Humboldt State University ® Department of Chemistry

Richard A. Paselk

Science 331
Fall 2004 Lecture/Activity Office: SA560a
Notes: 25 October Phone: x 5719
Home: 822-1116
e-mail: rap1

Conservation of Mass

Law of Conservation of Mass: Mass is neither created nor destroyed during a chemical change. (Strictly speaking there is no measurable change.) For example, if we burn gasoline (octane) in air we will get carbon dioxide and water:

C8H18 + 12 1/5 O2Æ 8 CO2 + 9 H2O

If we were to weigh (determine the mass of) the carbon and oxygen vs. the carbon dioxide and water we would find them to be identical - the masses are the same on both sides of the equation (that's why its called a chemical equation, the two sides indicate are equal).

Looked at another way, if you count the atoms, the numbers of each kind of atom on each side are identical - so we can also say that atoms are conserved in chemical processes.



Law of Conservation of Energy: Energy is neither created nor destroyed in chemical processes. The problem here of course is - What is energy? Energy is the capacity to do work. So what's work? Work occurs when an object (mass) is moved against a force. Some common forms of energy important to our study include:

Another form of energy we need to be familiar with is:

Note that these forms of energy are readily interconverted.


Atoms and Atomic Structure

Elements are substances which cannot be broken down further into simpler substances.

Atoms are the smallest constituents of elements. The first successful atomic theory was that of John Dalton (1803). Dalton's atomic theory states:

  1. All matter is composed of ultimately small particles, called atoms.
  2. Atoms are permanent and indivisible - they can neither be created nor destroyed.
  3. Elements are characterized by their atoms. All atoms of a given element are identical in all respects. Atoms of different elements have different properties.
  4. Chemical change consists of a combination, separation, or rearrangement of atoms.
  5. Chemical compounds are composed of atoms of two or more elements in fixed ratios.

All of these statements are close to reality, and nearly describe chemical behavior. But here are exceptions. Thus atoms can be created and destroyed via nuclear processes. They consist of different forms called isotopes. Atoms are not the smallest particles, etc.


Atoms are now known to consist of three different types of particles: electrons, protons and neutrons (the common form of one very important atom, hydrogen, has only two kinds: a proton and an electron). The protons and neutrons reside in a small inner portion called the nucleus while the electrons reside in a relatively large cloud centered on the nucleus. Important properties of these particles are listed in the table below:

 Particle Charge Relative Mass Mass
Electron (e-) -1 1/1840 9.11 x 10-28g
Proton (p or H+) +1 ª1 1.67 x 10-24g
Neutron (n) 0  ª1  1.67 x 10-24g

Some important terms which you must know are:



Isotopes are forms of elements which differ only in the number of neutrons. This means different isotopes of the same element have essentially the same chemical properties but slightly different physical properties. They can also different substantially in terms of their nuclear stability. Let's look at some examples of isotopes:

 Symbol Z A p n e-
14 6 14 6 8 6
238U6+ ? ? ? ? ?
35Cl- ? ? ? ? ?
?O? ? 18 ? ? 10 

You should be able to fill in the blanks in a table like this with, the aid of a periodic table, on a quiz.


Chemical Periodicity

Look at the Periodic Chart on the wall. The pattern arises due to a repetition or periodicity of chemical properties. The vertical columns of the charts are called groups, while the rows are referred to a periods.

Note the numbering of the groups. The numbers from 1 - 18 are the internationally accepted numbers. We will also use the I - VIII "American" numbering system. Note that the "tallest" columns comprise what are referred to as the "representative elements" (IA - VIIIA).


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© R A Paselk

Last modified 27 October 2004