Calcium has 6 different isotopic forms, listed with their fractional composition values, 40Ca-0.96941; 42Ca-0.00647; 43Ca-0.00135; 44Ca-0.2086 ; 46Ca-0.00004; and 48Ca-0.00187. Identify the most and least abundant isotopes of calcium.

The most abundant isotope is ⁴⁰Ca and the least abundant is ⁴⁶Ca

Explanation:

The mass, in percentage, of eah isotope of Calcium is their fractional composition multiplied by 100:

40Ca-0.96941*100 =  96.941% of ⁴⁰Ca

42Ca-0.00647*100 = 0.647% of ⁴²Ca

43Ca-0.0013*100 = 0.13% of ⁴³Ca

44Ca-0.02086*100 = 2.086% ⁴⁴Ca

46Ca-0.00004*100 = 0.004% ⁴⁶Ca

48Ca-0.00187*100 = 0.187% of ⁴⁸Ca

That means the most abundant isotope is ⁴⁰Ca and the least abundant is ⁴⁶Ca

it's c. elements are placed in rows based on how may valencies electrons have.