CHEMISTRY

TOPIC: ATOMIC MODELS Part 1

By Kingsley Idiagbor, B.Sc.(Hon), PGDCs, NCE, MNSM

After the discovery of the fundamental sub-particles such as electrons and protons, many scientists began to propose different models of the atomic structure sometimes based on plain guesswork and for relatively contemporary scientists; their proposals were based on the result of painstaking experiments.

Rutherford's Atomic Model

Earnest Rutherford postulated that the atom consist of basically nucleus at the core of the atom [See Atomic Structure-canal rays & protons] in which protons are found, while the negatively charged particles or electrons revolved around this nucleus. According to Ernest Rutherford, the electrons radiated energy continuously as they revolved around the nucleus.

This constituted the Rutherford's Atomic Model (See Fig. 1)

DEFECTS OF THE MODEL

There are two major defects of this model:

If it were to be correct, the electrons would spiral into the nucleus, in accordance with the laws of electrostatics;

Again, the Rutherford's model failed to account for the existence of the atomic spectra.

Bohr's Model

Neil Bohr pointed out the strength in the Rutherford's model but postulated that electrons are found in several energy levels. He also suggested that electrons do not emit radiation continuously as proposed by Rutherford but only do so when they move between the energy levels.

When an electron moves from a lower energy level to a higher one, the atom is said to be excited and the process is known as excitation.

The energy level from where the electron is leaping from, is called the ground level (or state) while the higher energy level is known as the excited level (or state)

As you know from elementary science, energy is needed to do work. Thus, the energy that is required for an electron to jump from the ground level to the excited level must be absorbed by the electron and is called absorption energy.

Absorption energy is therefore defined as the amount of energy that is required for an electron to move from a lower energy level to a higher one.

Electrons at the excited level are generally unstable and soon drop back to a lower level in some cases. When this happens, energy in the form of light is radiated. This is known as emission energy.

Emission energy is simply defined therefore as the amount of energy that is emitted or lost in the form of light when an electron returns to lower energies.

DEFECT OF THE MODEL

Bohr's model did not explain the atomic spectra emitted when electric current was passed through gaseous elements at low pressure in discharge tubes [See Atomic Structure-(Cathode rays)];

To distinguish the Bohr's model from the Ernest Rutherford's model, it is obvious to see that in the latter electrons emitted radiation continuously as they revolved round the nucleus whereas in the Bohr's model radiation is only emitted by electrons when transition occurs. By transition, we mean those movements of electrons between the energy levels.

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