Unit 3

What is an isotope?Atoms of the same element can have different numbers of neutrons; the different possible versions of each element are called isotopes.

   For example, the most common isotope of hydrogen has no neutrons. There is another version of hydrogen with 1 neutron, and another version with 2 neutrons.  

Hydrogen-1 Hydrogen-2 Hydrogen - 3

What makes one isotope different from another isotope?

__________________________________

Symbols for Isotopes

I Do: Write the isotope symbol and name for an atom with 42 protons, 40 electrons, and 54 neutrons

Symbol______Name__________________

 

We Do: Write the isotope symbol and name for an atom with 42 protons, 42 electrons, and 58 neutrons

Symbol______Name________________________

You Do: Write the isotope symbol and name for an atom with 50 protons, 50 electrons, and 62 neutrons

Symbol______Name________________________

 

How many isotopes can one element have? Can an atom have just any

number of neutrons?There ARE "preferred" combinations of

neutrons and protons– Light elements tend to have about as many

neutrons as protons– Heavy elements apparently need more

neutrons than protons in order to stick together.

– Atoms with a few too many neutrons, or not quite enough, can sometimes exist for a while, but they're unstable.

How many isotopes can one element have? Can an atom have just any

number of neutrons?

• Do atoms just fall apart if they don't have the right number of neutrons? – YES! Unstable atoms are radioactive: their

nuclei change or decay by spitting out radiation, in the form of particles or electromagnetic waves.

Common Uses for Isotopes

Smoke detectors - Many contain a small amount of americium-241. By utilizing the radioactive properties of this material, smoke from a fire can be detected at a very early stage.

Common Uses for IsotopesRadioactive dating - If the half life of a given

isotope (usually Carbon-14) is known, the amount of that isotope and the product of its decay can be analyzed to determine the age of once-living organisms

Example : If an organism such as a tree contained 1 gram of carbon-14 while it was living, then after 5700 years it would contain half that amount, or 0.5 grams of carbon-14.

Common Uses for Isotopes

Medicine - Radioactive isotopes can be injected into a patient’s body. These isotopes will then appear on a scan and can help doctors diagnose problems or disease

Common Uses for Isotopes

Nuclear Energy - Radioactive elements such as uranium can be utilized to create energy through fission (splitting apart).

Average Atomic Mass

Listed on the periodic table

• Is a weighted average of all the isotopes for that element.

• This is why atomic mass is not a whole number.

Steps for Determining Average Atomic Weights

1. Turn the percent into a decimal.

2. Multiply the decimal by the mass.

3. Repeat for each isotope

4. Add the masses up.

Example: MagnesiumIsotopes Mass Percent of Isotope Abundance

Mg-24 = 24.0 amu 78.70% = _____________

Mg-25 = 25.0 amu 10.13% = _____________

Mg-26 = 26.0 amu 11.17% = _____________

Atomic mass (average mass) Mg= ___________ amu