Atoms, elements and compounds - Isotopes (3)
Resources |
Revision Questions |
Chemistry
Login to see all questions
Click on a question to view the answer
1.
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.
2.
Carbon exists in several naturally occurring isotopes. Describe, using the definition of isotopes, what is meant by the term "isotope" in this context. Give an example of two carbon isotopes and their respective atomic masses.
In the context of carbon, the term "isotope" refers to different forms of the carbon atom that all have 6 protons (atomic number 6), but differ in the number of neutrons in their nucleus. This leads to different mass numbers.
For example:
- Carbon-12 (12C): 6 protons + 6 neutrons. Atomic mass approximately 12 amu.
- Carbon-14 (14C): 6 protons + 8 neutrons. Atomic mass approximately 14 amu.
Both 12C and 14C are isotopes of carbon because they both have 6 protons, but differ in their neutron numbers.
3.
A magnesium atom has an atomic number of 12. A magnesium ion has a charge of 2. State the mass number of the magnesium ion and write the symbol for the ion.
The magnesium atom has an atomic number of 12, so it has 12 protons and 12 neutrons. The mass number is the sum of protons and neutrons. Therefore, the mass number of the magnesium atom is 12. A magnesium ion with a charge of 2 means it has lost two electrons. This means it has 12 protons and 10 neutrons. The mass number is 12 + 10 = 22. The symbol for the magnesium ion is 22Mg2+.