Question

1. why do atomic numbers jump by 15 from left to right in the section of the periodic table shown above?

the missing elements form the lanthanide and actinide series, usually found below the periodic table.
the missing elements are all radioactive.
the missing elements do not exist.
the missing elements have not yet been discovered.

Explanation:

The lanthanide and actinide series are placed below the main body of the periodic table to keep the table more compact. These series consist of elements with atomic numbers that would otherwise cause large jumps in the main table layout. Radioactive elements are not the reason for the jump (many non - radioactive elements are in these series). The elements in these series do exist and have been discovered.

Answer:

The missing elements form the lanthanide and actinide series, usually found below the periodic table.