There are two isotopes of an unknown element, X-19 and X-21. The abundance of X-19 is 12.01%. Now that you have the contribution from the X-19 isotope (2.282) and from the X-21 isotope (18.48), what is the average atomic mass (in amu) of this element using four significant figures

The average atomic mass of X is 16.53

Explanation:

Mass of isotope X-19  = 2.282  

% abundance of isotope X-19 = 12.01% =

Mass of isotope X-21 = 18.48

% abundance of isotope X-21 = (100-12.01)% =

Formula used for average atomic mass of an element :

Therefore, the average atomic mass of X is 16.53

answer is: nuclear fusion produces elements that are heavier than helium.

nuclei combine to form a heavier nucleus, releasing energy.  

for example, fusion of hydrogen atoms at the sun.  

this fusion nuclear reaction is in the inner core of the sun and hydrogen (atomic number 1) is conveted into helium (atomic number 2).  

the inner core is the central region of the sun and all solar energy is produced in the core by nuclear fusion.