous solution

The formation and existence of dipositive cations is due to the grater hydration energy of dipositive ions as compared to those of the monopositive ions. The dipositive ions of alkaline earth metals have much larger charge to size ratio and therefore exert a much stronger electrostatic attraction of the oxygen of water molecules surrounding them. Thus, the dipositive ions get heavily hydrated. During their hydration, a large amount of energy known as hydration energy is released. The large amount of energy released in the hydration of dipositive ions more than compensates the second ionization energy of the corresponding metal atom required for the formation of such ions. Thus the formation and existence of dipositive cations of alkaline earth metals in aqueous  solutions is due to their higher values of hydration energies. 

Nature of Dipositive Ions

 The dipositive ions of alkaline earth metals are formed by the loss of two valence electrons by their parent atoms. Therefore, M2+ ions have stable nearest noble gas configuration.

All the electrons present in them are paired. Due to the presence of all the electrons paired the dipositive ions of alkaline earth metals are colorless and diamagnetic.

Flame Coloration

Except beryllium and magnesium, the alkaline earth metals and salts impart characteristic colors to the flame. 

When an alkaline earth metal or its salt is heated in a non-luminous flame, the electrons get excited to higher energy levels. The excited states are short lived and the excited electrons return back to their normal state after some time. During de-excitation, they emit radiation having particular wavelengths corresponding to the visible region and this characteristic colors in the flame. 

Beryllium and magnesium atoms are smaller in size. Therefore, their electrons are more tightly bound to the nucleus and require large amounts of energy for each excitation to higher energy levels. The energy of the flame is not enough for this purpose. This is why beryllium and magnesium do not show any color when heated in a flame.


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