Atomic Structure and Chemical Bonding

Introduction

Understanding the structure of the atom is fundamental to chemistry. ICSE Class 9 explores the evolution of atomic models, subatomic particles, and how atoms bond together to form compounds.

Daltons Atomic Theory

Postulates:

All matter is made up of tiny indivisible particles called atoms
Atoms of the same element are identical in all respects
Atoms of different elements are different
Atoms combine in simple whole-number ratios to form compounds
Atoms can neither be created nor destroyed in a chemical reaction

Limitations: Dalton believed atoms were indivisible. Later discoveries showed atoms contain subatomic particles.

Discovery of Subatomic Particles

Electron

Discoverer: J.J. Thomson (1897)
Charge: -1.6 × 10⁻¹⁹ C
Mass: 9.1 × 10⁻³¹ kg (negligible)
Location: Outside the nucleus
Experiment: Cathode ray tube experiment

Proton

Discoverer: E. Goldstein (1886)
Charge: +1.6 × 10⁻¹⁹ C
Mass: 1.67 × 10⁻²⁷ kg
Location: Inside the nucleus
Experiment: Anode (canal) ray experiment

Neutron

Discoverer: James Chadwick (1932)
Charge: 0 (neutral)
Mass: 1.67 × 10⁻²⁷ kg (similar to proton)
Location: Inside the nucleus

Rutherfords Model (1911)

The Gold Foil Experiment:

Alpha particles were fired at a thin gold foil
Most passed through; a few were deflected; a few bounced back

Conclusions:

Atom has a small, dense, positively charged nucleus (from deflections)
Most of the atom is empty space (from alpha particles passing through)
Electrons revolve around the nucleus

Limitations: Could not explain stability of the atom (electrons should spiral into nucleus)

Bohrs Model (1913)

Postulates:

Electrons revolve in fixed energy levels (orbits/shells)
Each shell has a fixed energy (K, L, M, N shells)
Electrons can jump between shells by absorbing or emitting energy

Bohrs model successfully explained the stability of the atom and line spectra.

Atomic Number and Mass Number

Atomic Number (Z)

Number of protons in the nucleus. Each element has a unique atomic number.

Mass Number (A)

Total number of protons and neutrons in the nucleus. A = Z + n

Notation: ^A_Z X

Isotopes

Atoms of the same element with the same atomic number but different mass numbers (different numbers of neutrons).

Isotope of Hydrogen	Protons	Neutrons	Mass Number
Protium (¹H)	1	0	1
Deuterium (²H)	1	1	2
Tritium (³H)	1	2	3

Examples: Carbon-12 (⁶C¹²), Carbon-14 (⁶C¹⁴)

Chemical Bonding

Electrovalent (Ionic) Bonding

Definition: Complete transfer of electrons from one atom to another, forming ions held by electrostatic attraction.

Conditions for formation:

Metal (electron donor) + Non-metal (electron acceptor)
Difference in electronegativity > 1.7

<ICSEExample title="Formation of NaCl"> <Solution> Na (2,8,1) loses 1 electron to become Na⁺ (2,8) Cl (2,8,7) gains 1 electron to become Cl⁻ (2,8,8) Electrostatic attraction forms NaCl. Electron dot structure: Na + Cl → Na⁺ [Cl]⁻ </Solution> </ICSEExample>

Covalent Bonding

Definition: Sharing of electrons between atoms.

Types:

Single bond: 1 pair shared (e.g., H₂, Cl₂, H₂O)
Double bond: 2 pairs shared (e.g., O₂, CO₂)
Triple bond: 3 pairs shared (e.g., N₂)

<ICSEExample title="Electron Dot Structure of Water (H₂O)"> <Solution> Oxygen (2,6) has 6 valence electrons Each H (1) has 1 valence electron O shares 1 electron with each H Structure: H-O-H with two single covalent bonds </Solution> </ICSEExample>

Comparison: Ionic vs Covalent Bonds

Feature	Ionic	Covalent
Electron transfer/sharing	Transfer	Sharing
Strength	Strong	Moderate
Melting point	High	Low
Solubility in water	Generally soluble	Generally insoluble
Electrical conductivity	Conducts when molten/dissolved	Does not conduct
Examples	NaCl, CaO	H₂O, CO₂, CH₄

Common Mistakes With Fixes

Mistake	Correction
Bohr model is the current model	The modern quantum mechanical model is more accurate
Protons and neutrons are equal in number	In small atoms yes, but larger atoms have more neutrons
Isotopes have different chemical properties	Isotopes have same chemical but different physical properties
Hydrogen bond is the same as covalent bond	Hydrogen bond is a weak intermolecular force, not a covalent bond

ICSE Exam Focus

Topic	Marks (approx.)	Frequency
Rutherford and Bohr models	4-5 marks	Very common
Ionic and covalent bonding	4-5 marks	Very common
Isotopes and their uses	3 marks	Common
Electron dot structures	3-4 marks	Frequently asked

Self-Test

Q1: State the postulates of Bohrs model of the atom.

Q2: Differentiate between ionic and covalent bonds with examples.

Q3: What are isotopes? Give two examples.

Q4: Draw the electron dot structure of methane (CH₄).

Q5: Write the atomic number, mass number, and number of neutrons in Carbon-12 and Carbon-14.

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