Structure of Atom: Complete Guide for Class 11 NCERT Chemistry

Discovery of Subatomic Particles: Protons, Neutrons, and Electrons

The atom, once thought indivisible, is made up of smaller particles:

• Electron: Discovered by J.J. Thomson through cathode ray experiments, electrons are negatively charged particles with very small mass.
• Proton: Identified from canal rays in modified cathode ray tubes, protons carry a positive charge (+1) and have a mass approximately 1836 times that of electrons.
• Neutron: Discovered by James Chadwick in 1932 by bombarding beryllium with alpha particles, neutrons are neutral particles with mass close to protons.

These discoveries explained the atom's positive charge and mass distribution, leading to the concept of the nucleus containing protons and neutrons, with electrons orbiting around it.

Atomic Number and Mass Number: Defining an Element

Two important numbers define any atom:

• Atomic Number (Z): Number of protons in the nucleus. It determines the element's identity.
• Mass Number (A): Total number of protons and neutrons in the nucleus.

For example, a carbon atom with 6 protons and 6 neutrons has $Z=6$ and $A=12$. This distinction helps in understanding isotopes and atomic behavior.

Millikan’s Oil Drop Experiment: Measuring Electron Charge

Millikan’s oil drop experiment was crucial in determining the fundamental charge of the electron:

• Tiny oil droplets were sprayed into a chamber and allowed to fall under gravity.
• X-rays ionized the air, charging the droplets.
• By applying an electric field, the motion of droplets was controlled and observed through a microscope.
• Balancing gravitational and electrical forces allowed calculation of the charge on droplets.

Millikan concluded that the charge $q$ on any droplet is an integral multiple of the elementary charge $e$:

$$ q = n e, \quad n = 1, 2, 3, ... $$

where $e = 1.6 \times 10^{-19}$ coulombs.

Atomic Models Explaining the Structure of Atom

Several atomic models have been proposed to explain the arrangement of subatomic particles:

• Dalton’s Model: Atom as indivisible particles.
• Thomson’s Plum Pudding Model: Electrons embedded in a positively charged sphere.
• Rutherford’s Nuclear Model: Atom has a small, dense positively charged nucleus with electrons orbiting around.
• Bohr’s Model: Electrons move in fixed energy orbits or shells around the nucleus.

Bohr’s model introduced the concept of quantized energy levels, explaining atomic spectra. This model laid the foundation for quantum mechanics.

Quantum Numbers and Electron Arrangement

Quantum numbers describe the unique state of an electron in an atom:

• Principal Quantum Number ($n$): Energy level or shell number.
• Azimuthal Quantum Number ($l$): Shape of the orbital.
• Magnetic Quantum Number ($m$): Orientation of the orbital.
• Spin Quantum Number ($s$): Electron spin direction (+1/2 or -1/2).

For example, the 11th electron of sodium has quantum numbers:

$$ n=3, \quad l=0, \quad m=0, \quad s=+\frac{1}{2} $$

These numbers help predict chemical properties and bonding behavior.

Worked Example: Calculating Number of Neutrons in an Atom

Problem: An atom has atomic number $Z=17$ and mass number $A=35$. Calculate the number of neutrons.

Solution:

Number of neutrons = Mass number - Atomic number

$$ = A - Z = 35 - 17 = 18 $$

So, the atom has 18 neutrons.

This calculation is essential to understand isotopes and nuclear stability.