Atomic Structure

The philosophical inquiry into the atom originated as early as $600\text{ BCE}$ , with ancient Indian and Greek thinkers acting as pioneers in conceptualizing matter.
In ancient India, Maharishi Kannada propounded that all matter is composed of extremely small, indestructible particles termed Paramanu.
In ancient Greece, philosophers including Democritus, Epicurus, and Leucippus theorized that matter consists of minute particles called atoms.
The term 'atom' originates from the Greek language and translates directly to the 'incapability of being cut', reflecting the historical assumption that these fundamental units were entirely indestructible.
These classical philosophical ideas were speculative concepts rather than empirically verified theories.
Later scientific developments shifted toward experimentation, culminating in John Dalton's systematic formulation of an atomic theory.
Subsequent discoveries of subatomic particles demonstrated that atoms are divisible, which led to intensive research into the internal arrangement of these constituents and the formulation of modern atomic models.

In $1808$ , John Dalton proposed a formal atomic theory structured around the empirical laws of chemical combination known at the time.
The theory was widely accepted throughout the $19\text{th}$ century because it provided an explanation for the established laws of chemical combination.
The theoretical framework of Dalton's model is primarily based on:
- The law of conservation of mass (formulated by Lavoisier).
- The law of definite proportions.
- The law of multiple proportions.
Biographical detail: John Dalton was red-green colourblind, a physiological condition historically designated as 'Daltonism'.

Matter is composed of minute, indivisible particles termed atoms, which can neither be created, destroyed, nor transformed into atoms of another element.
All atoms of a given element are completely identical in all respects, including size, shape, and mass.
Atoms of different elements possess different properties and vary from one another.
Chemical reactions involve the combination of atoms of different elements in simple integral ratios to produce compounds.
When elements combine to form more than one compound, their atoms can combine in multiple simple whole-number ratios.

Advancements in experimental physics and chemistry contradicted several key premises of Dalton's model, prompting substantial modifications:
- Divisibility of the Atom: The discovery of subatomic particles—specifically electrons, protons, and neutrons—disproved the assertion that atoms are indivisible.
- Existence of Isotopes: The discovery of isotopes established that atoms of the same chemical element can possess different atomic weights, demonstrating that atoms of an element are not identical in every respect.
- Existence of Isobars: Certain atoms of entirely different elements possess identical mass numbers (e.g., calcium and argon).
Modern Validity and Enduring Legacy:
- Although Dalton could not explain subatomic divisibility, isotopic variation, or isobaric phenomena, his framework served as the foundation for modern atomic theory.
- Modern atomic theory retains Dalton's core postulate that atoms represent the smallest fundamental units of matter that participate directly in chemical reactions.
- The phenomenon of radioactivity was instrumental in revealing the internal subatomic and fundamental particles of the atom.

In $1879$ , Sir William Crookes conducted electrical discharge experiments using a specialized sealed glass apparatus known as a discharge tube.
When high voltage was applied across the two metal electrodes within the tube, distinct visible rays travelled between them.
These emissions were designated as Crookes's rays or cathode rays, providing direct evidence of subatomic components.
The eventual identification of the electron, proton, and neutron firmly categorized these constituents as fundamental subatomic particles.

An atomic model serves as a conceptual and visual description depicting the specific internal arrangement of fundamental particles inside an atom.
Thomson's Model ( $1903$ ):
- J. J. Thomson proposed the plum pudding atomic model in $1903$ .
- This model was formulated prior to the formal discovery and isolation of the proton.
Rutherford's Model ( $1911$ ):
- In $1911$ , Ernest Rutherford introduced the nuclear model of the atom based on empirical data collected from the $\alpha$ -particle scattering experiment.
- The experiments revealed a dense, central, positively charged core termed the nucleus.
- A major limitation of Rutherford's nuclear model was its inability to account for the overall stability of an atom.
Bohr's Model ( $1913$ ):
- In $1913$ , Niels Bohr introduced a revised atomic model featuring quantized electron orbits, successfully explaining the physical stability of the atom.

Atomic Number ( $Z$ ):
- The atomic number represents the total number of protons located within the nucleus of an atom.
- It is designated symbolically by the uppercase letter $Z$ .
Mass Number ( $A$ ):
- The mass number represents the aggregate sum of protons and neutrons residing within the nucleus of an atom.
- It is designated symbolically by the uppercase letter $A$ .

Isotopes:
- Isotopes are defined as atoms belonging to the same chemical element that possess identical atomic numbers but contain different numbers of neutrons (and consequently exhibit different mass numbers).
Isobars:
- Isobars are defined as atoms of completely different chemical elements that share the same mass number ( $A$ ) while possessing different atomic numbers ( $Z$ ).
- An example of an isobaric pair is calcium and argon.

Bohr-Bury Scheme:
- The maximum population of electrons that can be accommodated within any given electron orbit or shell is defined by the Bohr-Bury scheme of electronic configuration.
Geometrical Representation:
- The schematic or diagrammatic illustration showing the spatial distribution and arrangement of electrons across successive concentric shells in an atom is termed its geometrical representation.
Valence Shell and Valence Electrons:
- The valence shell of an atom is defined as its outermost occupied electron shell.
- Any electrons occupying this outermost shell are designated as valence electrons.