How would you describe the relationship of the atomic number and mass of an element?

Answer 1

atomic number is the number of protons in the nucleus. Atomic mass is the sum of protons and neutrons (weighted average if you have isotopes with different number of neutrons).

atomic mass = atomic number + # neutrons.

Answer 2

The "relationship" between atomic number and the mass (or "atomic weight") of elements is this:

First, the atomic number is simply the number of protons in an atom's nucleus.

Second, the "atomic weight" is defined in terms of the number of "atomic units" of weight that an atom posesses. CRUDELY speaking, this is the same as the combined number of both protons and neutrons in the nucleus. This is "crude" for several reasons: (a) the neutron and proton DIFFER from one another in their masses by a small, but nevertheless measurable amount; (b) different kinds of atoms, particularly those for heavier atoms and or other isotopes of given atoms, DIFFER in the ratio of neutrons to protons in their nuclei; and (c) although they weigh less than 1/1800th of the masses of protons or neutrons, both ELECTRON masses AND their electronic BINDING ENERGIES also contribute to the measured chemical masses of atoms.

For all these reasons, the masses of atoms CANNOT, strictly speaking, be simple integer multiples of some absolutely fixed, standard number. Consequently, as a kind of compromise that makes the atomic weights of most of the most common elements close to such integer multiples, the atomic mass unit has been set to be one twefth (1/12th) of the mass of a Carbon 12 atom.

With the exception of hydrogen (just one proton in its nucleus), the most common isotope of the light elements has equal numbers of protons and neutrons.

(Quite remarkably, the STRONG FORCE between protons and neutrons alike JUST neutralizes the repulsive force between protons when there are equal numbers of them, for relatively small numbers --- to me, this is currently a deep mystery for which, as far as I know, no satisfactory explanation has ever been found. Could it hold the key to yet more unification?!)

Thus, as one proceeds to consider atoms beyond hydrogen, one finds Helium (2p, 2n), ... Carbon (6p, 6n), ... Oxygen (8p, 8n), and so on ... .

However, eventually the inter-proton repulsion grows in such a way that more, and then still more, neutrons are needed to hold them in. That's why one ultimately sees more neutron-dominated atomic nuclei at higher atomic numbers. That's how you find elements at the upper end of the Periodic Table of the Elements like "Uranium 235" : U(92p, 143n).

So: except for hydrogen (just one proton), the most common isotopes of the elements have a NUMERICAL "relationship" between the atomic weight that starts out being about a factor of 2 ("about" for reasons given above), but rising to the order of 2.6 or more at the upper end of the periodic table.

In detail, of course, the situation is a bit more complicated. Many intermediate and more massive elements have quite a large number of possible isotopes. Nuclear physicists talk in terms of "the valley of beta-stability," a sometimes preferred path through a graphical way of representing all the possible isotopes of ALL the elements. One of the tasks of nuclear astrophysicists is to understand and explain how certain regularities or anomalies in the relative abundances of the known isotopes could have come about, through known or possibly not yet known astrophysical processes. It's a fascinating, forensic aspect of modern science.

But at its heart is the quest to understand, in the sense described above, the relationship between the atomic numbers and atomic weights of the known nuclei, and in addition how their relative abundances came about.

Live long and prosper.

Answer 3

E=mc^2

Answer 4

atomic mass is the atomic number plus the number of neutrons in an atom...

Answer 5

'astatine' already answered.