State the assumptions of Shell Model.

ANSWER : The shell model, also known as the Bohr–Sommerfeld Model, was primarily developed by Danish physicist Niels Bohr in 1913. Bohr's model was an extension of the earlier Rutherford model, which depicted the atom as a small, dense nucleus surrounded by orbiting electrons. Bohr's shell model laid the foundation for understanding atomic structure and electron behaviour. The shell model is an atomic model which primarily describes the arrangement of electrons in an atom. Its assumptions include :

1. Electron Shells : Electrons occupy distinct energy levels called shells, denoted by the principal quantum number (n). The innermost shell is closest to the nucleus and has the lowest energy.
2. Energy Levels : Each shell has a maximum number of electrons it can hold, given by 2n². For example, the first shell (n = 1) can hold up to 2 electrons, the second shell (n = 2) can hold up to 8 electrons, and so on.
3. Aufbau Principle : Electrons fill the lowest available energy level first before occupying higher energy levels.
4. Pauli Exclusion Principle : No two electrons in an atom can have the same set of quantum numbers. This means each electron must have a unique combination of its four quantum numbers (n, l, m_l, m_s).
5. Hund's Rule : When filling degenerate orbitals (orbitals with the same energy level), electrons will occupy separate orbitals with parallel spins before pairing up.
6. Electron Orbitals : Within each shell, there are subshells or orbitals, which are regions where electrons are likely to be found. The number of subshells in a shell is equal to the principal quantum number (n). The subshells are labeled using letters: s, p, d, f, and so on.

These assumptions help us understand the distribution of electrons in different energy levels, subshells, and orbitals within an atom, providing a basis for predicting and explaining various atomic properties and chemical phenomena.

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