Isotopes affect mass number because isotopes of the same element have different numbers of neutrons.
Here's a more detailed explanation:
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Isotopes Defined: Isotopes are variants of a chemical element which have the same number of protons, but different numbers of neutrons. All isotopes of a given element have the same atomic number (number of protons), but different mass numbers (total number of protons and neutrons).
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Atomic Number vs. Mass Number: The atomic number (Z) is the number of protons in an atom's nucleus, defining the element. The mass number (A) is the total number of protons and neutrons in the nucleus.
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Neutrons and Mass Number Variation: Since isotopes of the same element have the same number of protons (same atomic number), the difference in mass number stems solely from the different number of neutrons. Adding or subtracting neutrons changes the mass of the atom, hence changing its mass number.
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Illustrative Example: Consider hydrogen (H). It has three naturally occurring isotopes:
Isotope Name Symbol Protons Neutrons Mass Number (A) Protium ¹H 1 0 1 Deuterium ²H 1 1 2 Tritium ³H 1 2 3 As you can see, each isotope of hydrogen has one proton, but the number of neutrons varies. This variation in neutron count directly influences the mass number.
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Impact of Different Mass Numbers: The different mass numbers affect the atomic mass of the element (the weighted average of the masses of all its isotopes). While chemical properties remain largely similar amongst isotopes of the same element, physical properties like density and nuclear stability can vary.
In summary, isotopes, which are atoms of the same element with varying neutron numbers, directly influence the mass number (A) of an atom by changing the total count of protons and neutrons in its nucleus.
