## Chem – Isotopes

What sections should I know before attempting to learn this section?

—> Neutrons

What are isotopes?

So far we have been talking about the number of neutrons using only the mass numbers on the periodic table. However, those are truly only the average number of neutrons. If we were to look at a random atom, chances are it would have the number of neutrons we have been calculating so far, but that is not always the case. Some atoms of an element don’t have the same atomic mass as they do on the periodic table because they have more or less neutrons than the average number shown on the periodic table. Isotope is a word used to describe how two atoms of the same element can have different numbers of neutrons. Sometimes in a chemistry class, the definition of an isotope is not taught very well so I will also phrase it another way because it could be important to your class. If you have one helium atom with a mass of 4, like it is on the periodic table, and another helium atom with a mass of 5. Those two are isotopes of one another. It is not that the helium with a mass of 4 is the only isotope or that the helium with a mass of 5 is the only isotope. When using the word isotope, you are already suggesting that different atoms of the same element can have different numbers of neutrons. So an isotope will never be used to describe only one atom. It will always be used to describe the differences in neutrons between two or more atoms. Whenever you hear the word isotope you want to immediately think “I am comparing some kind of difference in neutrons”.

Remember your equation for counting neutron from the previous section.  neutrons = atomic mass – atomic number.

Examples: Make sure you have this periodic table link open when answering these questions.  VIDEO Isotopes Examples 1.

Argon – 42 (Argon that has a atomic mass of 42) is one isotope of argon. How many neutrons does it have?

Antimony – 124 (antimony that has a atomic mass of 124) is one isotope of antimony. How many neutrons does it have?

If you have Nitrogen – 15 and Oxygen – 17 and Sulfur – 34 and Nitrogen – 13 how many of these are isotopes of one another?

Answer: The two nitrogens are isotopes of one another because they have to have the same number of protons but a different number of neutrons.

PRACTICE PROBLEMS: Make sure you have this periodic table link open when answering these questions.

Magnesium – 25 is one isotope of magnesium. How many neutrons does it have?