NEET Important Chapter: The s-Block Elements

In the Modern Periodic Table, the elements of Group 1 and Group 2 are known as the s-block elements. The s-block elements are those elements whose valence shell electronic configuration is restricted within the s-orbital. The elements having general electronic configuration ns1 are called Group 1 elements or Alkali Metals (except hydrogen).

While the elements having general electronic configuration ns2 called Group 2 elements or Alkaline Earth Metals. The members of the s-block lie on the extreme left of the Periodic Table.

The ‘s-block Elements’ is an important topic of the NEET 2022 exam. Physical properties, chemical properties, anomalous properties, biological importance of Na, K, Ca and Mg, some important compounds of the s-block elements and so on are all significant topics.

We will explore everything about the characteristics of Group 1 and Group 2, in this article. This article provides an example of the types of questions that might be asked about this subject.

Important Topics of the s-Block Elements

• Electronic Configuration

• Physical Properties Trend

• Chemical Properties Trend

• Anomalous Behaviour

• Diagonal Relationship

• Occurrence

• Industrial Use of Lime and Limestone

• Biological Importance of Na and K

• Biological Importance of Mg and Ca

• Preparation and Properties of Sodium Carbonate

• Preparation and Properties of Sodium hydrogencarbonate

• Preparation and Properties of NaCl, NaOH

Electronic Configuration of the s-Block Elements

Occurrence of the s-Block Elements

• Elements of s -block do not occur in a free state in nature. These are always found in the combined state, usually carbonates, sulphates, silicates, phosphates, etc.

• All the Group 1 elements are soft silvery-grey metals. The alkali metals are the most reactive of all metals so, being reactive in nature, are never found free in nature but always found in combined state. 

• Of all the alkali metals, only sodium and potassium are found in abundance in nature, i.e are 7th and 8th most abundant elements by weight in earth's crust. Their compounds are commonly known and used from very early times. The last member, Fr, occurs only in traces as a radioactive decay product because its half life period is very small.

• Among Group 2 elements, beryllium is not much abundant, but magnesium and calcium are quite abundant.

Anomalous Properties of the First Element of Group 1 and 2

Anomalous Behaviour of Lithium (Li)

Lithium, being the first member of the alkali group, exhibits the characteristic properties of alkali metals but it differs at the same time in many respects from them.

• The Li atom and its ion has very small size. Lithium ion (Li+) on account of its small size exerts high polarizing effect on anions in its compounds resulting in increased covalent character. 

• Lithium atom has highest ionization enthalpy and low electropositive character as compared to other alkali metals.

• Non-availability of d-orbitals in its valence shell.

• Lithium has strong metallic bonding which is responsible for its low reactivity to some extent. 

Anomalous Behaviour of Beryllium (Be)

• Beryllium differs from the rest of the alkaline earth metals on account of its small atomic size, high electronegativity and slight difference in electronic configuration. Be2+ is very small. It exerts a high polarizing effect on any anion associated with it. That is why beryllium compounds show covalent character. Compounds of Be have low melting points and are soluble in organic solvents. These are hydrolysed in water. 

Diagonal Relationship

Similarities Between Lithium and Magnesium

Lithium, an element of group 1 shows resemblance with magnesium, an element of group 2. This resemblance is called a diagonal relationship. 

• Electronegativities of Li and Mg are quite comparable. 

• Atomic radii and ionic radii of Li and Mg both are not very much different. 

• Atomic volumes of Lithium and Magnesium are quite similar. 

• Both have high polarizing power (ionic potential), and also both are hard metals.

Similarities Between Beryllium and Aluminium

Beryllium (Be) shows some similarities in properties with the aluminium (Al), the second  element of group 13 of the next higher period. 

• Both have similar polarizing power and both the metals are stable in air.

• Be and Al both have a strong tendency to form covalent compounds.

• Both do not impart any colour to the flame.

• Oxides and hydroxides of B and Al are amphoteric in nature.

• Anhydrous chlorides of both Be and Al i.e. BeCl2 and AlCl3 both act as Lewis acids.

Trends in the Variation of Properties

Trends in Chemical Reactivity With Water, Oxygen, Hydrogen and Halogens

Preparation and Properties of Some important Compounds of Na (Sodium Carbonate, Sodium Chloride, Sodium Hydroxide and Sodium Hydrogencarbonate)

Sodium Carbonate (Na2CO3)

• By electrolytic process, NaCl is first converted into NaOH. In the Nelson cell used for the manufacture of sodium hydroxide, carbon dioxide gas under pressure is blown along with steam. Hence, sodium hydroxide produced will then react with carbon dioxide to form sodium carbonate.

            2 NaOH + CO2 → Na2CO3 + H2O

• It is a white colour crystalline solid and is also known in several hydrated forms. The common form is decahydrate, Na2CO3.10H2O. This form is called washing soda. 

Sodium Chloride (NaCl)

• NaCl is obtained by evaporation of seawater in sun but due to the presence of impurities like CaSO4, CaCl2 and MgCl2, it is deliquescent and then purified by passing HCl gas through the impure saturated solution of NaCl then due to common ion effect, pure NaCl gets precipitated.

• NaCl is a white crystalline solid and is slightly hygroscopic. It is soluble in water. NaCl dissolves in water with the absorption of heat while insoluble in alcohol.

Sodium Hydroxide (NaOH)

• Causticisation (Gossage) process depends on the reaction between suspension ·of lime (milk of lime, calcium hydroxide) and sodium carbonate. This reaction is reversible. 

Na2CO3 + Ca(OH)2  ⇌  CaCO3 + 2NaOH

• NaOH is a white crystalline solid and has a soapy touch. It is highly deliquescent and is highly soluble in water and bitter in taste.  Its solubility is comparatively less in alcohol.  Also, it is corrosive in nature. 

Sodium Hydrogencarbonate (NaHCO3)

• NaHCO3 is obtained as the intermediate product in the Solvay ammonia soda process. Also, normal carbonate can be changed to bicarbonate by passing carbon dioxide through its saturated solution.

Na2CO3 + CO2 + H2O → 2NaHCO3 

• NaHCO3 is a white crystalline solid, sparingly soluble in water. The solution is alkaline in nature due to hydrolysis so the solution is weakly basic. On heating, NaHCO3 loses carbon dioxide and water forming sodium carbonate. 

Biological Importance of Sodium and Potassium

• ‘Sodium ions’ are found in the human cells such as Nerve cells and they regulate the flow of water across the membrane. Also, ‘Na’ is needed to transport the sugars and amino acids in the cells.

• Potassium ions are found in the cells. Also, regulate the stomata like opening and closing stomata. Potassium ions help to maintain the osmolarity of the cell. 

• Sodium and Potassium both maintain the electrolyte balance of the body. If the consumption of Na is high and consumption of K is low then there is a chance of heart disease and risk of death too.

• The function of the Sodium Potassium Pump is that it pumps the sodium and potassium ions against the concentration gradient. The sodium ions are pumped out while potassium enters in. A sodium potassium pump is important for nerve impulse transmission in the human body.

Industrial Use of Lime and Limestone 

• Limestone is a carbonate sedimentary rock that is formed predominantly on the seafloor where materials rich in calcium carbonates accumulate. Its main component is calcium carbonate (CaCO3) and also called limestone. 

• Limestone is used in industries as a flux in steel making and also for soil conditioning and latex treatment. And used in the manufacture of quicklime, slaked lime, cement, washing soda and glass.

• Calcium oxide (CaO) is also called quicklime or burnt lime. Quick lime is a commercial compound being used in various chemical industries such as caustic soda, in the purification of sugar, manufacture of dye stuff, bleaching powder, calcium carbide, calcium salts, mortar, cement, glass, etc. 

Biological Importance of Mg and Ca 

1. Calcium

• The presence of Calcium is generally found in vast quantities in our bones and also our teeth.

• Ca helps in the clotting of blood during an injury. Without an adequate amount of Calcium (Ca), this clot will take an exceptionally long time to form. 

• The presence of Ca helps in moderating the metabolic process of nitrogen found in plants. Without an adequate supply of Ca, the entire growth process of the plants will get impacted as well as the number of chloroplasts in them. 

• Calcium helps in stabilizing the permeability of cell membranes and also helps in the proper functioning of the heart and nerves.

2. Magnesium

• Chlorophyll, which is responsible for photogenic reactions in green plants containing magnesium, plays an important role in the process.

• Magnesium (Mg) is also essential for the proper functioning and stability of DNA and also for its synthesis.

• Mg is essential for maintaining electrolytes in the human body and plays an important role in biochemical reactions which are catalyzed by enzymes.

• Magnesium (Mg) is also essential in the manufacturing of adequate energy in the cells in our bodies.

Solved Examples From Chapter

Example 1: On the Basis of lattice energy and other considerations, which one of the following alkali metal chlorides is expected to have the highest melting point?

(a) LiCl                           (b) KCl                       (c) NaCI                   (d) RbCI

Solution: 

On the basis of lattice energy, the melting point decreases from top to bottom in the alkali metal group as lattice energy decreases with the increase of the atomic number. So, LiCl should have the highest melting point but LiCI has a covalent character due to the very small size of the Li+ ion. Therefore, the melting point of NaCl is the highest among the above chlorides. 

Thus, the correct answer is option (c) NaCl.

Key point to remember: When we move from top to bottom in the alkali metal group, lattice energy decreases therefore melting point also decreases. But here exception is with Li as it shows covalent character due to its small size.

Example 2: Which of the following compounds has the hydration energy higher than the lattice energy? 

(a) MgSO4                       (b) BaSO4                   (c) SrSO4                     (d) CaSO4

Solution: Smaller the ion, the higher the degree of hydration i.e., high is hydration energy. Thus, the hydration energy decreases as the size of the ion increases from Mg2+ to Ba2+. Here, the Mg2+ ion is the smallest ion and hence has the highest hydration energy. Thus, the correct answer is option (a) MgSO4.

Key point to remember: Hydration energy is defined as the energy released when one gram mole of an ion is dissolved in water to get it hydrated. The hydration energy of alkaline earth metal ions also decreases from top to bottom. 

Solved Questions From Previous Year Question Papers

Question 1:  From the following, which oxide is most acidic in nature?   

(a) BeO                                              (b) MgO                       

(c) CaO                                              (d) BaO

Solution: The given oxides are of group 2 elements which are metals. On moving down the group, the metallic character increases therefore basic character also increases. Hence, the order of basic character of given oxides will be: BeO < MgO < CaO < BaO. So, the most acidic oxide should be BeO. In fact, BeO is an amphoteric oxide while MgO, CaO, and BaO are basic oxides. 

Thus, the correct answer is option (a) BeO. 

Trick: Generally oxides of metals are basic in nature. When we move from top to bottom in a group, the basic character of oxides increases.

Question 2: Ionic mobility of which of the following alkali metal ions is lowest when the aqueous solution of their salts is put under an electric field?

(a) Na                   (b) Rb                  (c) Li                       (d) K

Solution: Generally, ionic mobility of alkali metal ions increases down the group. And Li+ being the smallest ion, has maximum charge density. Li+ is the most heavily hydrated among all given alkali metal ions. The effective size of Li+ in an aqueous solution is the largest. Hence, Li+ ion moves slowest under an electric field. 

Thus, the correct answer is an option (c) Li. 

Trick:  Ionic mobility of alkali metal ions increases from top to bottom in a group. Also, the smaller the size of the ion, the greater the degree of hydration, hence the lesser the ionic mobility.

Question 3: In which of the following, false statement is:

(a) Mg2+ ions are important in the green parts of the plants.

(b) Mg2+ ions form a complex with ATP.

(c) Ca2+ ions are important in the blood clotting.

(d) Ca2+ ions are not important in maintaining regular beating of the heart.

Solution: We know that the Mg2+ ion is the central atom in chlorophyll which imparts green colour to the plants. And we know that ATP is the most important form of energy currency in living systems. Mg2+ ions bind with ATP and form a complex in order to make ATP biologically active. 

Hence, options (a) and (b) are true. Ca2+ ions are  important  in  blood  clotting  and  are  also important  in  maintaining  the  regular  beating  of  the heart so that it can contract and pump out blood to the rest of the body. Hence, option (c) is true but option (d) is a false statement which is the correct answer.

Trick: Ca2+ ions are  important  in  blood  clotting  and  also important  in  maintaining  the regular beating of the heart.

Practice Questions

Question 1: Chemical ‘X’ is used for water softening to remove temporary hardness. Then ‘X’ reacts with sodium carbonate to generate caustic soda. When CO2 is bubbled through ‘X’, it changes into cloudy. What is the chemical formula of ‘X’?

(a) CaO                     (b) CaCO3                    (c) Ca(OH)2                      (d) Ca(HCO3)2 

Answer: (c) Ca(OH)2

Question 2: Which of the properties of alkali metals is not correct?

(a) The least electronegative metal: Cs

(b) A natural radioactive metal: Fr 

(c) The alkali metal with the lowest density: K

(d) The most abundant alkali metal in the earth's crust: Na

Answer: (c) The alkali metal with the lowest density: K

Conclusion

In this article, we studied about the alkali and alkaline earth metals and why they are called s-block elements. And we studied the diagonal relationship between second and third-period elements and the abnormal behaviour of the first element of each group. We also know the physical and chemical properties of the alkali and alkaline earth metals as well as their some common compounds.

Hence, this chapter is important not only for competitive exams like JEE or NEET but also for a better understanding of the study of s -block elements and their compounds.