Comprehensive NEET Study Notes: Structure of the Atom

The atom consists of three primary subatomic particles: protons, neutrons, and electrons.
Proton: - Symbol: $p^+$ - Charge: $+1$ - Relative Mass: $1$
Neutron: - Symbol: $n^0$ - Charge: $0$ (Neutral) - Relative Mass: $1$
Electron: - Symbol: $e^-$ - Charge: $-1$ - Relative Mass: $\frac{1}{1836}$

The model posits that the atom contains a small, dense, and positively charged nucleus located at its center.
The nucleus is composed of protons ( $p^+$ ) and neutrons ( $n^0$ ).
Electrons revolve around this central nucleus in specific circular paths.
A significant observation of this model is that most of the volume within an atom is empty space.

This model suggests that electrons revolve in fixed circular orbits characterized by definite energy levels.
Stationary States: In a stable orbit, no radiation is emitted by the electron.
Energy Transitions: Radiation (energy) is emitted or absorbed only when an electron undergoes a transition or "jump" from one orbit to another.
Energy Formula: The energy of the $n^{th}$ orbit is calculated using the following equation: - $E_n = - \frac{13.6 Z^2}{n^2} \, eV$ - Where $Z$ represents the Atomic Number and $n$ represents the Principal Quantum Number.

Atomic Number ( $Z$ ): This is equal to the number of protons in the nucleus. In a neutral atom, it also equals the number of electrons.
Mass Number ( $A$ ): This is the sum of protons and neutrons in the atom. - $A = \text{Protons} + \text{Neutrons}$
Isotopes: These are atoms of the same element that have the same Atomic Number ( $Z$ ) but different Mass Numbers ( $A$ ). - Example: Carbon-12 ( $^{12}_{6}C$ ) and Carbon-14 ( $^{14}_{6}C$ ).
Isobars: These are atoms of different elements that have the same Mass Number ( $A$ ) but different Atomic Numbers ( $Z$ ). - Example: Argon ( $^{40}_{18}Ar$ ) and Calcium ( $^{40}_{20}Ca$ ).

Mass Distribution: The mass of the electron is considered negligible. Consequently, the nucleus contains almost all the mass of the atom.
Atomic Size: The approximate size (diameter) of an atom is $10^{-10} \, m$ .
Nuclear Size: The approximate size of the nucleus is $10^{-15} \, m$ .

Electronic configuration describes the distribution of electrons in different shells or energy levels.
Case Study: Boron ( $Z=5$ ): - K shell ( $n=1$ ): Contains $2e^-$ - L shell ( $n=2$ ): Contains $3e^-$ - Final Configuration: $2, 3$

Understanding the structure of the atom is essential for several reasons: - It serves as the foundation for understanding chemical bonding. - It provides the explanation for periodic properties of elements in the Periodic Table. - It acts as the fundamental basis for modern atomic theory.

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Isotopes: These are atoms of the same element that have the same Atomic Number ( $Z$ ) but different Mass Numbers ( $A$ ).
- Example: Carbon-12 ( $^{12}_{6}C$ ) and Carbon-14 ( $^{14}_{6}C$ ).
Isobars: These are atoms of different elements that have the same Mass Number ( $A$ ) but different Atomic Numbers ( $Z$ ).
- Example: Argon ( $^{40}_{18}Ar$ ) and Calcium ( $^{40}_{20}Ca$ ).
Isoelectric: Atoms or ions that have the same number of electrons and therefore the same electronic structure but belong to different elements.
- Example: $Na^+$ and $Ne$ are isoelectric.
Isotones: Atoms of different elements that have the same number of neutrons but different numbers of protons (and thus different atomic numbers).
- Example: Carbon-14 ( $^{14}_{6}C$ ) and Boron-14 ( $^{14}_{5}B$ ) are isotones.