Magnesium (Mg): A Comprehensive Study Guide

Introduction

Magnesium (Mg) is a critical element with diverse applications, essential for both industrial processes and biological systems. Its light weight, reactivity, and role in fundamental life processes make it a frequently tested topic in chemistry examinations. From providing structural integrity to alloys to powering photosynthesis in plants, Magnesium’s significance extends across various domains.

CBSE/JEE Quick Revision Notes

Key properties and facts for rapid recall:

• Symbol: Mg
• Atomic Number: 12
• Atomic Mass: 24.305 u
• Group: 2 (Alkaline Earth Metals)
• Period: 3
• Block: s-block
• Valency: +2 (forms Mg²⁺ ion)
• Oxidation State: Primarily +2
• Nature: Silvery-white, lightweight, relatively soft, reactive metal.
• Electronegativity (Pauling scale): 1.31
• Density: 1.738 g/cm³ (at 20°C)
• Occurrence: Abundant in nature, primarily found in minerals like magnesite (MgCO₃), dolomite (CaCO₃·MgCO₃), and carnallite (KCl·MgCl₂·6H₂O). Also present in seawater.

Electron Configuration & Bonding Behavior

Magnesium’s electronic structure dictates its chemical reactivity.

• Electronic Configuration: 1s² 2s² 2p⁶ 3s² or [Ne] 3s²
• Valence Electrons: 2 electrons in the 3s orbital.
• Ionization Energy: Magnesium has relatively low first and second ionization energies due to the ease of removing two valence electrons to achieve a stable noble gas configuration (isoelectronic with Neon).
  • Mg → Mg⁺ + e⁻ (First Ionization Energy)
  • Mg⁺ → Mg²⁺ + e⁻ (Second Ionization Energy)
• Bonding: Due to its tendency to readily lose two electrons, Magnesium primarily forms ionic compounds where it exists as the Mg²⁺ ion. For example, MgCl₂ and MgO are ionic. In some organometallic compounds, such as Grignard reagents (RMgX), the Mg-C bond exhibits significant covalent character.

Crucial Chemical Reactions

Understanding Magnesium’s reactions is vital for examination success. All equations must be balanced.

1. Reaction with Oxygen (Air)

Magnesium burns in air with a dazzling white flame to form magnesium oxide.

• 2Mg(s) + O₂(g) → 2MgO(s)

2. Reaction with Nitrogen

At high temperatures, Magnesium reacts with nitrogen to form magnesium nitride.

• 3Mg(s) + N₂(g) → Mg₃N₂(s)

3. Reaction with Water

Magnesium reacts slowly with cold water, but vigorously with steam.

• With Cold Water: Mg(s) + 2H₂O(l) → Mg(OH)₂(s) + H₂(g)
• With Steam: Mg(s) + H₂O(g) → MgO(s) + H₂(g)

4. Reaction with Acids

Magnesium reacts readily with dilute acids, producing magnesium salt and hydrogen gas.

• Mg(s) + 2HCl(aq) → MgCl₂(aq) + H₂(g)
• Mg(s) + H₂SO₄(aq) → MgSO₄(aq) + H₂(g) (Note: Reaction with dilute nitric acid varies based on concentration, producing H₂, N₂O, NO, or NH₄NO₃.)

5. Reaction with Halogens

Magnesium reacts with halogens to form magnesium halides.

• Mg(s) + Cl₂(g) → MgCl₂(s)

6. Reaction with Carbon Dioxide

Once ignited, Magnesium can continue to burn in carbon dioxide, reducing CO₂ to carbon.

• 2Mg(s) + CO₂(g) → 2MgO(s) + C(s)

Industrial and Biological Importance

Industrial Applications

• Alloys: Magnesium’s low density makes its alloys (e.g., with aluminium) crucial for lightweight construction in aerospace, automotive, and sports equipment industries.
• Pyrotechnics and Flares: Due to its bright white flame upon burning, magnesium powder is used in fireworks, flares, and incendiary devices.
• Reducing Agent: Used in the metallurgy of other metals, such as in the Kroll process for the production of titanium from titanium tetrachloride: TiCl₄ + 2Mg → Ti + 2MgCl₂
• Grignard Reagents: Organomagnesium compounds (RMgX, where R is an alkyl or aryl group and X is a halogen) are vital in organic synthesis for forming new carbon-carbon bonds.
• Sacrificial Anodes: Magnesium’s high reactivity makes it an effective sacrificial anode for cathodic protection of iron and steel structures (e.g., pipelines, storage tanks) against corrosion.

Biological Importance

• Chlorophyll: Magnesium is the central metal ion in the chlorophyll molecule, the pigment essential for photosynthesis in plants. Without magnesium, plants cannot photosynthesize.
• Human Body: It is an essential mineral nutrient and a co-factor for over 300 enzyme systems, regulating diverse biochemical reactions in the body, including:
  • ATP synthesis and energy metabolism
  • Protein synthesis
  • DNA and RNA synthesis
  • Muscle contraction and nerve function
  • Blood glucose control
  • Blood pressure regulation
• Bone Structure: Magnesium contributes to the structural development of bone and is involved in the transport of calcium and potassium ions across cell membranes.