Question

State which two elements from the first twenty elements of the modern Periodic Table are not arranged in order of increasing atomic mass.

Answer 1

The two elements not arranged by increasing atomic mass in the first twenty elements of the Periodic Table are hydrogen and helium, ordered instead by atomic number.

Step-by-step explanation:

The two elements from the first twenty elements of the modern Periodic Table that are not arranged in order of increasing atomic mass are hydrogen (H) and helium (He). Despite having a lower atomic mass, hydrogen is placed before helium because the Periodic Table is arranged by atomic number, which reflects the number of protons in an atom's nucleus.

For example, hydrogen, with an atomic number of 1, has one proton, and thus comes before helium, which has an atomic number of 2, despite helium having an atomic mass of about four times that of hydrogen.

The discovery by H.G.J. Moseley that the atomic number, not atomic mass, was the basis for the organization of elements resolved discrepancies like these in the Periodic Table.