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The atomic number is the number of protons in the nucleus of an atom. The number of protons in an atom's nucleus determines the chemical identity of that atom. It tells us which element it is. The elemental identity of an atom is determined by the number of protons in that atom's nucleus. This was bit repetitive, but it is a critical concept in chemistry and physics. Atomic Mass might be thought of as the "weight" of an atom. Atomic mass is found by adding up the masses of all the protons, neutrons, and electrons in the atom. As electrons weigh less than 1/1800th of what a proton weighs, they contribute only the smallest amount to the weight of the atom (compared to the protons and neutrons), regardless of which atom it is. These weights are expressed in atomic mass units because the "normal" terms for the expression of weight (mass) are far too large to be "easy to work with" on the atomic scale. Let's do a quick example with the two stable isotopes of lithium. Lithium is element number three on the Periodic Table. It has 3 protons in its nucleus, and 3 is its atomic number. (Repeating, the number of protons in the nucleus is equal to the atomic number, and this identifies the element under investigation.) Lithium has one "version" or isotope that has 3 neutrons. The atomic mass of 3Li is about 6.015 or so. Note: subtracting the atomic number from the atomic mass gives us the number of neutrons in the nucleus of that atom. The 6.015 minus the 3 yields an answer of 3 for the number of neutrons. There is also a lithium atom that has 4 neutrons in its nucleus. It's 4Li and its atomic mass is about 7.016 or so. Same math. Take the 7.016 and subtract the 3 and get 4 as the answer - and the number of neutrons in the nucleus of that atom. Those are the two stable isotopes of lithium. (There are a number of other isotopes of lithium, but they're all unstable and disappear fairly quickly after being made in the physics lab.)

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