Fundamentals of Atomic Structure and Particle Discovery

Subatomic Particles: Components of the Atom

Atoms are composed of tiny particles called subatomic particles:

• Electron

  Discovered by J.J. Thomson in 1897 while studying cathode rays produced in an evacuated glass tube with a voltage of about 10,000 volts. The electron is a negatively charged particle (on the order of $10^{-19}$ coulombs) with a mass of approximately $10^{-31}$ kg.

• Proton

  Discovered by Ernest Rutherford in 1919. It is a positively charged particle with a magnitude equal to the electron's charge and a mass approximately 1,840 times greater than the electron's mass.

• Neutron

  Discovered by James Chadwick in 1932. He worked with radiation emitted by beryllium when bombarded with alpha particles (originating from radioactive emissions). The neutron is a neutral particle with a mass nearly equal to that of the proton.

Historical Atomic Models

As subatomic particles were discovered, scientists needed models to explain how these components were arranged within the atom.

Thomson's Plum Pudding Model

Thomson proposed a model where the atom was regarded as a compact sphere of positive charge with electrons embedded throughout it.

Rutherford's Nuclear Model

In 1911, Ernest Rutherford discovered that when alpha particles were fired at a thin layer of gold, most passed straight through without deflection, while only a few changed trajectory significantly. He concluded that the atom was mostly empty space, possessing a dense, positive central nucleus around which electrons revolved in circular orbits.

Modern Atomic Theory and Electron Distribution

Bohr's Model and Energy Levels

Niels Bohr, studying the light emitted by heated atoms, proposed the distribution of electrons in specific layers in 1913. According to Bohr, electrons exist only in certain permitted energy levels, which he labeled K, L, M, and so on. The greater the distance from the nucleus, the higher the energy level.

Electrons can move between these levels. When they absorb energy, they jump to a higher level; when they return to their original layer, they emit the energy difference as light.

This model was very useful for the hydrogen atom but could not explain certain features of the light emitted by other, more complex atoms.

Sommerfeld and Quantum Mechanics

Arnold Sommerfeld later suggested that orbits were not always circular but could also be elliptical. Furthermore, within the same energy level, electrons could occupy regions of different shapes and inclinations.

The electrons are localized in specific regions called orbitals, designated s, p, d, and f.

Atomic Structure and Charge

Generally speaking, neutral atoms are formed by the same number of electrons and protons, along with a certain number of neutrons.

The Nucleus

Protons (positive charge) and neutrons (neutral charge) reside together in the nucleus.

The Electron Shell (Crust)

Electrons form the outer shell (sometimes called the 'crust') and are arranged in energy levels and, within those levels, in different orbitals. Electrons can move from one level to another by absorbing or releasing energy.