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Kate McEvoy

on 11 November 2013

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Transcript of Isotopes

What are isotopes?
Atoms of the same element that have different numbers of neutrons
Atomic Weight on the Periodic Table
On the Periodic Table, there is an atomic weight, the decimal number in the box for the element.
Using Atomic Weight
Think about averages. Averages of numbers tend to be in between the numbers that you averaged.
Naming Isotopes
Since isotopes are the same element, how do scientists tell them apart?
Finding Isotopes
Elements can have anywhere between 2 and 20 isotopes.
Mass Spectrometer?
A mass spec (cool scientist lingo) measures the mass of individual atoms in a sample and reports a count of how many atoms it measured at each mass.
Atomic Weight reveals isotopes
Look at carbon, with an atomic weight of 12.011.
Atomic Weight reveals most common isotope
We're not simply averaging 2 numbers; there are more atoms of one isotope than another.

The weight is closest to the mass of the more common isotope.
Different neutrons??
Wait a minute...
How can these atoms be the same element and yet not be the same??
Protons and Electrons determine the identity and reactivity of an element, so different masses do NOT affect the chemical properties of an element.

Isotopes have essentially identical chemical properties.
This atomic weight is the average mass of an atom of that element.

We find this using the mass spec's data of how many atoms of each isotope there are in a sample.
Atomic Weight of B
In a sample of 100 atoms of Boron, 20 have a mass of 10 and 80 have a mass of 11.
Most Common Isotope
Going back to carbon, with an atomic weight of 12.011, we can see that the most common isotope of carbon is one with a mass of 12.
Atom Symbol
We can also use the symbol for the element, with the mass number and atomic number on top of each other to the left of the element symbol.
Remember that the neutrons only change the mass of the atom, NOT what element the atom is.
Mass Spec Data
Atomic Weight of B
Average this out to give an atomic weight of 10.8.
Notice that this average is closer to 11 than to 10 - because 80% of the sample has a mass of 11.
Isotopes of C
It probably has isotopes with a mass of 12 and a mass of 13, since the average mass is 12.011.
Most likely, C has a mass of 12
The average mass is 12.011, so MOST atoms of C have a mass of 12.
Mass Spec Data
What is the most common isotope of Copper?
What are the isotopes of fluorine likely to be?
How correct am I??
Educated guesses are really good guesses, but there is room for error.
Cl has an atomic weight of 35.45.

It has 2 isotopes: 35 and 37.
What are the names & symbols for the isotopes of gold?
Full transcript