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# Naturally occurring europium (Eu) consists of two isotopes with a mass of 151 and 153. 151/63 Eu has an abundance of 48.03% and 153/63 Eu has an abundance of 51.97%. What is the atomic mass of europium?

The of europium is 152 u.

**Method 1**

Assume that you have 10 000 atoms of Eu.

Then you have 4803 atoms of ##""_63^151"Eu"## and 5197 atoms of ##""_63^153"Eu"##.

Mass of ##""_63^151"Eu"## = 4803 × 151 u = 725 253 u

Mass of ##""_63^153"Eu"## = 5197 × 153 u = 795 141 u

Mass of 10 000 atoms = 1 520 394 u

Average mass = ##(1520394"u")/10000## = 152 u

**Method 2**

##""_63^151"Eu"##: 48.03 % × 151 u = 72.5253 u

##""_63^153"Eu"##: 51.97 % × 153 u = 79.5141 u

Total = "152.0394 u" = 152 u

Note: In both methods, the answer can have only three , because that is all you gave for masses of the two .

Hope this helps.