Without oxygen, there can be no fire. The atomic number for oxygen is 8. O O 2 O 3 O 4 Oxygen exists in air in molecules of O 2 and O 3 . The air we breathe out contains more of a certain gas than the air we breathe in. The air we breathe out contains more carbon dioxide, than the air we breathe in.
Animals breathe in oxygen from the air.
We need oxygen to break down the food we eat and to release energy. When oxygen reacts with food, it forms carbon dioxide, which we breathe out. So we breathe out less oxygen and more carbon dioxide, than we breathe in. Iron is an element and all elements contain only one kind of atom. Iron contains only iron atoms.