Which statement about the periodic table is correct?A.     The elements with atomic numbers 8, 16 and 34 have the same number of main energy levels.B.     The elements with atomic numbers 8, 9 and 10 have similar chemical properties.C.     The elements with atomic numbers 20, 21 and 22 are in the same group.D.     The elements with atomic numbers 20, 38 and 56 have the same number of electrons in their outer energy level.

The correct statement about the periodic table is D. The elements with atomic numbers 20, 38 and 56 have the same number of electrons in their outer energy level.

Here's why:

A. The elements with atomic numbers 8, 16 and 34 do not have the same number of main energy levels. The number of main energy levels is determined by the period number in the periodic table. Oxygen (8), Sulfur (16), and Selenium (34) are in periods 2, 3, and 4 respectively.

B. The elements with atomic numbers 8, 9 and 10 do not have similar chemical properties. These elements are Oxygen, Fluorine, and Neon. Oxygen and Fluorine are both highly reactive, while Neon is a noble gas and is very stable.

C. The elements with atomic numbers 20, 21 and 22 are not in the same group. These elements are Calcium, Scandium, and Titanium, which are in groups 2, 3, and 4 respectively.

D. The elements with atomic numbers 20, 38 and 56 are Calcium, Strontium, and Barium. These elements are all in group 2 of the periodic table, meaning they have the same number of electrons in their outer energy level, which is 2.