![]() 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. ![]() The atomic mass unit (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. 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 extent of the deflection depends on the mass-to-charge ratio of the ion. ![]() When an electric field is applied, the ions are accelerated into a separate chamber where they are deflected from their initial trajectory by a magnetic field, like the electrons in Thomson’s experiment. First, electrons are removed from or added to atoms or molecules, thus producing charged particles called ions. The technique is conceptually similar to the one Thomson used to determine the mass-to-charge ratio of the electron. Scientists can measure relative atomic masses very accurately, however, using an instrument called a mass spectrometer. Although the difference in mass is small, it is extremely important because it is the source of the huge amounts of energy released in nuclear reactions.īecause atoms are much too small to measure individually and do not have charges, there is no convenient way to accurately measure absolute atomic masses. ![]() For example, the ratio of the masses of 1H (hydrogen) and 2H (deuterium) is actually 0.500384, rather than 0.49979 as predicted from the numbers of neutrons and protons present. Once the masses of atoms were determined, the amu could be assigned an actual value:ġ amu = 1.66054 x 10 -24 grams conversely: 1 gram = 6.02214 x 10 23 amuĪlthough the masses of the electron, the proton, and the neutron are known to a high degree of precision ( Table 2.3.1), the mass of any given atom is not simply the sum of the masses of its electrons, protons, and neutrons. Thus, the mass of the hydrogen atom ( 1H) is 1.0080 amu, and the mass of an oxygen atom ( 16O) is 15.995 amu. The atomic mass unit ( amu) was not standardized against hydrogen, but rather, against the 12C isotope of carbon ( amu = 12). As we saw earlier, it is convenient to use a reference unit when dealing with such small numbers: the atomic mass unit. ![]() We now know that a hydrogen atom has a mass of 1.6735 x 10 -24 grams, and that the oxygen atom has a mass of 2.6561 X 10 -23 grams. Thus, oxygen was assigned an atomic mass of 16. Hydrogen, the lightest element, was assigned a relative mass of '1', and the other elements were assigned 'atomic masses' relative to this value for hydrogen. ![]()
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