Mendeleev’s principle of chemical periodicity states that when the elements are arranged according to their atomic numbers. this periodic law allows elements with similar properties to appear at regular intervals. predict the group of elements and their number of valence electrons that will combine in a 2: 1 ratio to with sodium. a)
bromine, chlorine, iodine, 7
b)
sulfur, selenium, polonium, 6
eliminate
c)
hydrogen, fluorine, potassium, 1
d)
calcium, phosphorus, tellurium, 2

The main part of this question is the "2:1 ratio."

On the periodic table, bromine, chlorine, and iodine have 7 valence electrons and sulfur, selenium, and polonium have 6, so these choices are viable based on their valence electrons. However, fluorine does not have 1 valence electron and phosphorus and tellurium do not have 2 valence electrons, so C) and D) are automatically out, leaving A) and B).

If we know that the sodium will combine in a 2:1 ratio, we know that the charge of the ions must have a 2:1 ratio too; that's how ionic compounds work. Sodium (Na) has an ion with a charge of +1 (Na+), so that means whatever ion it combines with in a 2:1 ratio must have a charge of -2   -   a 2:1 ratio.

Bromine, chlorine, and iodine have a charge of -1, but sulfur, selenium, and polonium have charges of -2. This gives us the correct answer as C).