Atoms, elements and compounds - Isotopes (3)
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1.
Define isotopes as different atoms of the same element that have the same number of protons but different numbers of neutrons.
Isotopes are atoms of the same element (same atomic number, meaning the same number of protons) that have different numbers of neutrons. This results in different mass numbers for the isotopes. The key defining feature is the shared atomic number, distinguishing them from different elements.
2.
A student states: "Isotopes are atoms with different numbers of protons." Explain why this statement is incorrect, using the definition of isotopes.
This statement is incorrect because isotopes are defined as atoms of the *same* element. Atoms of the same element have the same number of protons, which defines the element. Isotopes differ only in the number of neutrons. Therefore, isotopes share the same atomic number (number of protons) but have different mass numbers (number of protons + neutrons). The student's statement neglects the crucial similarity in the number of protons that defines isotopes.
3.
The isotope carbon-12 is the most abundant isotope of carbon. Carbon-13 is also present in smaller quantities. The relative mass of carbon-12 is 12.000 amu and the relative mass of carbon-13 is 13.001 amu. Calculate the relative atomic mass of carbon.
Solution:
The relative atomic mass (Ar) is calculated using the following formula:
Ar = Σ (Isotopic Mass × Isotopic Abundance)
We are given:
- Isotopic Mass (Carbon-12) = 12.000 amu
- Isotopic Abundance (Carbon-12) = 98.9% = 0.989
- Isotopic Mass (Carbon-13) = 13.001 amu
- Isotopic Abundance (Carbon-13) = 1.1% = 0.011
Therefore:
Ar(C) = (12.000 × 0.989) + (13.001 × 0.011)
Ar(C) = 11.868 + 0.143
Ar(C) = 12.011 amu
The relative atomic mass of carbon is 12.011.