Atoms, are one of the smallest things in the world, they are the building blocks of matter and all things living. If you where to enlarge an atom that the center/core would be the size of orange or basketball, and the electrons would be the size of a grain of rice. The atom would be the size of a football stadium. Most of a atom is dead air space. There are few ways to split an atom, and one is to put it through a nuclear reaction. This would split the atoms apart. That is how a nuclear bomb works.
A pure element is made up of just one kind of atom. If you have two different atoms in one object, it is not a element. Helium is a element, water is not. Water is two hydrogen, and one oxygen. Another name that is not as familiar is DiHydrogen Monoxide. On the periodic table there is 118 different elements, some are strange and others not so strange.
If you add vinegar and baking soda they for a reaction that produces carbon dioxide. The reaction is chemical. A nuclear reactor creates new atoms by ripping old ones apart, thus creating heat. Oxygen on iron creates rust or iron oxide which is a reaction, but to speed things up, just add water.
On the element you see a number. That number is the atomic number. The letters for example Au= gold. The letters are in Italian. Au has the atomic level of 79. As you have probably noticed there is no steel on the table, and why? Because steel is not a element, it is man made from Iron carbon allo.