Chapter 4: The Periodic Table of Elements

4.1 Development of the Periodic Table of Elements

The modern Periodic Table arranges elements in order of increasing proton number, allowing systematic study of physical and chemical properties.

Historical Contributions

• Antoine Lavoisier: Classified elements into four groups (gases, non-metals, metals, earths). Limited because heat and light were included as elements.
• Johann W. Döbereiner: Arranged elements into Triads (groups of three elements with similar chemical properties).
• John Newlands: Arranged elements in order of increasing atomic mass and discovered the Law of Octaves (properties repeated every eighth element).
• Dmitri Mendeleev: Arranged elements in order of increasing atomic mass and grouped elements with similar properties together. Left blank spaces for undiscovered elements and predicted their properties.
• Henry Moseley: Arranged elements based on increasing proton number, which forms the basis of the modern Periodic Table.

4.2 Arrangement of Elements in the Modern Periodic Table

• Groups (Vertical Columns): 18 vertical columns (Group 1 to Group 18). Elements in the same group have the same number of valence electrons, resulting in similar chemical properties.
• Periods (Horizontal Rows): 7 horizontal rows (Period 1 to Period 7). The period number indicates the number of electron shells occupied with electrons.

4.3 Elements in Group 18 (Noble Gases)

Group 18 elements include Helium, Neon, Argon, Krypton, Xenon, and Radon.

Physical and Chemical Properties

• Inert and chemically unreactive because they have achieved a stable duplet (Helium, $2$) or octet ($2.8$, $2.8.8$) electron arrangement.
• Do not gain, lose, or share electrons under standard conditions; exist as monatomic gases.
• Physical properties (density, melting point, boiling point) increase down the group due to increasing atomic size and stronger van der Waals forces.

Uses of Noble Gases

• Helium: Filling weather balloons and deep-sea diving gas tanks.
• Neon: Advertising sign lights.
• Argon: Filling filament light bulbs and providing an inert atmosphere in welding.
• Krypton: Flash bulbs for photography.
• Xenon: Strobe lights and lighthouse lamps.
• Radon: Radiotherapy for treating cancer.

4.4 Elements in Group 1 (Alkali Metals)

Group 1 elements include Lithium ($\text{Li}$), Sodium ($\text{Na}$), Potassium ($\text{K}$), Rubidium ($\text{Rb}$), Caesium ($\text{Cs}$), and Francium ($\text{Fr}$).

Physical Properties

• Soft metals with low density and low melting/boiling points compared to other metals.
• Going down the group: Atomic radius increases, melting/boiling points decrease (metallic bonding weakens).

Chemical Properties and Reactivity

• All have 1 valence electron and readily lose 1 electron to form $+1$ charged cations ($\text{M} \rightarrow \text{M}^+ + e^-$).
• Reactivity increases down the group: As atomic radius increases, valence electron is located further from nucleus; attraction force weakens, making it easier to lose the valence electron.
• Reactions:
  • Reaction with Water: Produces alkaline metal hydroxide solution and hydrogen gas. $$2\text{M}(s) + 2\text{H}_2\text{O}(l) \rightarrow 2\text{MOH}(aq) + \text{H}_2(g)$$
  • Reaction with Oxygen Gas: Produces solid white metal oxide. $$4\text{M}(s) + \text{O}_2(g) \rightarrow 2\text{M}_2\text{O}(s)$$
  • Reaction with Chlorine Gas: Produces white solid metal chloride. $$2\text{M}(s) + \text{Cl}_2(g) \rightarrow 2\text{MCl}(s)$$

4.5 Elements in Group 17 (Halogens)

Group 17 elements include Fluorine ($\text{F}$), Chlorine ($\text{Cl}$), Bromine ($\text{Br}$), Iodine ($\text{I}$), and Astatine ($\text{At}$). Exist as diatomic molecules ($\text{X}_2$).

Physical Properties and Trends Down Group

• Physical states at room temperature: Fluorine and Chlorine (gas), Bromine (liquid), Iodine (solid).
• Color darkens down the group: Fluorine (pale yellow), Chlorine (greenish-yellow), Bromine (reddish-brown), Iodine (purplish-black solid/purple vapor).
• Melting/boiling points increase down the group due to larger molecular size and stronger van der Waals forces.

Chemical Properties and Reactivity

• All have 7 valence electrons and gain 1 electron to form $-1$ halide anions ($\text{X} + e^- \rightarrow \text{X}^-$).
• Reactivity decreases down the group: Atomic radius increases, nuclear attraction force on incoming electron weakens.
• Reactions:
  • Reaction with Water: Dissolves to form acidic solutions (e.g., $\text{Cl}_2 + \text{H}_2\text{O} \rightleftharpoons \text{HCl} + \text{HOCl}$).
  • Reaction with Metals (e.g., Iron wool): Forms brown iron(III) halides ($2\text{Fe} + 3\text{X}_2 \rightarrow 2\text{FeX}_3$).
  • Reaction with Sodium Hydroxide: Forms water, metal halide, and metal halate(I).

4.6 Elements in Period 3

Period 3 contains: Sodium ($\text{Na}$), Magnesium ($\text{Mg}$), Aluminium ($\text{Al}$), Silicon ($\text{Si}$), Phosphorus ($\text{P}$), Sulfur ($\text{S}$), Chlorine ($\text{Cl}$), and Argon ($\text{Ar}$).

Trends Across Period 3 (Left to Right)

• Atomic Radius: Decreases across the period because proton number increases (increasing positive nuclear charge), drawing electron shells closer to the nucleus.
• Electronegativity: Increases across the period.
• Oxide Properties:
  • Basic Oxides: $\text{Na}_2\text{O}$, $\text{MgO}$ (react with acids to form salt + water).
  • Amphoteric Oxide: $\text{Al}_2\text{O}_3$ (reacts with BOTH acids and alkalis).
  • Acidic Oxides: $\text{SiO}_2$, $\text{P}_4\text{O}_{10}$, $\text{SO}_2$, $\text{Cl}_2\text{O}_7$ (react with alkalis to form salt + water).

4.7 Transition Elements

Elements in Groups 3 to 12. All transition elements are metals with specific characteristic properties.

Special Characteristics of Transition Elements

• Form colored compounds (e.g., $\text{Cu}^{2+}$ is blue, $\text{Fe}^{2+}$ is green, $\text{Fe}^{3+}$ is brown, $\text{MnO}_4^-$ is purple).
• Exhibit variable oxidation states (e.g., $\text{Fe}^{2+}$ and $\text{Fe}^{3+}$, $\text{Cu}^+$ and $\text{Cu}^{2+}$).
• Form complex ions (e.g., hexaaquairon(III) ion).
• Act as catalysts in industrial processes:
  • Iron ($\text{Fe}$): Haber Process (synthesis of ammonia, $\text{NH}_3$).
  • Vanadium(V) oxide ($\text{V}_2\text{O}_5$): Contact Process (manufacture of sulfuric acid, $\text{H}_2\text{SO}_4$).
  • Platinum ($\text{Pt}$): Ostwald Process (manufacture of nitric acid, $\text{HNO}_3$).
  • Nickel ($\text{Ni}$): Hydrogenation of vegetable oils to margarine.