Chapter Two, Section Two


  1. Learn about the particles of the atom, mass number, atomic number.
  2. Learn about ions, isotopes, and atomic weight.
  3. Learn about the periodic table, metals, nonmetals, and metalloids.


The atom consists of a nucleus with a diameter of about 10-13 cm and electrons that move about the nucleus at an average distance of about 10-8 cm. (Corresponds to a marble of diameter 0.5 inch --> Dome with a radius of about 547 yards -- check this calculation yourself). As we will see later, the chemistry of atoms is mainly due to the electrons and therefore we can make do with a crude model of the nucleus (in nuclear chemistry this will not suffice) which is that the nucleus consists of protons (positive charges) and neutrons (no charge):

Electron9.11 x 10-31kg-1
Proton1.67 x 10-27kg+1
Neutron1.67 x 10-27kgNone

From our Dome model we see that the atom is mostly space with the electrons occupying most of that volume. It is the "intermingling" of the electrons which causes the binding that occurs when atoms combine to form molecules. Since elements can contain very different numbers of electrons then we expect there to be significant differences in the chemical reactivity of atoms. We indicate the number of electrons and protons in an atom with a symbol:


where X is the symbol for the element (C for carbon, O for oxygen, etc), A is called the mass number (the total number of protons and neutrons in the nucleus), and Z is called the atomic number (the number of electrons which is also equal to the number of protons). Some examples follow.

147N This is the symbol for nitrogen showing that it has 7 electrons, 7 protons, and 7 neutrons (14-7).

5626Fe This is the symbol for Fe showing that iron has 26 electrons, 26 protons, and 30 neutrons (56-26).

Rather then using the very small gram numbers as above, another unit is used called the aomic mass unit (amu) where 1 amu = 1.67 x 10-24 g. Thus the proton has a mass of approximately 1 amu.


When we get to carbon we find experimentally that two possible symbols exist: 126C and 136C. We see that each have 6 electrons and 6 protons, but one has 6 neutrons and one has 7 neutrons. Experimentally we also see that about 91% of all carbon atoms have the first set and about 9% have the second set. Since each has the same number of electrons, each will behave the same chemically. This observation leads to the definition of isotopes :atoms of the same element which have the same number of electrons but differ in the number of neutrons .

Most isotopes are referred to simply by their element and their mass number such as U-235 (uranium 235) instead of 23592U. Since the symbol tells us the atomic number (from the periodic table -- see later), we can omit the atomic number and just use the mass number: 235U. However some isotopes have specific names such as 1H is hydrogen, 2H is deuterium, and 3H is tritium. When water is made from deuterium rather than hydrogen, it is referred to as "heavy water".

Most elements do exist as combinations of isotopes. The atomic weight is the weighted average of the masses of the isotopes. For example, above we said that carbon existed as about 91% C-12 and about 9% C-13. To calculate the atomic weight of carbon we proceed as follows:

(91/100)* 12amu + (9/100)*13amu = 12.09 amu

If we had been more accurate with the percents, we would have gotten 12.011 amu, which is the number below C in the periodic table. We see that there is no such thing as a C atom with a mass of 12.011 amu, but we will use that number in our calculations for the statistical sample of elements used will have on average that mass.

Ions are particles with an uneven number of protons and electrons so that it will have either a positive charge (cation) or a negative charge (anion).


A periodic table shows most of the elements known -- depends upon when the table was prepared as to how many are there. The letters in the boxes of the table show are the symbols for the elements which come from either the original names or the current names. Some examples are below:
Current NameOriginal NameSymbol

I don't recommend that you try to memorize all of the original names, but just to memorize the element symbols for each of the elements. Fortunately many of them are easy like H,He,Li,Be,B,C,N,O,F,Ne. You might notice that I just went through the first two rows of the periodic table.

The numbers above the symbols are the atomic numbers (number of electrons which is equal to the number of protons) of the elements. Remember that I said that the chemical properties of the elements are determined by the electrons? Good. Now you can know why the elements are arranged the way that they are. All of the elements in a particular column have very similar chemical properties. We will see why that is later when we learn more about the electronic structure of the elements. These columns are called groups or families. The rows are called periods.

Looking at a periodic table like the one above, you will see that there are also descriptive names for sets of the elements: alkali metals, alkaline earth metals, transition metals, non-metals, halogens, noble gases, lanthanides, and actinides. You will see that most of the elements are metals. The noble gases have very little chemical reactivity. It was not until about 1962 that the noble gases were found to be chemically reactive when XeF4 was discovered. Metals tend to lose electrons to form cations, conduct heat and electricity, be malleable (easily formed), and tarnish easily. Nonmetals tend to gain electrons to form anions.

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.

Also note the numbers above each column like IA, IIA, etc. The Group IA metals all form +1 cations and the Group IIA metals all form +2 cations. You will learn more about the periodic table as we continue our study.


  1. Here is an excellent Periodic Table
  2. Here is a little more information on atomic structure.
  3. Here is a site developed by some high school students illustrating atomic structure.
  4. How many electrons, protons, and neutrons are present in the following: K- with a mass number of 39, Cl with a mass number of 37, and Pb2+ with a mass number of 207.There are two naturally occuring isotopes of antimony, 121Sb and 123Sb. What are the abundances of the two isotopes? Post your answers on WebCT.
  5. wct


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

Table Of ContentsE-Mail

Web Author: Dr. Leon L. Combs
Copyright 2001 by Dr. Leon L. Combs - ALL RIGHTS RESERVED