How did Mendeleev decide which elements should be placed in the same group of the periodic table?

How will the emission of an alpha particle affect the atomic number of an atom

How many liters of hydrogen gas will be produced at stp from the reaction of 7.179×10^23 atoms of magnesium with 54.219g of phosphoric acid (h3po4) the equation is 3mg + 2h3(> mg(po4)2+3h2

Acontainer holds 35.8 moles of gas under 10.0 atm of pressure at 70.0 c. what is the volume of the container?