You cannot directly find the mass of an atom from its mass number alone. The mass number only provides the total number of protons and neutrons in an atom's nucleus. To obtain the mass, you need additional information.
Understanding Mass Number and Atomic Mass
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Mass Number: This is a whole number representing the sum of protons and neutrons in an atom's nucleus. It's denoted by A. For example, Carbon-12 has a mass number of 12 (6 protons + 6 neutrons).
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Atomic Mass: This is the weighted average of the masses of all isotopes of an element, taking into account their relative abundances. It's expressed in atomic mass units (amu) and is typically found on the periodic table. It's not the same as mass number.
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Isotopes: Atoms of the same element with the same number of protons but different numbers of neutrons. They have the same atomic number but different mass numbers.
Calculating the Mass of a Single Atom
To calculate the mass of a single atom, you'd need to know the isotopic mass (the mass of a specific isotope). While the mass number gives a close approximation, it doesn't account for the mass defect (the small difference in mass between the sum of the constituent nucleons and the actual mass of the nucleus). The isotopic mass is usually given in atomic mass units (amu), where 1 amu is approximately the mass of a proton or neutron.
Example: The mass number of Carbon-12 is 12. Its isotopic mass is approximately 12 amu. However, this is an approximation. The precise mass of a Carbon-12 atom is slightly less than 12 amu due to the mass defect.
Using Avogadro's Number to Calculate the Mass of a Mole
Avogadro's number (approximately 6.022 x 10²³) is the number of atoms or molecules in one mole of a substance. If you know the atomic mass of an element, you can calculate the mass of one mole of that element in grams (which is numerically equal to the atomic mass).
Example: The atomic mass of Carbon is approximately 12 amu. Therefore, one mole of Carbon weighs approximately 12 grams.
In summary: The mass number helps determine the number of nucleons, offering a rough estimate of an atom's mass. However, it's crucial to distinguish between mass number and atomic mass (or isotopic mass), as only the latter allows precise mass calculations for single atoms or moles. The provided video clip supports this by showing how to calculate mass number from the number of protons and neutrons.
