Sodium (Na): Comprehensive Study Guide

Introduction

Sodium (Na) is an essential element, crucial not only in fundamental chemistry but also in various industrial applications and biological systems. From its presence in common table salt to its vital role in human physiology and its use in advanced technologies, understanding sodium’s properties and reactions is fundamental for any chemistry student.

CBSE/JEE Quick Revision Notes

• Symbol: Na
• Atomic Number (Z): 11
• Atomic Mass (A): 22.99 g/mol
• Electronic Configuration: $1s^2 2s^2 2p^6 3s^1$ or [Ne] 3s^1
• Period: 3
• Group: 1 (Alkali Metals)
• Block: s-block
• Valency: +1
• Common Oxidation State: +1
• Nature: Soft, silvery-white, highly reactive metal. Excellent conductor of heat and electricity.
• Density: Less dense than water (0.97 g/cm³).
• Melting Point: Low ($97.8 ^\circ C$).
• Boiling Point: Low ($883 ^\circ C$).
• Flame Colour: Golden yellow (due to excitation of 3s electron).
• Occurrence: Never found free in nature due to high reactivity. Found extensively as NaCl (common salt), NaNO₃ (Chile saltpetre), Na₂CO₃ (Trona).
• Storage: Stored under kerosene or paraffin oil to prevent reaction with air and moisture.

Electron Configuration & Bonding Behavior

Sodium’s electron configuration $1s^2 2s^2 2p^6 3s^1$ indicates a single valence electron in its outermost 3s orbital.

• Valence Electron: 1
• Ionization Energy: Sodium possesses a very low first ionization energy ($495.8 \text{ kJ/mol}$) due to the single electron being far from the nucleus and shielded by inner electron shells. This makes it easy for sodium to lose this electron.
• Electronegativity: It has a low electronegativity (0.93 on the Pauling scale), signifying its tendency to lose electrons.
• Bonding: Sodium readily loses its single valence electron to form a unipositive cation, $Na^+$, achieving a stable noble gas configuration (that of Neon, [Ne]). Consequently, sodium predominantly forms ionic bonds with non-metals, particularly with highly electronegative elements like halogens and oxygen.

Crucial Chemical Reactions

1. Reaction with Air/Oxygen

Sodium reacts vigorously with oxygen. Depending on the availability of oxygen, it forms different oxides.

• With limited oxygen (forming oxide): $4Na(s) + O_2(g) \rightarrow 2Na_2O(s)$ (Sodium oxide)
• With excess oxygen (forming peroxide): $2Na(s) + O_2(g) \rightarrow Na_2O_2(s)$ (Sodium peroxide)

2. Reaction with Water

Sodium reacts violently and exothermically with water, producing sodium hydroxide and hydrogen gas. The reaction is so vigorous that the released hydrogen gas can ignite due to the heat generated. $2Na(s) + 2H_2O(l) \rightarrow 2NaOH(aq) + H_2(g) + \text{Heat}$

3. Reaction with Halogens

Sodium reacts readily with halogens to form ionic halides.

• With Chlorine: $2Na(s) + Cl_2(g) \rightarrow 2NaCl(s)$ (Sodium chloride)
• With Bromine: $2Na(s) + Br_2(l) \rightarrow 2NaBr(s)$ (Sodium bromide)

4. Reaction with Acids

Sodium reacts extremely vigorously and dangerously with dilute acids, producing salt and hydrogen gas. $2Na(s) + 2HCl(aq) \rightarrow 2NaCl(aq) + H_2(g)$

5. Reaction with Ammonia

• With liquid ammonia (forming a blue solution): Sodium dissolves in liquid ammonia to form a deep blue solution which is electrically conductive. This solution contains solvated electrons and solvated sodium ions. $Na(s) + (x+y)NH_3(l) \rightarrow [Na(NH_3)_x]^+ + [e(NH_3)_y]^-$
• With gaseous ammonia (at high temperature, forming sodamide): At elevated temperatures, sodium reacts with gaseous ammonia to produce sodamide ($NaNH_2$) and hydrogen gas. $2Na(s) + 2NH_3(g) \xrightarrow{573K} 2NaNH_2(s) + H_2(g)$

Industrial and Biological Importance

Industrial Importance

• Production of Sodium Compounds: Sodium is the primary raw material for manufacturing essential industrial chemicals such as sodium hydroxide (NaOH, Caustic Soda), sodium carbonate (Na₂CO₃, Washing Soda), sodium bicarbonate (NaHCO₃, Baking Soda), and sodium peroxide (Na₂O₂).
• Sodium Vapour Lamps: Used in street lighting due to their characteristic bright golden-yellow light, which has high luminosity and penetrates fog well.
• Reducing Agent: Employed as a strong reducing agent in various organic and inorganic reactions (e.g., Wurtz reaction for alkane synthesis, reduction of esters in Bouveault-Blanc reduction).
• Coolant in Nuclear Reactors: Liquid sodium is an excellent heat transfer fluid and is used as a coolant in fast breeder nuclear reactors due to its low melting point, high thermal conductivity, and low neutron absorption cross-section.
• Production of Metals: Historically used in the extraction of certain metals like titanium and zirconium from their compounds.

Biological Importance

• Major Extracellular Cation: Sodium ions ($Na^+$) are the most abundant cations in the extracellular fluid of animals, playing a crucial role in maintaining osmotic balance and fluid volume outside cells.
• Nerve Impulse Transmission: Essential for the generation and transmission of nerve impulses through the action of the sodium-potassium pump ($Na^+/K^+$ pump), which actively transports $Na^+$ out of cells and $K^+$ into cells.
• Muscle Contraction: Involved in the electrochemical gradients necessary for muscle cell depolarization and contraction.
• Nutrient Absorption: Facilitates the absorption of glucose, amino acids, and water in the intestines and kidney tubules via co-transport mechanisms.
• Blood Pressure Regulation: Plays a significant role in regulating blood pressure. Imbalances in sodium levels can lead to hypertension (high blood pressure) or hypotension (low blood pressure).