The atom is the basic part of matter and a basic understanding of its structure is probably the best place to start this unit.
We should keep in mind that atoms are small and consist mainly of empty space. When atoms combine to make different molecules the spaces between molecules of different substances vary. To stress this point put 50ml. of water in a graduated cylinder and 50ml. of ditto fluid in another cylinder. Ask the students to predict the combined volumes of the liquids. The volume will not be 100ml. which will help get across the point of spaces between the molecules. The actual volume will be +98ml.
Since atoms are too small to be studied directly, models are used to describe them and their activities. The average diameter of an atom is about 2 x 10 to the minus 10 meters.
To describe the internal structure of the atom, we need to consider the proton, neutron, and electron.
The proton carries a (+)charge and is located in the nucleus along with the neutron which has a (0) charge. The electron has a (-) charge and circles the nucleus.
(figure available in print form)
There are approximately 108 elements both natural (92) and artificial with the number of protons distinguishing one element from another. An atom also has an equal number of protons and electrons which prevents it from having an electrical charge. The Periodic Table arranges the elements in an orderly fashion which aids us in understanding and predicting how they will react with one another.
Let us use symbols for the elements and think of these symbols as letters of the alphabet. Some letters can be combined to form words while others will not, just as some atoms will combine to form new substances and others will not combine.
Except for hydrogen and helium, the atoms of the lower order strive to have 8 electrons in their outer shell. This can be accomplished by lending or borrowing electrons and in some cases sharing them to stabilize there outer shell.
We will use groups IA IIA IIIA IVA VA VIA VIIA to explain a basic method of predicting a chemical reaction.
(figure available in print form)
Let us use a (+) sign for the tendency to give up electrons and a (-) sign for the tendency to borrow electrons. The roman numeral in front of the letter “A” indicate the number of electrons in the outer shell which aids us in determining their valence.
IA=+ IIA=++ IIIA=+++ IVA=++++/ÐÐÐÐ VA=ÐÐÐ VIA=ÐÐ VIIA=Ð
The number of (+) or (Ð) signs will not change for an atom thus each time it enters into a chemical reaction it will have the tendency to follow the pattern of lending or borrowing the number of electrons in its group.
In order to have a chemical reaction we need something to lend and something to borrow electrons. The number of (+) signs must equal the number of (Ð) signs in the completed reaction.
When an atom loses or gains an electron it takes on an electrical charge and becomes an ion. If it loses an electron the protons outnumber the electrons and it becomes a (+) ion. If the reverse happens and it gains an electron, the electrons will out number the protons and it will become a (Ð) ion. The results of one atom becoming a (+) ion and the other a (Ð) ion causes and electrical attraction between them and a chemical reaction may occur.
Let’s combine some elements using this method. examples:
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Li=+
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Cl=Ð
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rxn=LiCl
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H=+
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O=ÐÐ
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H=+
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rxn=H20
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Al=+++
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O=ÐÐ
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Al=+++
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O=ÐÐ
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O=ÐÐ
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rxn=Al2 03
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Notice the number of (+) signs equal the number of (Ð) signs in each completed reaction. The subscript is determined by the number of times each atom has to be written down to balance the number of (+) and (Ð) signs to balance the reaction.
Now that we have a basic understanding of why certain atoms will combine with one another and others will not combine, let us concern ourselves the measuring of matter.
