Electrons are found outside the nucleus of an atom because of the fundamental forces at play in atomic structure. The behavior of electrons and their distribution around the nucleus is governed by the principles of quantum mechanics and the electromagnetic force.
In an atom, the nucleus is composed of positively charged protons and uncharged neutrons. According to the electromagnetic force, particles with opposite charges attract each other. Therefore, one might expect the negatively charged electrons to be attracted to the positively charged nucleus, causing them to collapse into it.
However, the behavior of electrons is better described by quantum mechanics. Quantum mechanics introduces the concept of energy levels or orbitals, which represent the probability distribution of finding an electron in a specific region around the nucleus. These orbitals are associated with specific energy states, and electrons occupy them in accordance with the Pauli exclusion principle and the aufbau principle.
The Pauli exclusion principle states that no two electrons within an atom can have the same set of quantum numbers. This principle leads to the organization of electrons into different energy levels and sublevels, forming electron shells and subshells around the nucleus. Each shell can hold a specific number of electrons.
The aufbau principle dictates that electrons fill the lowest energy orbitals first before occupying higher energy levels. This filling pattern results in electrons occupying different energy levels and being distributed around the nucleus in a specific manner.
The electron distribution is such that electrons occupy orbitals that minimize their overall energy within the atom. Electrons are found in different shells and subshells, which have specific shapes and orientations. The electron density is highest in regions around the nucleus where the electron orbitals are most likely to be found.
In summary, the electron distribution around the nucleus is a result of the complex interplay between quantum mechanics, the electromagnetic force, and the organization of electrons into energy levels and orbitals. While the electromagnetic force attracts electrons to the nucleus, the quantum mechanical principles determine their distribution in specific energy levels and orbitals outside the nucleus.