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Physical Science Notes 416-436
Isotopes
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The "true" purpose to neutrons

Isotopes

 

-What is an Isotope?

                -An Isotope is an atom of an element with a different number of neutrons (nē) in its nucleus

 

-Drawing Isotopes

-To draw an Isotope you use a Bohr Model (If you do not know how to draw Bohr models look up in the notes for the section called Bohr Models). The only piece of information that changes in the atom of Isotopes is the number of nēs in the nucleus. Remember, if you change the number of protons (p+) you get a new element.

E.g.: There are two Isotopes of Cl (chlorine)

                                                Cl-35       and         Cl-37

                                                35 and 37 = mass # or atomic mass

 

                                                                Cl-35

                                Atomic #: 17

                                Atomic Mass: 35

                                Per: 3 = 3 EL

                                Gr: 7A = 7 valence (val) electrons (e-)

isotope.jpg

Cl-37

                        Atomic #: 17

                        Atomic Mass: 37

                        Per: 3 = 3 EL

                        Gr: 7A = 7 valence (val) electrons (e-)

isotope2.jpg

-But thats not all. You also have to know in what percentage these Isotopes occur in nature. With that information you can find out the average atomic mass of an element. The only way to find this out is to use a bit of mathematics.

            1)The first step is to write down the percentage in which the Isotopes occur.

                        75%                      25%

Of Cl atoms       Of Cl atoms

 are Cl-35          are Cl-37

2)The second step is to convert our percentages into decimals. A simple mathematical equation does the trick. Take your percentage and divide it by 100.

            75/100              25/100

              .75                    .25

3)Next take your new decimal and multiply it by its Atomic Mass.

            .75x35              .25x37

 26.25                                9.25

4)After the multiplication you are left with two decimals. Add them together to find your average atomic mass for the element you are dealing with.

            26.25 + 9.25 = 35.5

5)Finally you are left with that atomic mass units (amu) of the element you were dealing with.

                        35.5 amu

-To help make sense of the work you did following a procedure can help in remembering steps and aid in organization. Here is a final product following a structured procedure:

           

Cl

(Chlorine)                               Atomic #         Atomic Mass

            Cl-35   - 75%                17                     35

            Cl-37   - 25%                17                     37

 

            75/100                        25/100

              .75                                .25

 

            .75x35                        .25x37

 26.25                                                    9.25

 

          26.25 + 9.25

                                                 35.5amu

 

If you need a simple way to remember the procedure of writing Isotopes memorize this:

%

% --> decimal

Decimal x Atomic Mass

Decimal + Decimal + ...

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