Atomic Structure and Evolution of Atomic Models

Understanding Atoms and Subatomic Particles

All matter consists of particles called atoms. An atom is composed of smaller elements known as subatomic particles:

• Protons: Possess a positive electric charge and are located in the nucleus.
• Neutrons: Carry no electric charge and are located in the nucleus alongside protons.
• Electrons: Carry a negative electrical charge and orbit in the electron cloud.

Dalton's Atomic Theory

In 1808, John Dalton retrieved the atomic theory of Democritus. He proposed that atoms were indivisible particles and the ultimate constituents of matter that combined to form compounds.

The Plum Pudding Model

Experiments conducted in 1897 on the conduction of electricity through gases led to the discovery of the negatively charged electron. Cathode rays were composed of electrons jumping from gas atoms when subjected to electric shocks, proving that atoms were not indivisible. Consequently, J.J. Thomson proposed the first atomic model: electrons were embedded in a cloud of positive charge that exactly compensated for the negative charge, rendering the atom electrically neutral.

The Rutherford Model

Rutherford conducted a series of bombardment experiments on thin metal films. He directed alpha particles (positively charged) at a thin gold foil and observed their paths on a screen:

• The majority of particles passed through the foil without deviation.
• Very few (approximately one in 10,000) deviated at an angle greater than 10 degrees.
• In rare cases, particles bounced back.

Based on these results, Rutherford demonstrated that atoms are not solid but are mostly empty space with a tiny, dense kernel at the center. This established the Rutherford Nuclear Model, defining the atom as consisting of two parts: the nucleus and the electron crust.