Structure of the Atom - Rediscovering the Roots of Atomic Theory

Dalton's Atomic Theory originally suggested that atoms were indivisible; however, the discovery of subatomic particles like electrons ($e^-$) and protons ($p^+$) led to its revision.

J.J. Thomson's Model (Plum Pudding Model) proposed that an atom consists of a positively charged sphere with electrons embedded in it, ensuring the atom is electrically neutral.

Rutherford's $\alpha$-particle scattering experiment revealed that most of the atom's space is empty and that the mass is concentrated in a tiny, positively charged center called the nucleus.

Bohr's Model of the Atom introduced discrete orbits or shells ($K, L, M, N$) where electrons revolve without radiating energy.

The Atomic Number ($Z$) is the total number of protons in the nucleus of an atom. For a neutral atom, $Z = \text{number of protons} = \text{number of electrons}$.

The Mass Number ($A$) is the sum of the total number of protons and neutrons (collectively called nucleons) in the nucleus: $A = Z + n$.

Valency is the combining capacity of an atom, determined by the number of electrons in its outermost shell. For atoms with $1$ to $4$ valence electrons, valency equals the number of valence electrons; for those with $5$ to $7$, it is often calculated as $8 - \text{valence electrons}$.

Isotopes are atoms of the same element with the same atomic number but different mass numbers (e.g., ${}_{1}^{1}\text{H}$, ${}_{1}^{2}\text{H}$, and ${}_{1}^{3}\text{H}$).

Isobars are atoms of different elements with the same mass number but different atomic numbers (e.g., ${}_{18}^{40}\text{Ar}$ and ${}_{20}^{40}\text{Ca}$).