The Periodic Table & Atomic Structure Unit Notes: Part I Learning Target 1: Development of the Atomic Theory

GO TO-->Chemistry-->Unit 2: Atomic Theory-->Concept 2.5: Development of the Atomic Theory-->Explore Tab 1

Essential Question: How Did the Atomic Theory evolve from Ancient Greece to Modern Day, and Which Key Scientists Made Which Contributions?
Ancient Greek philosophers were the first to discuss atoms and matter. This became known in time as. Atomic theory states that matter is composed of units called atoms.
Democritus (460–370 BCE) proposed that all matter in the world consisted of invisible and indivisible corpuscles (or small particle-like objects). He called these corpuscles .
John Dalton (1766–1844) is best known for his work on , which came about during his development of partial pressure gas laws.
John Dalton's atomic theory had five points:
of particles called atoms.
Atoms of a given element have unique properties. While atoms of the are identical, atoms of elements differ in size, mass, and properties.
Atoms cannot be subdivided, , or .
Atoms of different elements combine in simple ratios to form chemical .
During chemical reactions, atoms are combined, separated, or .
Vocabulary
Law of Conservation of Mass -

Early Developments in Atomic Theory
J. J. Thomson discovered the first subatomic a little more than a century ago. He used a cathode ray tube to determine that atoms contain charged particles, or electrons.
Video: Thomson's Atomic Model
Question: Why is Thompson’s model called the plum pudding model? What does it say about the negative and positive charges in an atom?

Ernest Rutherford’s Gold Foil Experiment
--considered by some to be the “Father of .” He is perhaps best known for his experiment.
Rutherford directed a narrow beam of charged particles toward a thin piece of .
Rutherford predicted that most particles would . He also predicted that a few would be by the electrons in the atoms of the gold foil, but that the path of these particles change much.
Some of the particles, however, were at the particle source! This proved that something larger than an electron was blocking its path through the foil.
Rutherford concluded that an atom’s and its mass are concentrated in a very small area of the atom.
He believed the electrons occupied the space around the , his model was called the model of the atom.
He determined the radius of the nucleus to be at least times smaller than the radius of the entire atom.
Rutherford showed that the nucleus contains positively (+) charged particles we now call .
Refining the Atomic Theory
Niels Bohr, studied the changes that occurred within an atom when was added to it or removed. He found that the amounts of energy absorbed and emitted by hydrogen atoms were related to the energies of their .
His theory claimed that orbit the nucleus of an atom.
The amount of energy determines the electron's from the nucleus.
When Bohr studied the light energy given off by an atom of hydrogen, he found that each electron is associated with a of energy. These are referred to as .
When an electron absorbs a quantum of energy, it becomes excited and moves to a energy level, one that is from the nucleus. When the electron falls from the excited state back to its ground state, it emits energy in the form of radiation (light).
Vocabulary. Define the term below
Energy Levels -

Video: Structure of the Bohr Model
Question: What are Bohr’s 4 ‘postulates’?
1.
2.
3.
4.

Bohr’s theory gave rise to the planetary or model of the atom, because it stated that electrons orbited the atom’s nucleus in circular , similar to those of planets in the solar system. Bohr’s model was later proven .
Modern Quantum Theory
Today’s understanding of the atom is built on Bohr’s model. It is now believed that electrons move around the nucleus in a “” instead of circular paths. Quantum theory attempts to explain the fact that energy and electrons have properties similar to particles and .
Video: Electron Cloud Model
Question: Why can’t we say that electrons are in orbits?

Explain Question For Learning Target 1: How did scientists use the work of earlier scientists and the contributions of their peers to develop the atomic theory?