Which element has the greatest density at stp?
(1) scandium (3) silicon
(2) selenium (4) sodium

            Selenium has the greatest density at STP.

                   According to Ideal gas equation,

                                                P V  =  n R T

where;

             n =  m/M

Therefore,

                                                P V  =  m R T / M

Rearranging,

                                                P M  =  (m/V) R T

Also,

             m/V  =  density  = d

So,

                                                P M  =  d R T

Solving for density,

                                                d  =  P M / R T

According to this equation density is directly proportional to Molar mass of a substance. Hence, the Atomic masses of given compounds are listed below,

                                                Scandium =  44.96 g.mol⁻¹

                                                Selenium  =  78.97 g.mol⁻¹

                                                Silicon  =  28.09 g.mol⁻¹

                                                Sodium  =  22.99 g.mol⁻¹

Therefore, selenium having the greatest Atomic Mass will have the greatest density. The densities of given elements are as follow;

                                                Scandium =  2.98 g.cm⁻³

                                                Selenium  =  4.81 g.cm⁻³

                                                Silicon  =  2.33 g.cm⁻³

                                                Sodium  =  0.96 g.cm⁻³

< > according to newton's third law of motion, forces always act in equal but opposite pairs. another way of saying this is for every action, there is an equal but opposite reaction.< >

chemical changes occur when a substance combines with another to form a new substance, called chemical synthesis or, alternatively, chemical decomposition into two or more different substances. these processes are called chemical reactions and, in general, are not reversible except by further chemical reactions. some reactions produce heat and are called exothermic reactions and others may require heat to enable the reaction to occur, which are called endothermic reactions. understanding chemical changes is a major part of the science of chemistry.