0-17, an isotope of oxygen, has 9 neutrons. The of oxygen is 8.
isotope number
atomic number
mass number
atomic mass

=

100

% ofO

16

×mass+% ofO

17

×mass+% ofO

18

×mass

​

16.12=

100

90×16+17(a)+18(10−a)

a=8

% of O

17

=8%

% of O

18

=10−8=2%      

the answer is learned

the density(ratio between mass and volume as m/v) decreases from top to bottom in periodic table group as the size increases and nuclear charge decreases. but in periods density increases from left to right as atom size decreases and nuclear charge increases.