Acertain element x has four isotopes. 94.93% of x has a mass of 31.97207 amu . 0.7640% of x has a mass of 32.97146 amu . 4.290% of x has a mass of 33.96786 amu . 1.600×10−2% of x has a mass of 35.96709 amu . what is the average atomic mass of element x ?

Average atomic mass is the weighted average atomic mass of an element with regard to the abundance of all its isotopes.
average atomic mass of X can be calculates as follows;
average atomic mass of X = sum of the products of mass of isotopes by their abundance percentages
average atomic mass of X = (94.93 % x 31.97207 amu) + (0.7640% x 32.97146 amu) + (4.290% x 33.96786 amu) + (1.600×10−2% x 35.96709 amu) 

average atomic mass of X = 30.35 + 0.2519 + 1.457 + 0.005755
therefore average atomic mass of X = 32.06 

Determine the  wavelength of light emitted when an electron  in an li2+ ion  makes a transition from an orbital with n  =  6 to an orbital with n  = 5.

Both models are effective because they show the valence electrons, and these valence electrons indicate the reactivity of an element.