To solve this problem, we can look it up on the periodic table.
Barium has a mass number of 56, also the 56th element on the table. Mass number actually is equal to the number of protons, proton number of an element is like an ID card number, which no other elements have the same. So, the number of protons is 56.
Electrons of an atom equals to the number of proton too. So the number of electrons of barium atom is also 56. If the number of electrons is changed, the atom is no longer an atom, it'll become ions.
A substance that has a fixed chemical composition throughout and a mixture is when two or more substances are combined, but they are not combined chemically.
