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Isotopes and Atomic How to write it The of an element can be indicated in two ways: An element is distinguished by always having the same number of in the nucleus. Using the elements symbol (numbers can either be to the left or the right of the symbol): However, there can be a variable number of . 54 54 An isotope of an element has the Fe Fe 26 (may not be present) 26 same atomic number but has a different number of neutrons.

This will affect the average mass Using the elements of an of an element. name:

If we take a specific example it will become clear. Next > - 54 Next >

Example Question 1

Iron, atomic number 26, has four has an atomic number of 6. Its two naturally naturally occurring . Atomic occurring isotopes are carbon 12 and carbon 13. 54 (number of ) They are: How many neutrons do carbon 12 and 13 have? Fe Atomic number Fe-54, Fe-56, Fe-57, and Fe-58. 26 (number of protons) Carbon -12 Carbon -13 From the table, we can A) 6 7 see the number of protons neutrons nucleons B) 12 13 neutrons in each Fe-54 26 28 54 isotope’s nucleus. Fe-56 26 30 56 C) 7 6 A , is a Fe-57 26 31 57 D) 13 12 particle that is in the Fe-58 26 32 58 nucleus (either a or a ). Next > Next >

Question 1 Abundance of each Isotope of Iron

Carbon has an atomic number of 6. Its two naturally If a sample of iron was occurring isotopes are carbon 12 and carbon 13. examined with a mass How many neutrons do carbon 12 and 13 have? , the abundance of each Carbon -12 Carbon -13 isotope could be A) 6 7 measured.

B) 12 13 Luckily, tables exist giving Mass spectrometer us this information from Abundance in % C) 7 6 previous experiments. Fe-54 5.8 Fe-56 91.72 D) 13 12 How does this give us Fe-57 2.2 the average mass of Fe-58 0.28 Next > an atom? Next >

1 Question 2 Question 2

Copper has two naturally occurring isotopes (Cu-63 has two naturally occurring isotopes (Cu-63 and Cu-65). and Cu-65). If copper 63 has an abundance of 69.17% what is If copper 63 has an abundance of 69.17% what is the abundance of copper 65? the abundance of copper 65?

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Calculation of Atomic Mass Number Calculation of Atomic Mass Number

What is the atomic mass of each isotope? Multiply the mass Abundance Atomic mass Atomic mass of each isotope in % abundance Fe-54 53.93612 5.8 3.12829 Atomic mass by the Again, tables give us that Fe-56 55.93439 91.72 51.30302 Fe-54 53.93612 abundance, to data. (This is the atomic Fe-57 56.935396 2.2 1.25258 Fe-56 55.93439 give the mass relative to 1/12 of a Fe-58 57.933278 0.28 0.16221 Fe-57 56.935396 contribution of carbon atom). Total 55.8461 Fe-58 57.933278 each isotope.

Summing the individual abundances, gives the total relative (average) atomic mass of iron.

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Calculation of Atomic Mass Number

The gives the of iron as 55.85.

If we round up our calculation to 2 d.p. we also get 55.85.

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