atomic structures about isotopes and the relative atomic mass

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kellly
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a sample of copper contains 30%Cu-65 and 70%Cu-63. Describe the structure of the two forms copper and explain why in this sample boron has a relative atomic mass of 63.5
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LukeT333
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Okay so not sure if this is right but:

Structure:
The numbers next to the Cu represent the mass number, the mass number is the total number of protons + the total number of neutrons. Since isotopes are the same element but just have more/less neutrons:
Cu - 65 has 29 protons and (65-29) 36 neutrons
Cu - 63 has 29 protons and (63-29) 34 neutrons
They both have the same number of electrons as the proton number doesn't change (therefore they both have 29 electrons)

Atomic mass of 63.5:
because elements have multiple isotopes (forms of the same element with different no.neutrons) you need to work out an average mass called the Relative atomic mass (Ar):

the relative atomic mass can be worked out using:

RAM (ar) = sum of (isotope abudance x isotope mass number) / sum of abdundances of all isotopes (which should equal 100)
so just substitute the values in
RAM (ar) = (70 x 63) + (65 x 30) / 100 = 63.6(but the percentages will be more like 30.85% which will give 63.5

Hope this was helpful
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