Bohr model

The Bohr model describes an atom as having a small, positively charged nucleus at its centre, with electrons arranged in fixed shells around it.

The earlier nuclear model established the central nucleus. Niels Bohr improved the model by suggesting that electrons cannot orbit at just any distance from the nucleus. Instead, they occupy particular orbits, also called shells or energy levels, at fixed distances from it.

In this model, an electron belongs to one of these permitted energy levels rather than being anywhere around the nucleus. This arrangement helped explain experimental observations more successfully than a model that allowed electrons to be at any distance from the nucleus.