Mass is a constant; density (amount of matter in an object).  Matter is a composition of atoms.  Carbon (C), hydrogen (H), oxygen (O), and nitrogen (N) make up 96% of human body matter.  Twenty-five of ninety-two naturally-occurring elements are of value to organisms.  Atoms are the basic structure of matter.

Protons are the atomic number of an element; have positive charges; 1 amu mass; and are located in the nucleus.  Neutrons have equal mass of a proton and are located in the nucleus; however, they have no charge.  Electrons (e^-) are negatively charged particles and revolve around the nucleus in orbitals.

Bonds can be ionic (involving a transfer of electrons), covalent (involving the sharing of electrons equally, to achieve stable orbital configurations), polar covalent (involving the sharing of electrons unequally, due to differences in the affinities of the atoms involved in the covalent bond.  Hydrogen bonds only occur between H, N, O, and F (fluorine)and are very weak - the weakest type of bonding, in fact.  Thus they conserve a large amount of energy because when they are broken only a small amount of energy is released.  Hydrogen bonds stabilize water molecular associations to provide cohesiveness.

Compounds, Molecule, or Mixture?  A compound is two or more elements in a chemical bond (usually ionic) at a specified ratio.  A molecule is two or more atoms participating in covalent bonding (polar or nonpolar).  A mixture is two or more substances put together with no regularity whatsoever.