Why are electrons arranged in shells?

Electrons are arranged in shells due to the energy levels and the rules of quantum mechanics.

Electrons are tiny, negatively charged particles that orbit the nucleus of an atom. The arrangement of these electrons in shells, also known as energy levels, is determined by the principles of quantum mechanics. Quantum mechanics is a branch of physics that deals with phenomena on a very small scale, such as atoms and subatomic particles like electrons.

The first shell, closest to the nucleus, has the lowest energy level and can hold up to two electrons. The second shell can hold up to eight electrons, and the third can hold up to eight as well. This pattern continues with increasing energy levels and capacities in the outer shells. However, the distribution of electrons among these shells follows the 'Aufbau Principle', which states that electrons occupy the lowest energy level available first.

The arrangement of electrons in shells is crucial for the chemical behaviour of an atom. The outermost shell, also known as the valence shell, determines an atom's reactivity. If the valence shell is full, the atom is stable and less likely to react with other atoms. If it is not full, the atom can react with others to fill its valence shell, leading to the formation of chemical bonds and molecules.

In summary, the arrangement of electrons in shells is a fundamental aspect of atomic structure, governed by the principles of quantum mechanics. This arrangement determines the energy levels of the electrons and plays a key role in the chemical reactivity of atoms.