Atomic Theory
Student Expectation
The student is expected to understand the experimental design and conclusions used in the development of modern atomic theory, including Dalton’s Postulates, Thomson’s discovery of electron properties, Rutherford’s nuclear atom, and Bohr’s nuclear atom.
Key Concepts
The concepts of the atom and the nature of matter originated with Greek philosophers more than two thousand years ago. These ideas, though not scientifically tested, formed the basis for later scientists to build upon and develop modern atomic theory.
John Dalton extensively investigated the nature of gases in the early 1800s in order to gain a better understanding of the laws of conservation of mass and of multiple proportions. His resulting five postulates of atomic theory helped to define the structure and nature of the atom. The scientific community accepted his postulates due to his sound experimental evidence.
J.J. Thomson experimented with particles known as cathode rays. In the late 1800s he discovered that these rays were actually negatively charged particles with a charge of 1-, and that they were much smaller than atoms. He conducted further experiments to investigate the relationship between electric charge and matter. Thomson developed the “plum pudding model,” in which electrons were embedded in a positively charged sphere.
Ernest Rutherford developed the nuclear model of the atom in the early 1900s. He came to his conclusion after analyzing the results of his experiment, known as the Rutherford scattering experiment, in which he found that atoms contained an extremely small, dense, and positively charged nucleus and that the area around the nucleus was mostly empty space with a few negative electrons.
Niels Bohr refined the findings of Rutherford in the early 1900s. Bohr’s used spectral light emissions to conclude that electrons had specific energy levels, known as quantized energy levels. His atomic model consisted of spherical shells of electrons on various states surrounding the positively charged nucleus.
ATOMIC THEORY
Greek Contributions to Atomic Theory
The term atom originated from the Greek word atomos. The concepts of the atom and the nature of matter originated with Greek philosophers more than two thousand years ago. These ideas, based on philosophical reasoning but not scientific testing, formed the basis for later scientists to build and develop modern atomic theory.
John Dalton’s Postulates
John Dalton extensively investigated the nature of gases in the early 1800s in order to gain a better understanding of the law of conservation of mass and the law of multiple proportions. Dalton’s law, published in 1804, states that the elements within a compound combine in ratios of small, whole numbers. Also known as the law of multiple proportions, this law was an important step toward the atomic theory that he would later propose. It also formed the basis for the modern chemical formulas of compounds. Dalton’s resulting postulates of atomic theory helped to define the structure and nature of the atom. The scientific community accepted his postulates due to his sound experimental evidence. Dalton’s postulates were:
Matter is composed of atoms.
All matter is composed of indivisible particles that cannot be created or destroyed.
Atoms of the same element are identical, but atoms of different elements have different properties.
Atoms group together in whole number ratios to form compounds. In any compound, the same mass ratio of the elements is always present.
Chemical reactions are the result of the rearrangement of atoms. Dalton used this information as evidence for his atomic theory.
J.J. Thomson’s Electron Properties
J. J. Thomson experimented with particles known as cathode rays. In the late 1800s, he discovered that these rays were actually negatively charged particles with a charge of -1, and that they were much smaller than atoms. In 1897, he conducted further experiments to investigate the relationship between electric charge and matter. He thought that the atoms could be further divided into smaller substances. Thomson developed the plum pudding model in which negatively charged electrons were embedded in a positively charged sphere.
Ernest Rutherford’s Nuclear Atom
In the early 1900s, a student of J.J. Thomson’s named Ernest Rutherford developed the nuclear model of the atom. He came to his conclusion after analyzing the results of an experiment known as the Rutherford scattering experiment. In this experiment, he found that atoms contained an extremely small, dense, and positively charged nucleus and that the area around the nucleus was mostly empty space with a few negative electrons. He described the atom as a planetary model.
Niels Bohr’s Nuclear Atom
With the development of quantum theory, Niels Bohr refined the discoveries of Rutherford in the early 1900s. Bohr used spectral light emissions to conclude that electrons had specific energy levels, known as quantized energy levels. His atomic model consisted of spherical shells of electrons surrounding the positively charged nucleus. In his model, electrons travel in circular orbits with specific energy and momentum.