Two elements, hydrogen and oxygen, make up water. You smell the pie. Some of the atoms of the pie had to move to your nose for you to smell the pie. Neutrons and protons contribute most to an atoms mass. The nucleus contains the neutrons and protons so you could also say that the nucleus contributes most to an atoms mass.
Electrons contribute most to an atoms size.
The nuclei are positively charged. Nuclei can't get too close together because the positive nuclei repel each other. The number of protons distinguishes different types of atoms. The atomic number of an atom, is the number of protons in the atom. The cat shed some molecules that the dog later smells.