1. Development of Atomic Theory
Development of Atomic Theory – Ancient Times | Dalton's Postulates | Thomson's Discovery of Electron Properties | Rutherford's Nuclear Atom | Bohr's Nuclear Atom | Modern Atomic Theory
Ancient Times | Dalton's Postulates | Thomson's Discovery of Electron Properties | Rutherford's Nuclear Atom | Bohr's Nuclear Atom | Modern Atomic Theory

2. Development of Atomic Theory
Learning Objectives
Understand the experimental design and conclusions used in the development of modern atomic theory
After this lesson you will be able to understand the experimental design and conclusions used in the development of modern atomic theory.

3. Ancient Times Democritus proposed atomos Aristotle’s beliefs
Indivisible, indestructible particles that made up matter
Different types possessed different properties
Changes in matter produced by changes in the composition of groups containing different types
Moved through empty space
Aristotle’s beliefs
Matter composed of earth, air, fire, water
No such thing as empty space
Idea of atom dismissed for thousands of years
Disagreement
Inability to prove ideas through experimentation
The Greek philosopher, Democritus, was the first to suggest that the building blocks of matter are extremely small and indivisible particles. He called these particles atomos (a Greek word meaning “indivisible”). The word “atom” is derived from atomos. Democritus believed atomos could not be created, destroyed, or split into smaller parts.
He also suggested that there were different kinds of atoms, each with different properties.
Rather than proposing that atomos themselves changed to produce changes in matter, Democritus proposed that changes in matter were produced by changes in the composition of groups containing different types of atomos.
Democritus also believed that atoms moved through empty space. These ideas clashed with the ideas of Aristotle, who was a more influential contemporary of Democritus.
Aristotle believed that matter was composed of earth, air, fire, and water.
He also believed that empty space did not exist and criticized Democritus for his ideas because they did not line up with his own.
Although many of Democritus’ ideas were closer to the truth, they were not widely accepted because of Aristotle’s influence. Also, the technology needed to perform experiments that could prove or disprove the validity of these ideas did not exist at the time. Due to these limitations, the idea of the atom was dismissed for thousands of years.

4. Dalton’s Postulates Dalton’s Atomic Theory Today’s revisions
Based on experiments
Elements made of small indivisible particles called atoms
Atoms of an element are identical to each other and different from atoms of other elements
Chemical reactions occur when atoms are combined, separated, or rearranged in simplified whole-number ratios
Atoms not created or destroyed in chemical reactions
The scientific development of modern atomic theory began in 1808, when John Dalton introduced the first atomic theory based on scientific research. Dalton performed experiments in which he studied the behavior of atoms during reactions.
The postulates Dalton proposed as a result of these experiments can be summarized into four statements. First, elements are made of small and indivisible particles called atoms.
Second, atoms of a given element are completely identical to one another but different from atoms of other elements.
Third, chemical reactions occur when atoms are combined, separated, or rearranged in simplified whole-number ratios.
Fourth and finally, atoms are neither created nor destroyed in chemical reactions.
While many parts of Dalton’s theory have been confirmed, the theory was not without its shortcomings. Today, the definition of an atom has been revised to be the smallest unit of an element that demonstrates the properties of that element.
Further experimentation revealed that atoms of an element are not always identical. Atoms of the same element can have masses that are slightly different from one another. These atoms would later become known as isotopes.
It was also later discovered that atoms could be further divided into subatomic particles.
Today’s revisions
Atom – the smallest unit of an element that demonstrates the properties of that element
Atoms of the same element can have different masses
Atoms can be divided into subatomic particles

5. Thomson’s Discovery of Electron Properties
Cathode ray – a stream of electrons that emanate from a negative conductor (
Electron – a negatively charged subatomic particle
J.J. Thomson proved the existence of electrons
Mass of electrons is much smaller than the least massive atom
Proved subatomic particles exist
The road to the discovery of subatomic particles started when scientists began studying electrical patterns in atoms. Scientists observed that some substances decomposed when exposed to electrical current. This led to the idea that electrical forces hold elements together. Scientists soon discovered when two oppositely charged electrodes are placed in a tube under low-pressure, a ray of radiation will pass between the two electrodes. Since the rays emanated from the cathode (or negative conductor), they were named cathode rays. While experimenting, scientists found that the rays in the cathode tube were attracted to a metal plate that had a positive charge. Since opposite charges attract, scientists concluded that those rays must be streams of negatively charged particles.
A breakthrough occurred when J.J. Thomson found a way to calculate the electric charge-to-mass ratio of the particles in cathode rays. Thomson found that the mass of these particles was much less than hydrogen, the atom with the least mass, proving that there were smaller particles than atoms. These negative subatomic particles came to be known as electrons.
This was a major discovery because it disproved Dalton’s postulate that atoms could not be divided into smaller particles.

6. Thomson’s Discovery of Electron Properties
Plum pudding model postulated by Thomson
Atoms made of a positively charged mass
Electrons scattered throughout the mass
Failed to correctly describe the structure of the atom
After his discovery of the electron, Thomson formulated the “plum pudding” model of the atom.
According to the model, atoms were made of a positively charged mass and electrons were scattered throughout the mass, like plums mixed in a pudding.
The plum pudding model, however, failed to correctly establish the structure of the atom.

7. Rutherford’s Gold Foil Experiment
5th period: Start here on Monday, October 27

8. Rutherford’s Gold Foil Experiment
Nucleus – the small, dense, positively charged area in the center of an atom where protons and neutrons are found
Discovered during Ernest Rutherford’s gold-foil experiment
Nuclei of atoms repelled some positively charged (alpha) particles, while other alpha particles traveled through empty space
Thomson’s plum pudding model was disproved when Ernest Rutherford performed what is known today as the gold-foil experiment. In his experiment, Rutherford bombarded a thin gold foil with alpha particles, which have a +2 charge. The results showed that most alpha particles penetrated the foil, but a few were deflected or bounced back. The scattering pattern of alpha particles revealed that an atom is mostly empty space, but the center contains a dense region with a positive charge. This region is now called the nucleus. Since the nucleus is a small region with positive charge, it makes sense that the majority of the alpha particles penetrated the empty spaces within the atoms, while a few particles collided with nuclei and were deflected or bounced back because of the repulsion between the positive charges of the alpha particle and the nuclei.
6th Period: Start here 10/27/2014

9. Rutherford’s Gold Foil Experiment
Nucleus
Small but accounts for most of an atom’s mass
Contains protons
Proton – a positively charged subatomic particle that has a mass of approximately one atomic mass unit
Contains neutrons
Neutron – a neutrally charged subatomic particle that has a mass of approximately one atomic mass unit
Rutherford’s model showed the nucleus surrounded by orbiting electrons
Rutherford estimated that the diameter of the nucleus was very small (only about 1/10,000 of the size of the atom itself). However, the nucleus accounted for almost all of an atom’s mass.
Rutherford concluded that the nucleus contained protons, or positively charged subatomic particles.
Later, one of Rutherford’s colleagues discovered that the nucleus also contained neutrally charged subatomic particles, now called neutrons. Together, protons and neutrons account for the mass of the nucleus.
Rutherford’s version of the nuclear atom showed a positive nucleus orbited by electrons.

10. Bohr’s Nuclear Atom
Discovery that the emission spectrum of hydrogen contains only certain frequencies of light
Prompted Niels Bohr to propose another atomic theory
Attempted to describe the relationship between energy and movement of electrons
In response to the puzzling discovery that the emission spectrum of the hydrogen atom included only some light frequencies, a scientist named Niels Bohr proposed another atomic theory.
Bohr’s model of the atom attempted to explain the discontinuous spectrum of hydrogen by describing the relationship between energy and the movement of electrons in atoms.

11. Bohr’s Nuclear Atom
Electrons are arranged in circular orbits around the nucleus
Electrons
In each orbit have a fixed amount of energy
Nearest nucleus are in their lowest energy state
Gain and lose fixed amounts of energy
Release energy as photons that correspond to specific frequencies of light
According to the Bohr model, electrons are arranged in circular orbits around the nucleus and do not lose energy as they orbit.
Also, electrons in each orbit have a fixed amount of energy.
Electrons in the closest possible orbitals to the nucleus are in their lowest energy states.
Electrons may gain and lose certain, fixed amounts of energy called quanta. When they receive a quantum of energy, electrons move to a higher-energy orbit. Electrons release a quantum of energy when they return back to a lower-energy orbit.
The quantum of energy released is in the form of a particle called a photon. The quantum of energy that a photon possesses corresponds to a certain frequency. The specific energies of the photons emitted by the hydrogen atom explained the specific frequencies of light observed in the hydrogen emission spectrum.

12. Modern Atomic Theory Bohr’s shortcomings Quantum mechanical model
Only explained emission spectrum of hydrogen
Did not explain chemical properties
Electrons do not revolve around nucleus in circular orbits
Quantum mechanical model
Electrons exhibit wave and particle properties
Impossible to know both the speed and location of electrons at the same time
Region around the nucleus where finding an electron is most probable is called an orbital
However, like the other theories, there were shortcomings to Bohr’s nuclear atom. Bohr’s model did not explain emission spectrums of elements other than hydrogen.
It also failed to explain the chemical properties of elements.
Further experimentation also indicated that electrons don’t revolve around the nucleus in circular orbits. We now know that the movement of electrons is much more complex; so complex, in fact, that we cannot fully predict their motion today.
The complexity of the behavior and distribution of electrons within atoms prompted scientists to think about an alternative to Bohr’s model of the atom. This model is called the quantum mechanical model of the atom. According to this model, first proposed by Louis de Broglie, electrons exhibit the properties of both a wave and a particle.
Werner Heisenberg, a contemporary of de Broglie, contributed to the quantum mechanical model by theorizing that it was impossible to know both the exact location and speed of an electron at the same time.
Although these cannot be known, another scientist, Erwin Schrödinger, developed an equation that was able to help predict the probability of finding an electron in a certain region in space. An orbital is defined as a region in space around the nucleus where finding an electron is most probable.

13. Development of Atomic Theory
Learning Objectives
Understand the experimental design and conclusions used in the development of modern atomic theory
You should now be able to understand the experimental design and conclusions used in the development of modern atomic theory.