Atomic Structure: Developing the Model of the Atom

Discovering the Atomic Model

Daltons’s Atomic Theory

John Dalton (early 19th Century) proposed that all matter is made up of tiny, indivisible particles called atoms.
He claimed atoms couldn’t be created, destroyed or divided, thereby initiating the idea of conservation of mass in a chemical reaction.
Dalton concluded that each element consists of a particular kind of atom with a unique weight.

Thomson’s Atomic Model

Later in the 1890s, J.J. Thomson discovered the electron, a particle with a negative charge and much lighter than an atom, disproving Dalton’s theory of an indivisible atom.
Conducting his famous cathode ray experiment, he concluded that atoms contain these smaller, negatively charged particles.
Thomson suggested the plum pudding model where negatively charged electrons were embedded in a positive sphere of the atom, much like plums in a pudding.

Rutherford’s Nuclear Model

In the early 20th century, Ernest Rutherford carried out the alpha particle scattering experiment which greatly contributed to the current understanding of the atom.
His experiment demonstrated that the mass and positive charge of an atom are concentrated in a nucleus at its centre.
This led to the nuclear model of the atom, in which a tiny, dense nucleus holding protons and neutrons is surrounded by electrons orbiting at a distance.

Bohr’s Quantum Model

After Rutherford, Niels Bohr further refined the atomic model by proposing that electrons move in fixed orbits around the nucleus at certain energy levels or shells.
This quantum model of the atom explained why atoms don’t collapse and the light emitted or absorbed by atoms in terms of energy level transitions.

Key Learning Points

Our understanding of the atomic model has developed through the work of various scientists over time.
Every proposed atomic model was subjected to experimental evidence which endorsed or refuted it, leading to its acceptance, modification or replacement.
The modern atomic model now accommodates the cloud-like region of electron probability, called the electron cloud, and the existence of particles within the nucleus, the protons and neutrons.

Remember, scientific theories and models evolve over time in light of new experimental evidence and the atomic model is a testament to this process of scientific investigation.