Bohr’s Model of the Atom

The scientist Niels Bohr improved Rutherford’s nuclear model. Bohr proposed a model that included a positively charged nucleus in the centre with electrons orbiting the nucleus in a similar way to how planets orbit the Sun.

  • The orbits occupied by electrons are called energy levels or shells

Bohr’s model is often referred to as the planetary model.

The series of energy levels is at increasing distances from the nucleus. Each energy level holds a maximum number of electrons.

  • The first energy level can hold a maximum of two electrons
  • The second energy level can hold a maximum of eight electrons
  • The third energy level can hold a maximum of eight electrons

The idea that electrons orbit the nucleus became more accepted than the notion of a general cloud of electrons. This is because it supported the results of experiments by other scientists.

Discovering the Neutron

After further experiments, Ernest Rutherford confirmed that the positive charge in the nucleus is due to small particles known as protons.

Not long after, in 1932, James Chadwick provided evidence for particles in the nucleus with no charge. These are particles we now know as neutrons.

  • James Chadwick also discovered that a neutron has roughly the same mass as a proton

The discovery of the neutron led to an improved model of the atom: