An unknown element X has the following isotopes: ²⁵X (80.5% abundant) and ²⁷X (19.5% abundant). What is the average atomic mass in amu of X?

25.39

Explanation:

Given parameters:

Abundance of X-25  = 80.5%

Abundance of X - 27  = 19.5%

Unknown:

Average atomic mass of X  = ?

Solution:

The average atomic mass of X can be derived using the expression below:

Average atomic mass = (abundance x mass of X - 25) + (abundance x mass of X - 27)

Average atomic mass  =  (80.5%  x 25) +   (19.5% x 27)  = 25.39

increased stability in the completion of fluorine's outer shell.