What is the density of the substance of it has a volume of 25ml and mass of 20.5g ?

820 kilogram/cubic meter

Explanation:

ρ =  m/V

=  20.5 gram/25 milliliter

=  0.82 gram/milliliter

=  820 kilogram/cubic meter

since the atomic number is the number of protons in the nucleus, as we move to the left on the periodic table, the electrostatic pull between the nucleus and the valence electron shell decreases. as we move down a group (vertical column), the valence shell energy level gets further and further from the nucleus.