Atomic Mass Standard mass unit is derived from carbon 12 Atomic mass unit – the mass equal to 1/12 the mass of one Carbon 12 atom.

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Presentation transcript:

Atomic Mass Standard mass unit is derived from carbon 12 Atomic mass unit – the mass equal to 1/12 the mass of one Carbon 12 atom

Atomic Mass Experiments show that H is 8.400% less massive than carbon x amu = amu

Average atomic mass The number on the periodic table is based on the average of isotopes Carbon is labeled as seems unlikely from above discussion Carbon comes in two main isotopes: 98.90% carbon 12 and 1.10% carbon 13 Mass of carbon 12 = amu carbon 13 = amu

Average atomic mass To get average (Percent/100) a x mass a + (Percent/100) b x mass b + …. For carbon (0.9890) ( amu) + ( ) ( ) = = 12.01

Problem Mg has three naturally occurring isotopes. Mg 24 ( amu) with % abundance, Mg 25 ( amu) with 10.00% abundance, and a third will 11.01% abundance. Using the atomic mass of magnesium from the periodic table, calculate the mass of the third isotope.

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