Fictitious element X has an a average atomic mass of 254.9 and only 2 isotopes, One of its isotopes has an abundance of 72.00 and a mass of 250.9 amu. What is the atomic mass of the second isotope? 265.2

20.80

245.9

251.9

none of the above

all of the above

265.2amu

Explanation:

Given parameters:

Atomic mass  = 254.9amu

Abundance of isotope 1 = 72%

Atomic mass of isotope 1  = 250.9amu

Abundance of isotope 2  = 100  - 72  = 28%

Unknown:

Atomic mass of isotope 2 = ?

Solution:

To find the atomic mass of isotope 2, use the expression below:

Atomic mass = (abundance of isotope 1 x atomic mass of isotope 1) + (abundance of isotope 2 x atomic mass of isotope 2)

 Now insert the parameters and find the unknown;

  254.9  = (0.72 x 250.9)  + (0.28 x Atomic mass of isotope 2)

 254.9  = 180.648 + 0.28x atomic mass of isotope 2

 254.9 - 180.648  = 0.28x atomic mass of isotope 2

    74.25  = 0.28 x atomic mass of isotope 2

    Atomic mass of isotope 2 = 265.2amu

atomic mass = 39

number of neutrons = 20

number of protons= 19

electronic configuration = 1s2 2s2 2p6 3s2 3p6 4s1

the number of electron in first shell = 2

number of electrons in n=3 shell = 8