How many variables should a scientist change at one time to have a good experiment?

When the earth was formed and cooled, why did nickel and iron end up in the center of the earth while basalt and granite ended up in the outer layers

How many moles of oxygen gas are produced from the decomposition of six moles of potassium

Choose all the answers that apply. fluorine (f) has an atomic number of 9 and an atomic weight of 18.99. fluorine has a. 9 protons b. 10 neutrons c. 18 electrons d. an atomic mass of 19 e. at least one isotope