THE ATOMIC THEORY

JOHN DALTON’S ATOMIC THEORY

Dalton’s atomic theory, developed in 1808, established the foundations of modern chemistry. The theory makes five critical assumptions:

1. All matter is made up of extremely small particles called atoms

2. Atoms cannot be further subdivided or destroyed. *

3. Atoms of the same elements have the same mass and other properties, while atoms of different elements differ in mass and other properties. *

4. Atoms can combine in simple, whole-number ratios to form chemical compounds.

5. Atoms can combine in multiple simple whole-number ratios

Although some of the postulates have been rendered incorrect due to advancements in modern science, Dalton’s atomic theory has nonetheless served as the foundation of modern science.

LIMITATIONS OF DALTON’S ATOMIC THEORY

As science progressed, it was eventually determined that postulates (2) and (3) were flawed.

·   Postulate (2) has been rendered false due to the discovery of subatomic particles-protons, neutrons, and electrons-which proved that atoms can, in fact, be subdivided. Additionally, it was proven that atoms can be destroyed in nuclear reactions.

·   Postulate (3) has been rendered incorrect due to the discovery of isotopes, which are atoms of the same element that have different mass numbers but identical atomic numbers. This means that these atoms have different masses, although of the same element.

Therefore:

1. All atoms of the same element are identical in chemical properties but can vary in mass and physical properties.

2. Atoms can be further subdivided into protons, neutrons and electrons.

DISCOVERY OF SUBATOMIC PARTICLES

1897-J.J. THOMPSON

·   By deflecting cathode rays with electric and magnetic fields, Thompson realized that cathode rays were streams of particles. As a result, the mass-to-charge ratios of these particles were measured, and it was discovered that they were about 2000 times lighter than a hydrogen atom. These particles were electrons

·   Thompson then proposed the ‘Plum Pudding’ model of the atom. He theorized that the atom is a positively charged sphere that encloses negatively charged electrons within it, thereby maintaining electrical neutrality.

1900-E. GOLDSTEIN

·   After observing rays in a cathode ray tube that were travelling in the opposite direction to the cathode rays (he called these canal rays), Goldstein discovered the proton.

1909-EARNEST RUTHERFORD

·   By bombarding a thin metal foil with alpha particles, it was discovered that some particles were deflected at very large angles, while most seemed to have passed through. This meant that a strong repulsive force existed within the foil, and therefore, Rutherford theorized that the mass and positive charge were concentrated in a central nucleus while electrons orbited this positive charge outside the nucleus, which was mainly empty space.  This theory did not account for why the electrons did not lose energy and spiral into the nucleus, or why the electrons were not pulled into the nucleus due to attraction between unlike charges.

1913-NIELS BOHR

·   Niels Bohr proposed that the electrons encircle the nucleus of the atom in fixed orbits or energy levels. Electrons need to emit or absorb a quantum of energy to move down/up an energy level or orbit. Higher energy levels are orbits further away from the nucleus, and lower energy levels are orbits closer to the nucleus. For example, if an electron in a hydrogen atom absorbs a quantum of energy, it moves up to a higher energy level. This electron is now said to be excited and emits energy to return to the lower, more stable orbit.

1913-HENRY MOSELEY

·   Suggested that particles which have mass but no charge exist within the atom, since an atom’s mass is greater than the mass of its constituent protons and electrons.

1932-JAMES CHADWICK

·   Chadwick bombarded Beryllium with alpha particles, which produced a highly penetrating stream of particles that contained no electrical charge, thus discovering the neutron.