When matter of two different elements combine, the resulting matter is a compound.
Like NaCl (one Sodium atom combined with one chlorine atom) makes one molecule of salt.
H₂O (two atoms of Hydrogen combined with one atom of Oxygen) makes one molecule of water.
(A compound doesn't have to be made of only two atoms or two elements. A molecule of the kind of alcohol in wine has 9 atoms of Carbon, Hydrogen, and Oxygen. A molecule of one kind of DNA has 34 atoms. And a molecule of Sugar has 45 atoms ! )
