Question

Not only does atomic number increase across a row and down a column, but so

does average atomic mass (with only a few exceptions).
In which three pairs of elements in the periodic table does the element with the
higher atomic number have a lower average atomic mass?
A. Thorium (Th) and protactinium (Pa)
B. Cerium (Ce) and praseodymium (Pr)
C. Platinum (Pt) and gold (Au)
D. Tellurium (Te) and iodine (1)
E. Cobalt (Co) and nickel (Ni)

Answer 1

In the periodic table, there are three pairs of elements where the element with the higher atomic number has a lower average atomic mass: Thorium (Th) and protactinium (Pa), Cerium (Ce) and praseodymium (Pr), and Cobalt (Co) and nickel (Ni).

Step-by-step explanation:

In the periodic table, there are three pairs of elements where the element with the higher atomic number has a lower average atomic mass:

1. Thorium (Th) and protactinium (Pa): Th has an atomic number of 90 and an average atomic mass of 232.038 g/mol, while Pa has an atomic number of 91 and an average atomic mass of 231.036 g/mol.
2. Cerium (Ce) and praseodymium (Pr): Ce has an atomic number of 58 and an average atomic mass of 140.116 g/mol, while Pr has an atomic number of 59 and an average atomic mass of 140.907 g/mol.
3. Cobalt (Co) and nickel (Ni): Co has an atomic number of 27 and an average atomic mass of 58.933 g/mol, while Ni has an atomic number of 28 and an average atomic mass of 58.693 g/mol.

Learn more about Elements with higher atomic number and lower average atomic mass