Where are the elements that have the same valence electrons found in the periodic table

What’s the scientific notation for the number 6,840,000,000

Kc = 0.040 for the system below at 450oc. if a reaction is initiated with 0.40 mole of cl2 and 0.40 mole of pcl3 in a 2.0 liter container, what is the equilibrium concentration of cl2 in the same system? pcl5(g) ⇄ pcl3(g) + cl2(g)

Calculate the number of molecules present in 0.750 mol of mgo.