The atomic number of this element is 9 and its mass is 20. how many neutrons does this element have?

11 neutrons

Explanation:

Mass no. = 20

atomic no. = 9

no. of neutrons = mass number - atomic number

                          =   20 - 9

                          =     11 neutrons

there all right good

explanation:

2h2o2--> 2h2o + o2

this reaction is of the spontaneous decomposition of hydrogen peroxide down into water and oxygen.

add 2 molecules of hydrogen peroxide and 2 molecules of water. since oxygen is naturally diatomic, the total number of atoms of each element is now the same on both sides of the equation so it is balanced.

this may not be entirely correct but let me know.