Comprehensive Study Notes on the Structure of the Atom

Everything observed, felt, or seen around us is classified as matter.
Matter is composed of extremely tiny particles known as atoms, which are so small that they cannot be perceived by the naked eye.
Both living entities (human body, organs, tissues, cells, proteins) and non-living objects (houses, rooms, walls, bricks, silicates) are ultimately constructed from atoms.
Fundamental questions that drove scientific inquiry include whether the atom is the smallest indivisible unit, what its constituents are, how they are arranged, and why atomic models have evolved over time.

Atomic theory dates back more than $2,000$ years to ancient India and Greece, where thinkers pondered the fundamental composition of existence.
Acharya Kanada (Ancient India):
- Suggested that repeated division of matter (dravya) eventually leads to a stage where particles can no longer be divided.
- He termed these smallest particles parmanus.
- These ideas were recorded in the Sanskrit text Vaisesika Sutras.
- Parmanus are infinitely small and cannot be detected by the senses.
- Combinations of parmanus form dyads (groups of two) and triads (groups of three); these combinations create the material universe and living bodies.
Leucippus and Democritus (Ancient Greece):
- Proposed similar indivisible particles and called them atomos, which means "indivisible" in Greek.
- It is critical to note that the early concept of the atom originated as an imaginary or philosophical idea rather than from experimental observation.

In $1808$ , John Dalton proposed the first scientific description of matter's composition based on experimental observations.
Key postulates included:
- All matter is composed of indivisible particles called atoms.
- Atoms are the fundamental building blocks of matter that cannot be broken down into smaller parts.
Dalton's theory served as the foundation for the current understanding of atomic structure.

Discovery of Radioactivity: In the late $19^{th}$ century, the discovery that certain elements emit invisible energy and particles (radiation) proved that atoms were not indivisible.
J. J. Thomson and Cathode Rays ( $1897$ ):
- Thomson studied the conduction of electric current through gases at very low pressure using a glass tube with two electrodes and high voltage.
- He observed rays moving from the cathode (negative electrode) to the anode (positive electrode), termed cathode rays.
- Studies in electric and magnetic fields showed these rays were streams of negatively charged particles with a mass much smaller than the atom.
- These particles were named electrons.
- The nature of cathode rays was found to be independent of the cathode material or the gas used, proving electrons are a fundamental component of all atoms.
- The charge of an electron is $-1.602 \times 10^{-19}\,C$ , taken as $-1$ by convention.
The Plum Pudding Model (Thomson’s Model):
- Thomson proposed the atom as a sphere of positive charge with electrons distributed throughout, balancing the charges to make the atom neutral.
- It is compared to a plum pudding with embedded plums or a watermelon where the red pulp is the positive charge and the seeds represent electrons.

The Alpha-Ray Scattering Experiment ( $1911$ ):
- Conducted by Hans Geiger and Ernest Marsden under the guidance of Ernest Rutherford.
- A narrow beam of alpha ( $\alpha$ ) particles (positively charged helium nuclei containing $2$ protons and $2$ neutrons) was aimed at a very thin gold foil.
- According to Thomson’s model, particles were expected to pass straight through or deflect slightly.
Observations and Conclusions:
- Most $\alpha$ -particles passed through undeflected, indicating that most of the atom is empty space.
- Small fractions were deflected by large angles, and a few ( $1$ in $12,000$ ) bounced back entirely.
- Rutherford concluded the positive charge and nearly all the mass are concentrated in a tiny region called the nucleus.
- The nucleus is approximately $10^5$ (one lakh) times smaller than the atom.
- Diameter of the atom is approximately $10^{-10}\,m$ ; diameter of the nucleus is approximately $10^{-15}\,m$ .
- Analogy: If an atom were the size of a cricket ground ( $100\,m$ across), the nucleus would be a tiny black pepper grain at the center.
The Planetary Model:
- Electrons revolve around the nucleus like planets orbiting the Sun.

Instability Issue: According to classical physics, an accelerating charged particle (like an electron in circular motion) should lose energy. This would cause it to spiral toward and eventually collapse into the nucleus, making matter unstable. Since matter is stable, Rutherford's model was considered incomplete.
Discovery of the Proton: Rutherford showed the nucleus carries a positive charge from particles called protons. Protons are much heavier than electrons but have an equal and opposite charge ( $+1$ ). For neutrality, the number of protons must equal the number of electrons.