The s-Block Elements: Comprehensive NEET Chemistry Notes

1. Introduction to the s-Block Elements

The s-block elements in the periodic table include the elements in Group 1 (alkali metals) and Group 2 (alkaline earth metals). These elements are characterized by the presence of their valence electrons in the s-orbital. The s-orbital can hold a maximum of two electrons, which is why these elements are found in the first two groups of the periodic table.

Did You Know?

Alkali metals are named because they form strong bases (alkalies) when they react with water, such as sodium hydroxide (NaOH).

2. Characteristics of Alkali Metals (Group 1)

2.1 General Properties

Alkali metals include lithium (Li), sodium (Na), potassium (K), rubidium (Rb), cesium (Cs), and francium (Fr). These elements are highly reactive, with one electron in their outermost s-orbital, leading to the general electronic configuration of [ $noble gas$ ] $n s^{1}$ .

2.2 Atomic and Ionic Radii

The atomic and ionic radii of alkali metals are the largest in their respective periods. The radii increase down the group due to the addition of an extra electron shell.

2.3 Ionization Enthalpy

Alkali metals have low ionization enthalpies, which decrease further down the group. This is because the outermost electron is farther from the nucleus, making it easier to remove.

2.4 Hydration Enthalpy

The hydration enthalpy of alkali metal ions decreases with increasing ionic size: $L i^{+} > N a^{+} > K^{+} > R b^{+} > C s^{+}$ . This explains why lithium salts are generally more soluble in water.

2.5 Physical Properties

Alkali metals are soft, silvery-white, and have low melting and boiling points. Their density increases down the group, with the exception of potassium, which is less dense than sodium.

Mnemonic:

"Little Naughty Kids Rub Cats Furiously" - This helps remember the order of alkali metals: Lithium, Sodium, Potassium, Rubidium, Cesium, Francium.

2.6 Chemical Properties

Alkali metals are highly reactive and form ionic compounds. They react vigorously with water to form hydroxides and hydrogen gas:

$2 M + 2 H_{2} O \to 2 MO H + H_{2}$

Common Misconception:

Alkali metals do not form stable diatomic molecules. Instead, they readily lose an electron to form cations.

2.7 Comparison with Alkaline Earth Metals

Compared to alkaline earth metals, alkali metals have larger atomic radii, lower ionization enthalpies, and higher reactivity. Alkaline earth metals, having two valence electrons, are generally less reactive and form less soluble hydroxides.

Visual Aid Recommendation:
Include diagrams comparing the reactivity, ionization energies, and solubility trends between alkali and alkaline earth metals.

Quick Recap:

Alkali metals have a single electron in their outermost shell, making them highly reactive.
They are characterized by large atomic and ionic sizes, low ionization energies, and strong basicity of their hydroxides.

3. Characteristics of Alkaline Earth Metals (Group 2)

3.1 General Properties

The alkaline earth metals include beryllium (Be), magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba), and radium (Ra). These elements have two electrons in their outermost s-orbital, leading to the general electronic configuration of [ $noble gas$ ] $n s^{1}$ .

3.2 Atomic and Ionic Radii

The atomic and ionic radii of alkaline earth metals are smaller than those of alkali metals in the same period. This is due to the higher nuclear charge, which pulls the electrons closer to the nucleus.

3.3 Ionization Enthalpy

The ionization enthalpies of alkaline earth metals are higher than those of alkali metals, but they still decrease down the group. The second ionization enthalpy is significantly higher than the first, which explains the +2 oxidation state.

3.4 Hydration Enthalpy

Similar to alkali metals, the hydration enthalpy decreases down the group for alkaline earth metals:

$B e^{2 +} > M g^{2 +} > C a^{2 +} > S r^{2 +} > B a^{2 +}$ .

3.5 Physical Properties

Alkaline earth metals are harder than alkali metals and have higher melting and boiling points. They are good conductors of electricity and exhibit a higher density.

3.6 Chemical Properties

These metals are less reactive than alkali metals but still react with water, though more slowly, to form hydroxides and hydrogen:

$M + 2 H_{2} O \to M (O H)_{2} + H_{2}$

Real-life Application:

Magnesium is used in the manufacture of lightweight alloys for aircraft and automobile industries, while calcium is crucial in the production of cement and steel.

Quick Recap:

Alkaline earth metals have two electrons in their outermost shell, which leads to a +2 oxidation state.
They are less reactive than alkali metals but still form strong bases when reacting with water.

4. Diagonal Relationship between Lithium and Magnesium

Lithium, the first element of the alkali metals, and magnesium, the second element of the alkaline earth metals, show similarities in their properties due to the diagonal relationship in the periodic table. Both have similar atomic and ionic radii, and both form compounds with significant covalent character.

Visual Aid Recommendation:
Include a periodic table diagram highlighting the diagonal relationship between lithium and magnesium.

Quick Recap:

The diagonal relationship is due to the similarity in ionic sizes and charge/radius ratios of the elements.
Lithium and magnesium show similar properties, such as forming covalent compounds and exhibiting low reactivity compared to their respective groups.

5. Practice Questions

Explain why lithium salts are generally more soluble in water compared to the salts of other alkali metals.
Compare the ionization energies of alkali metals and alkaline earth metals, explaining the trend down the group.
What is the diagonal relationship in the periodic table? Illustrate it using lithium and magnesium.
Predict the products and write a balanced equation for the reaction of potassium with water.
Describe the physical properties of alkaline earth metals and explain how they differ from alkali metals.