The Atomic Theory

Atomic Theories

The idea of an atom -- the smallest particle of matter -- has intrigued mankind since the beginning of civilization.  Throughout the centuries the "view" of the atom has changed.  New ideas, and new technologies have influenced the model of the  atom.  This view of the atom is still a Theory and therefore it is still subject to change.  The  modern model of the atom is called the Quantum Model. The chart below summarizes the various atomic models that have been developed during the course of history.

Scientist & approximate Date		Name of Model, Sketch and main idea of theory		Importance and Improvement on previous model	Shortcomings - Problems or why was it changed
Democritus   c.300 BC		Atom the indivisible particle Atomos (in ancient Greek) means "that which cannot be further broken down into smaller pieces".		Talks about the atom as the smallest particle of matter. Defines the atom as an indivisible particle Explains certain natural occurrences such as the existence of elements	Does not give a scientific view of the atom only a conceptual definition Does not talk about subatomic particles (Electrons, Protons, Neutrons)
Dalton   c.1800		The solid sphere model Atoms are seen as solid, indestructible spheres (like billiard balls)		Explains a lot of chemical properties such as how atoms combine to form molecules Explains chemical change better than the  Particle Theory Confirms the basic Laws of Chemistry: Conservation of Mass & definite Proportions	Does not include the existence of the nucleus Does not explain the existence of ions or isotopes Does not talk about subatomic particles (Electrons, Protons, Neutrons)
J.J. Thomson  c.1850		The raisin bun Model or the chocolate chip cookie model :  Atoms are solid spheres made-up of a solid positive mass (or core) with tiny negative particles embedded in the positive core.		Infers on the existence of electrons and protons Introduces the concept of the nucleus Infers on the relative nuclear density and atom mass of different atoms	Does not explain the existence of electrons outside the nucleus does not explain the role of electrons in bonding Does not talk about neutrons therefore can't explain radioactivity and the existence of isotopes
Rutherford c. 1905		The Planetary Model Famous Gold Leaf Experiment proves that the nucleus is positive and the electrons are outside the nucleus.		First real modern view of the atom Explains why the electron spins around the nucleus (Bohr's Contribution) Proposes that the atom is really mostly empty space	Does not place electrons in definite energy levels around the nucleus Doesn't include neutrons in the nucleus Does Not relate the valence electrons atomic charge
(Neils Bohr) Bohr- Rutherford c. 1920		Electrons in Definite energy Levels around  the nucleus Used atomic spectra to prove that electrons are placed in definite orbitals (called shells) around the nucleus.		Explains the role of valence electrons in bonding Relegates the number of valence electrons to the Periods of a periodic table Fully explains ionic and covalent bonding Places electrons in definite energy levels 2 e- in the first 8 e- in the second 8 e- in the third	It does not explain the shapes of molecules or other abnormalities that result form unevenly shared pairs of electrons (such as the abnormal behaviour of water, the difference in Carbon-Carbon Bonds between diamond and graphite etc..)
Modern Theory Many Scientists Contributed.  Some of the more famous  are: Schroedinger Einstein Luis De Broglie Max Planck Frank Hertz Maxwell Fermi		Quantum Mechanical Model or Electron Cloud Model The analogy here is that of a "beehive" where the bees are the electrons moving around the nucleus in a "cloud" of energy levels.			Advanced Theories will explain bonding and other facts about the behaviour of atoms and their chemical and physical properties in forming new compounds.

Other important facts about the particles of an atom:

Subatomic Particle	Symbol	Charge	Relative Mass*	Location
Proton	p+	positive	2000	nucleus
Electron	e-	negative	1	orbits around nucleus
Neutron	n0	neutral (zero)	2000	nucleus

*Relative mass means that is the electron has a mass of 1 unit, the proton and neutron will have a mass 2000 times that of the electron.

The mass number (also known as atomic mass or atomic weight) and the atomic number from the Periodic Table are very important numbers because they tell us how many subatomic particles are contained in a given atom.

The atomic number tells us the number of electrons and the number of protons., i.e. Atomic Number = Number of electrons = Number of Protons.  The atomic mass tells the total number of particles in the nucleus, i.e. Atomic Mass = # of protons + number of neutrons.

Branches of Chemistry

Organic Chemistry

This specific type of chemistry is concerned with elements containing carbon. Carbon is only the fourteenth most common element on earth, yet it creates the largest number of different compounds. This type of chemistry is important to the petrochemical, pharmaceutical, and textile industries. All living organisms contain at least some amount of carbon in their body.

Inorganic Chemistry

This branch of chemistry deals with substances not containing carbon and that are not organic. Examples of such substances are minerals found in the earth's crust and non-living matter. There are many branches of inorganic chemistry. They include bioinorganic chemistry, nuclear science and energy, geochemistry, and synthetic inorganic chemistry, just to name a few.

Physical Chemistry

This type of chemistry deals with the discovery and description of the theoretical basis of the behavior of chemical substances. This means also that it provides a basis for every bit of chemistry including organic, inorganic, and analytical. This chemistry is defined as dealing with the relations between the physical properties of substances and their chemical formations along with their changes.

Biochemistry

Biochemistry is a science that is concerned with the composition and changes in the formation of living species. This type of chemistry utilizes the concepts of organic and physical chemistry to make the world of living organisms seem much clearer. Some people also consider biochemsitry as physiological chemistry and biological chemistry. The scientists that study biochemistry are called biochemists. They study such things as the properties of biological molecules, including proteins, lipids, carbohydrates, and nucleic acids. Other topics they focus on are the chemical regulation of metabolism, the chemistry of vitamins, and biological oxidation.

Analytical Chemistry

This kind of chemistry deals mostly with the composition of substances.

Other Branches
Agrochemistry - This branch of chemistry may also be called agricultural chemistry. It deals with the application of chemistry for agricultural production, food processing, and environmental remediation as a result of agriculture.

Astrochemistry - Astrochemistry is the study of the composition and reactions of the chemical elements and molecules found in the stars and in space and of the interactions between this matter and radiation.

Electrochemistry - Electrochemistry is the branch of chemistry that involves the study of chemical reactions in a solution at the interface between an ionic conductor and an electrical conductor. Electrochemistry may be considered to be the study of electron transfer, particularly within an electrolytic solution.

Environmental Chemistry - Environmental chemistry is the chemistry associated with soil, air, and water and of human impact on natural systems.

Food Chemistry - Food chemistry is the branch of chemistry associated with the chemical processes of all aspects of food. Many aspects of food chemistry rely on biochemistry, but it incorporates other disciplines as well.

General Chemistry - General chemistry examines the structure of matter and the reaction between matter and energy. It is the basis for the other branches of chemistry.

Geochemistry - Geochemistry is the study of chemical composition and chemical processes associated with the Earth and other planets.

Green Chemistry - Green chemistry is concerned with processes and products that eliminate or reduce the use or release of hazardous substances. Remediation may be considered part of green chemistry.

Medicinal Chemistry - Medicinal chemistry is chemistry as it applies to pharmacology and medicine.

Nanochemistry - Nanochemistry is concerned with the assembly and properties of nanoscale assemblies of atoms or molecules.

Nuclear Chemistry - Nuclear chemistry is the branch of chemistry associated with nuclear reactions and isotopes.

Organic Chemistry - This branch of chemistry deals with the chemistry of carbon and living things.

Photochemistry - Photochemistry is the branch of chemistry concerned with interactions between light and matter.

Polymer Chemistry - Polymer chemistry or macromolecular chemistry is the branch of chemistry the examines the structure and properties of macromolecules and polymers and finds new ways to synthesize these molecules.

Thermochemistry - Thermochemistry may be considered a type of Physical Chemistry. Thermochemistry involves the study of thermal effects of chemical reactions and the thermal energy exchange between processes.