![]() of an element is a weighted average of the masses of the isotopes that compose an element. The atomic mass The weighted average of the masses of the isotopes that compose an element. Thus, although it is easy to speak of the mass of an atom, when talking about the mass of an element, we must take the isotopic mixture into account. What is the mass of an element? This is somewhat more complicated because most elements exist as a mixture of isotopes, each of which has its own mass. More exact masses are found in scientific references-for example, the exact mass of uranium-238 is 238.050788 u, so you can see that we are not far off by using the whole-number value as the mass of the atom. Thus, the mass of carbon-12 is about 12 u, the mass of oxygen-16 is about 16 u, and the mass of uranium-238 is about 238 u. There will not be much error if you estimate the mass of an atom by simply counting the total number of protons and neutrons in the nucleus (i.e., identify its mass number) and ignore the electrons. By this scale, the mass of a proton is 1.00728 u, the mass of a neutron is 1.00866 u, and the mass of an electron is 0.000549 u. (u some texts use amu, but this older style is no longer accepted) is defined as one-twelfth of the mass of a carbon-12 atom, an isotope of carbon that has six protons and six neutrons in its nucleus. ![]() The atomic mass unit One-twelfth of the mass of a carbon-12 atom. For macroscopic objects, we use units such as grams and kilograms to state their masses, but these units are much too big to comfortably describe the masses of individual atoms and molecules. Individual atoms and molecules, however, are very small, and the masses of individual atoms and molecules are also very small. Express the masses of atoms and molecules.īecause matter is defined as anything that has mass and takes up space, it should not be surprising to learn that atoms and molecules have mass.The table in Chapter 21 "Appendix: Periodic Table of the Elements" also lists the atomic masses of the elements. Carbon exists on Earth as about 99% 12C and about 1% 13C, so the weighted average mass of carbon atoms is 12.01 u. Similar average atomic masses can be calculated for other elements. (also commonly referred to as the atomic weight) of an element.įor example, boron exists as a mixture that is 19.9% 10B and 80.1% 11B. How, then, do we describe the mass of a given element? By calculating an average of an element’s atomic masses, weighted by the natural abundance of each isotope, we obtain a weighted average mass called the atomic mass A weighted average of the masses of all the element’s naturally occurring isotopes. Because most elements exist in nature as a mixture of isotopes, any sample of an element will actually be a mixture of atoms having slightly different masses (because neutrons have a significant effect on an atom’s mass). Note, however, that these masses are for particular isotopes of each element. For example, the mass of an atom of 1H is 1.008 u, the mass of an atom of 16O is 15.995 u, and the mass of an atom of 32S is 31.97 u. Masses of other atoms are expressed with respect to the atomic mass unit. (abbreviated u, although amu is also used) is defined as 1/12 of the mass of a 12C atom: 1 u = 1 12 the mass of 12 C atom ![]() ![]() ![]() The atomic mass unit One-twelfth the mass of a 12C atom. Their masses are so small, however, that chemists often use a unit other than grams to express them-the atomic mass unit. Define atomic mass and atomic mass unit.Įven though atoms are very tiny pieces of matter, they have mass. ![]()
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