The development of the model of the atom (common content with chemistry)
Notes for this section are being prepared.
Discovery of protons and neutrons
Later experimental evidence showed that the positively charged nucleus is not a single, indivisible object: it contains smaller particles.
The positive charge of a nucleus could be divided into a whole number of equal positive charges. These charges are associated with particles called protons. Every proton carries the same amount of positive charge, so a nucleus with more protons has a greater positive charge.
About 20 years after the nucleus became an accepted scientific idea, James Chadwick’s experimental work provided evidence for another particle in the nucleus: the neutron. Neutrons have no electric charge, so they do not contribute to the nucleus’s positive charge. This evidence refined the nuclear model: the nucleus contains both positively charged protons and uncharged neutrons.
Notes for this section are being prepared.
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.