What is the difference of the atomic mass and the atomic size?

Answer 1

Neutrons

The size of an atom is determined by the number of electrons since they orbit in the outer rings. The mass is pretty much the same as the weight (under 1 Gravity, like Earth). The atomic size is often called the atomic number, which is a very important number.

Atomic Mass is the total weight of an atom including the neutrons, protons and electrons.

Atomic Number is the number of protons in the atom. In all but ions it is the same number of electrons. Ions either temporally have fewer or more electrons then they should. The Atomic Number is the atom's rating on the table of periodic elements. Hydrogen is 1, Helium is 2, Lithium is 3...

If you go to the Periodic Table of Elements then you can find the atomic weight of the standard atom based on its atomic number. http://en.wikipedia.org/wiki/Periodic_table

According to Wikipeida: http://en.wikipedia.org/wiki/Atomic_mass
"The atomic mass (ma) is the mass of an atom at rest, most often expressed in unified atomic mass units.[1] The atomic mass may be considered to be the total mass of protons, neutrons and electrons in a single atom (when the atom is motionless). The atomic mass is sometimes incorrectly used as a synonym of relative atomic mass, average atomic mass and atomic weight; however, these differ subtly from the atomic mass."

According to Wikipeida: http://en.wikipedia.org/wiki/Atomic_number
"In chemistry and physics, the atomic number (also known as the proton number) is the number of protons found in the nucleus of an atom. It is traditionally represented by the symbol Z. The atomic number uniquely identifies a chemical element. In an atom of neutral charge, the number of electrons also equals the atomic number."

According to Wikipedia: http://en.wikipedia.org/wiki/Atomic_size
"Atomic radius, and more generally the size of an atom, is not a precisely defined physical quantity, nor is it constant in all circumstances. The value assigned to the radius of a particular atom will always depend on the definition chosen for "atomic radius", and different definitions are more appropriate for different situations.

The term "atomic radius" itself is problematic: it may be restricted to the size of free atoms, or it may be used as a general term for the different measures of the size of atoms, both bound in molecules and free. In the latter case, which is the approach adopted here, it should also include ionic radius, as the distinction between covalent and ionic bonding is itself somewhat arbitrary.

The atomic radius is determined entirely by the electrons: The size of the atomic nucleus is measured in femtometres, 100,000 times smaller than the cloud of electrons. However the electrons do not have definite positions—although they are more likely to be in certain regions than others—and the electron cloud does not have a sharp edge."