Chapter Two, Section One


  1. Learn the different classifications of matter.
  2. Learn Dalton's theory of atomic structure, Law of Conservation of Mass, and the Law of Definite Proportions.
  3. Memorize the symbols for some important elements. Recognize monatomic and diatomic elements.


Let us first look at some definitions:

An ELEMENT is a pure substance that cannont be broken down into simpler substances by chemical reactions. There are 88 elements in the periodic table that occur in nature. The other elements have been prepared by chemists.

An ATOM is the smallest particle of an element that retains the chemical properties of the element.

A COMPOUND is two or more atoms combined chemically in definite proportions such as H2O which always has two hydrogen atoms and one oxygen atom. The atoms in a compound are not easily separated -- you can't boil water and get hydrogen and oxygen.

A MOLECULE is the smallest unit of a compound that retains the chemical characteristics of the compound.

A MIXTURE can be either homogeneous (same composition throughout) or heterogeneous (not the same composition throughout).

A SOLUTION is a homogeneous mixture of two or more substances with the substance present in the largest amount categorized as the solvent and the other(s) is (are) called the solute(s). A solution of salt and water will always have water as the solvent because it is not possible to have such a solution with salt in the greater amount. However in a solution of water and ethanol, either one could possibly be the solvent.


In 1808 Dalton presented his atomic theory:

Using this basis Dalton was able to develop the first table of atomic masses which chemists call atomic weights since the mass is usually determined by comparison to a standard mass -- a process called weighing. Many of his masses were incorrect due to wrong assumptions, but it was a start toward a correct table of masses.

It was not until the eighteenth century that combustion was finally understood based upon the experiments of Lavoisier who discovered that oxygen was involved in combustion and also that life was supported by a process which involved oxygen. It observations led to his discovery of the Law of Conservation of Mass: Mass can be neither created nor destroyed..

In the early 1800's, the chemist Proust discovered what is now known as theLaw of Definite Proportion: A given compound always contains the exactly the same proportion of elements by mass.. This discovery led Dalton to consider that atoms might be the individual particles that make up the elements. Dalton noted that carbon and oxygen formed two different compounds with different properties and different masses -- one of them contains exactly twice as much oxygen as the other. This principle was found to apply to other elements as well and led to the Law of Multiple Proportions: When two elements form a series of compounds, the ratios of the masses of the second element that combine with 1 gram of the first element can always to reduced to small whole numbers..


You should memorize the elements from atomic number 1 through atomic number 17 and column VIIA. Some of the elements actually exist as diatomic molecules(or diatomics). The diatomics are H2, N2, O2, F2, Cl2, Br2, and I2. You will need to memorize these.



After you have studied this material and practiced some problems, take quiz one. If you score at least 80 on the test then you are ready to continue to the next section.

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Web Author: Dr. Leon L. Combs
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