Chlorine has an atomic number of 17. it often forms an ion by gaining 1 electron. what would its charge be?

The charge would be -1.

Explanation:

The atomic number refers to the number of protons an atom has. Chlorine is electrically neutral, so it has the same  number of positive charges (protons) than negative charges (electrons). Therefore, chlorine has 17 protons and 17 electrons. When it gains 1 electron, it will have an excess of one negative charge, so the charge of the ion will be -1, forming Cl⁻.

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.