S Block Elements

s-Block elements are the elements in which the last electron enters the s-orbital of the outermost shell. Because of this, their outer electronic configuration ends in ns¹ or ns². These elements are placed on the left side of the modern periodic table. e s-block consists mainly of Group 1 and Group 2 elements. Hydrogen and helium also have valence electrons in the s-orbital, although helium is placed in Group 18 because of its noble gas character.

1. Alkali Metals (Group 1)

• Group 1 elements are called alkali metals because they form strongly alkaline hydroxides when they react with water.
• Alkali metals are highly reactive because they can easily lose their single valence electron to form positive ions.
• The elements of Group 1 have 1 valence electron in their outermost s-orbital.

Examples: Lithium (Li) , Sodium (Na) , Potassium (K) , Rubidium (Rb)

2. Alkaline Earth Metals (Group 2)

• They are called alkaline earth metals because their oxides and hydroxides are basic (alkaline) in nature and they are found in the earth’s crust.

• The elements of Group 2 have 2 valence electrons in their outermost s-orbital.

Examples: Beryllium (Be) , Magnesium (Mg) , Calcium (Ca) , Strontium (Sr)

Physical Properties of s-Block Elements

The physical properties of s-block elements are largely influenced by their large atomic size, low ionization enthalpy, and the presence of only one or two valence electrons. As a result, these elements exhibit typical metallic characteristics such as lustre, good conductivity, and relatively low melting and boiling points.

1. Metallic Nature: Except hydrogen, all s-block elements are metallic in nature. They possess a shiny surface and are good conductors of heat and electricity. They are also malleable and ductile.

2. Softness: Alkali metals are soft and can be cut easily with a knife, whereas alkaline earth metals are comparatively harder. The softness generally increases down the group as atomic size increases.

3. Density: Most s-block elements have relatively low densities because of their large atomic sizes. However, the density generally increases down the group with a few irregularities.

4. Low Melting and Boiling Points: The melting and boiling points of s-block elements are generally lower than those of many other metals. Alkali metals have lower melting and boiling points than alkaline earth metals due to weaker metallic bonding.

5. Flame Colour: Many s-block elements impart characteristic colours to a flame because their electrons can be easily excited. These colours are useful in identifying the elements.

Chemical Properties of s-Block Elements

The chemical properties of s-block elements are mainly due to their low ionization energy and strong tendency to lose electrons. Their reactivity increases down the group because atomic size increases and ionization energy decreases.

1. Reaction with Oxygen: S-block elements readily react with oxygen to form oxides. The nature of the oxides formed varies among different elements and generally depends on atomic size.

Examples: Sodium
4Na + O₂ → 2Na₂O

Exception: Lithium forms normal oxide (Li₂O) , Sodium forms peroxide (Na₂O₂ ) , Potassium forms superoxide (KO₂ )

2. Reaction with Water: Most s-block elements react with water to produce hydroxides and hydrogen gas. The vigour of the reaction increases as we move down the group.

Examples: Calcium
Ca + 2H₂O → Ca(OH)₂ + H₂↑

Exception:

• Beryllium does not react with water.
• Magnesium reacts very slowly with cold water but reacts with steam:

3. Reaction with Halogens: These elements combine directly with halogens to form ionic halides. The resulting compounds are generally stable and possess high melting points.

Example: Magnesium reacts with Chlorine to form Magnesium Chloride

Mg + Cl₂ → MgCl₂

4. Formation of Basic Oxides and Hydroxides: The oxides and hydroxides of s-block elements are generally basic in nature. Their basic strength usually increases down the group.

Example: Calcium oxide react with water to form Calcium Hydroxide

CaO + H₂O → Ca(OH)₂

Exception: BeO is amphoteric (reacts with both acids and bases).

5. Formation of Ionic Compounds: Because of their strong tendency to lose electrons, s-block elements predominantly form ionic compounds. The compounds formed are usually stable and electrovalent in character.

Electronic Configuration

The electronic configuration of s-block elements explains how electrons are arranged in their atoms. These elements are called s-block elements because the last electron enters the s-orbital of the outermost shell. The s-orbital can hold a maximum of two electrons.

Their general outer electronic configuration is:

• Group 1 → ns¹
• Group 2 → ns²

Here, “n” represents the number of the outermost shell.

Diagonal Relationship of s-Block Elements

Diagonal relationship is the similarity in properties between certain elements that are placed diagonally in the periodic table.

• When we move across a period, atomic size decreases.
• When we move down a group, atomic size increases.

In the s-block, the most important diagonal relationship is between:

Lithium (Li) and Magnesium (Mg)

Although Li is in Group 1 and Mg is in Group 2, they show similar behavior.

• Both form nitrides when heated with nitrogen:

6Li + N₂ → 2Li₃N
3Mg + N₂ → Mg₃N₂

• Both preferentially form normal oxides rather than peroxides or superoxides.
• Their carbonates decompose on heating:

Li₂CO₃ → Li₂O + CO₂
MgCO₃ → MgO + CO₂

• Their hydroxides are less soluble compared to other group members.

Alkali Metals vs Alkaline Earth Metals

Related Articles:

• Important Compounds of Sodium
• Important Compounds of Calcium