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Question
What is the difference between atomic weight & number? thanks!!
Hello,
The atomic number is the number of protons (or electrons) of an atom. It also goes by the name of 'proton number'.
The 'atomic weight' is now generally called the 'relative atomic mass' and is the mass of an atom *relative* to the mass of the carbon 12- nuclide, which is defined as 12 units or 12 u. (Where 1 u = 1 atomic mass unit = 1/12 the mass of the isotope of one carbon -12 atom, or 1.66033 x 10^-27 kg)
Without a lot of fanfare or fuss, the easiest way to obtain the relative atomic mass for a given element to the nearest whole number (say as given in periodic tables) is to add the number of protons + the number of neutrons (taking each of these to have a mass of 1 u, though technically speaking the neutron has very slightly larger rest mass).
Thus, by way of example - Lithium has 3 protons and 4 neutrons, so:
# u (Li) = 3 u + 4 u = 7 u
or a relative atomic mass (atomic weight) = 7.
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Answers by Expert:
Philip Stahl
Expertise
I have forty years of experience in Astronomy, specifically solar and space physics. My specialties include the physics of solar flares, sunspots, including their effects on Earth and statistics as applied to astronomical investigations.
Experience
Astronomy: more than forty years experience starting with construction of my own simple telescopes. Worked at university observatory in college, doing astrographic measurements. M.Phil. degree in Physics/Solar Physics and more than ten years as researcher.
Organizations
American Astronomical Society (Solar Physics and Dynamical Astronomy divisions), American Mathematical Society, American Geophysical Union
Publications
Solar Physics (journal), The Journal of the Royal Astronomical Society of Canada, The Proceedings of the Meudon Solar Flare Workshop (1986), The Proceedings of the Caribbean Physics Conference (1985). Books: 'Selected Analyses in Solar Flare Plasma Dynamics', 'Physics Notes for Advanced Level'.
Education/Credentials
B.A. Astronomy, M. Phil. Physics
Awards and Honors
American Astronomical Society Studentship Award (1984), Barbados Government Award for Solar Research
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