The atomic mass is a weighted average of all of the isotopes of that element, in which the mass of each isotope is multiplied by the abundance of that particular isotope. The atomic mass of an element is the average mass of the atoms of an element measured in atomic mass unit (amu, also known as daltons, D). How is the atomic mass of an element measured? For example, the atomic mass of Lithium is 6.941 Da. The mass number of an element is a whole number whereas the actual mass of an atom is not a whole number except for carbon-12. We know that isotopes are atoms with the same atomic number but different mass numbers due to a different number of neutrons. Is the mass of an isotope the same as the atomic number? The atomic masses of most elements are not whole numbers, because they are a weighted average of the mass numbers of the different isotopes of that element, with respect to their abundances in nature. Why are the atomic masses for most elements not whole? An element’s or isotope’s mass number tells how many protons and neutrons in its atoms. What does the atomic mass tell us about an element?Īn element’s or isotope’s atomic number tells how many protons are in its atoms. Different isotopes of an element have the same number of protons in the nucleus, giving them the same atomic number, but a different number of neutrons giving each elemental isotope a different atomic weight. Isotopes of a given element contain different numbers of neutrons, therefore, different isotopes have different mass numbers. Isotopes of any given element all contain the same number of protons, so they have the same atomic number (for example, the atomic number of helium is always 2). If the number of neutrons in the given atom is different, than it is an isotope. This is the number of neutrons that the regular version of the atom has. Subtract the number of protons from the atomic mass. Look up at the atom on the periodic table of elements and find out what its atomic mass is. When can you tell if an element is an isotope? The difference in the number of neutrons between the various isotopes of an element means that the various isotopes have different masses. Isotopes are atoms of the same element that have different numbers of neutrons but the same number of protons and electrons. Why do the isotopes of an element have different atomic masses?īasic principles. The mass number (A) is the total number of nucleons (neutrons and protons). What does the atomic mass represent in an isotope?Ītomic number, mass number and isotopes The atomic number of an element (symbolized as Z) is the number of protons in the nuclei of its atoms. The atomic mass of an element is the weighted average of the masses of the naturally occurring isotopes. The atomic mass is the average number of protons and neutrons for all natural isotopes of an element.Įach isotope of a given element has the same atomic number but a different mass number (A), which is the sum of the numbers of protons and neutrons. Key Takeaways: Atomic Mass Versus Mass Number The mass number is the sum of the number of protons and neutrons in an atom. Is the mass number of an isotope the same as the atomic mass? The average atomic masses are the values we see on the periodic table. The atomic mass of an element is the weighted average of the atomic masses of the naturally occurring isotopes of that element. 8 What is the mass of two isotopes of boron?.7 Where does most of the mass of an atom come from?.6 Why are the atomic masses for most elements not whole?.5 What does the atomic mass tell us about an element?.4 When can you tell if an element is an isotope?.3 Why do the isotopes of an element have different atomic masses?.2 Is the mass number of an isotope the same as the atomic mass?.
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