Physical properties of s-block elements

The group 1 of the periodic table contains 6 elements namely Lithium ,Sodium ,Potassium, rubidium, caesium and francium.

They are usually referred to as alkali metals since their oxides and hydroxides are soluble in water and form strong bases or alkalies.

Alkali metals are highly reactive and hence do not occur in the free state but are widely distributed in nature in the combined state in the form of halides ,oxides ,silicates ,borates and nitrates.

Lithium is the thirty-fifth most abundant element by weight in the earth’s crust and mainly occur in form of silicate minerals.

Sodium and potassium are the 7th and 8th most abundant elements by weight in the earth’s crust.NaCl and KCl  occur in large amount in sea water.The largest source of sodium is rock salt.

Potassium mainly occurs as KCl , a mixture of KCl and NaCl and the double salt, KCl.MgCl₂ ·6H₂O and as K₂O·Al₂O₃·6SiO₂ (Felspar)

1) Electronic Configuration

Alkali metals are s-block elements, because last electron in them enters the s-orbital. They contain only one s electron outside the noble gas core. Their general electronic configuration is [noble gas]ns¹ where n=2 to 7. Since all these elements have similar valence shell or outer electronic configuration ,they have similar physical and chemical properties.

 

2) Large Atomic Radii

The alkali radii of alkali metals are the largest in their respective periods. These increase as we travel down the group from Li to Cs.

Reason: As the atomic number increases, the number of energy shells increases by 1 with each succeeding element. As a result, the screening effect of the inner filled shells on the valence s-electron increases and the electron cloud tends to expand.

The distance between the nucleus and the last shell increases and thus the atomic radius increases with increase in atomic number.

As the atomic number increases, the nuclear charge also goes up. This tends to decrease the atomic radii by attracting the electron cloud inward more forcibly. But the screening effect is so large that it overcomes the contraceptive effect of the increased nuclear charge.

The increase in atomic radius as we move down the group from Li to Cs primarily due to predominant screening effect of inner filled shells on the valences-electron.

3) Large Ionic Radii

The ionic radii of the cations formed by them are smaller in size than the corresponding atoms.

Reason: When the valence s-electron is lost from the alkali metal atom, the monovalent cation formed has one shell less than the corresponding metal atom.

With the loss of one electron ,the effective nuclear charge increases and thus the remaining electrons are pulled closer to the nucleus thereby decreasing the size of each individual energy shells. Due to contractive effect of the increased nuclear charge and lesser number of energy shells, the radii of cations are much smaller than the corresponding atoms.

The ionic radii increases as we move down the group from Li to Cs primarily due to the addition of a new energy shell with each succeeding elements.

4) Low Ionization Enthalpy

The first ionization enthalpies of the alkali metals are the lowest as compared to the elements in the other group.

Reason: Noble gas core shields the lone s-electrons in the valence shell from the direct attraction of the positive charge on the atomic nucleus. As a result, the valence s-electron is loosely held by the nucleus and can be easily knocked off by supplying only a small amount of energy.

The ionization enthalpies of alkali metal decreases progressively as we move down the group from Li to Cs.

 

The s-electron to be removed becomes farther and farther away from the nucleus because of the addition of a new shell with each succeeding elements. Due to an increase in the number of inner shells ,the screening effect also increases. As a result, the attraction of the nucleus for the valence s-electrons decrease and hence the ionization enthalpies decreases as we travel down the group from Li to Cs.

The second ionization enthalpy of alkali metals are very high.

Reason: When an electron has been removed from the alkali metal ,the resulting monovalent cation has the stable noble gas configuration. Since it is difficult to remove an electron from the stable noble gas configuration, therefore, the second ionization enthalpies of alkali metals are very high. Second ionisation enthalpy decreases as we move down the group due to increasing size and increasing shielding effect.

4) Hydration Enthalpy

The hydration enthalpy of alkali metal ions decreases with the increase in ionic radii i.e.

Li⁺ > Na⁺ > K⁺ > Rb⁺ > Cs⁺

Because of the smallest size, Li⁺ has maximum degree of hydration and it is because of this reason that lithium salts are mostly hydrated.

5) Unipositive ions

The alkali metal exhibit oxidation state of +1 in their compounds and are strongly electropositive in character. The electropositive character increases from lithium down to caesium in the group.

Reason: As the alkali metals have low ionisation enthalpies ,they have a strong tendency to lose the single valence s-electron to form the unipositive ions. Thus, they show an oxidation state of +1 and are strongly electropositive.

M———> M⁺ + e‾

The second ionisation enthalpies of these elements are fairly high and thus the loss of second electron is quite difficult. This is due to the fact that the second electron has to be lost from the most stable noble gas core i.e. ns² np⁶.

As we go down the group, the atomic size increases and the ionisation enthalpy of the elements decreases i.e. the tendency of the elements to lose electron increases. Consequently, the electropositive character increases.

6) Metallic Character

The elements of this group are typical metals which are soft ,malleable and ductile solids. These can be easily cut with a knife. They are silvery white and on exposure to air they are tarnished. the metallic character increases down the group.

Reason: The tendency of an element to lose Valence electrons to form positive ions determines its metallic character. As alkali metals have low ionisation energy ,they have high tendency to lose valence electrons and are ,therefore ,typical metals. The metallic character increases down the group metallic character increases down the group.

Since the atoms of alkali metals have bigger kernels and smaller number of Valence Electrons, the metallic bonds in them are very weak. Hence they are soft and can be cut with a knife.

7) Melting and Boiling Points

The melting and boiling points of alkali metals are very low and decrease with increase in atomic number.

Reason: As alkali metals have large size ,the inter metallic bonds in them are quite weak.Hence, they have low  melting and boiling point which decreases down the group with increase in the atomic size.

8) Nature of Bonds formed

All the alkali metals form ionic compounds. The ionic character increases from Li to Cs as we move down the group.

Reason: Because alkali metals have low ionisation enthalpies which decreases down the group.

 

9) Density

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

Reason: Because of their large size, their are atoms are less closely packed. They have low densities. As we move down the group from Li to Cs ,both the atomic size and atomic mass increases. But the increase in atomic mass more than compensates the bigger atomic size. As a result, the densities of alkali metal increases from Li to Cs.Potassium is ,however, lighter than sodium. This is probably due to an abnormal increase in atomic size of potassium.

10) Flame Colouration

All the alkali metals and their salts impart a characteristic colour to the flame .

Reason: On heating an alkali metal or its salt especially chlorides due to its more volatile nature in a flame, the electrons are excited easily to higher energy levels because of absorption of energy. When these excited electrons return to their ground state, they emit extra energy in form of electromagnetic radiations which fall in the visible region thereby imparting a characteristic colour to the flame.

The colour actually arises from electronic transitions in short lived species which are formed momentarily in the flame.

11) Photoelectric Effect

Alkali metals except Li exhibit photoelectric effect.

Photoelectric effect is the phenomenon of ejection or emission of electrons from the metal surface when electromagnetic radiation are need to strike against them.

Reason: Due to low ionisation enthalpies ,alkali metals ,specially potassium and caesium, ejects electrons when exposed to light. Lithium  which has the highest ionisation enthalpy amongst the alkali metals cannot be used in photoelectric cells because it does not release electron when exposed to light.