Journey Inside the Atom · Lesson 11 of 14
A Deeper Look into Atomic Structure — Isotopes
“Same proton number with different neutron count produces isotopes of one element”
• Define isotopes using atomic and mass numbers. • Explain why isotopes are the same element. • Compare hydrogen isotopes. • Compare carbon isotopes. • Explain similar chemical properties. • Describe uses of selected isotopes. • Explain the use of unified atomic mass units.
Dalton treated atoms of one element as identical in mass. Later discoveries showed a more subtle picture: two atoms can have exactly the same proton number and therefore be the same element while containing different numbers of neutrons and having different masses.
A Deeper Look into Atomic Structure
Isotopes
Atoms of the same element that have the same atomic number but different mass numbers.
Isotopes of Hydrogen
Protium, deuterium and tritium each have one proton, so each has atomic number one. Protium has no neutron, deuterium one neutron and tritium two. A neutral atom of each still contains one electron.
| Hydrogen isotope | Protons | Neutrons | Mass number |
|---|---|---|---|
| Protium | 1 | 0 | 1 |
| Deuterium | 1 | 1 | 2 |
| Tritium | 1 | 2 | 3 |
Isotopes of Carbon
Carbon-12, carbon-13 and carbon-14 each contain six protons and, when neutral, six electrons. Their neutron counts are six, seven and eight respectively. Carbon-12 is the most abundant of these in nature.
| Carbon isotope | Protons | Neutrons | Neutral electrons |
|---|---|---|---|
| Carbon-12 | 6 | 6 | 6 |
| Carbon-13 | 6 | 7 | 6 |
| Carbon-14 | 6 | 8 | 6 |
Why Chemical Properties Are Similar
Chemical properties depend strongly on electron arrangement, especially valence electrons. Neutral isotopes of the same element have the same electron number and electronic configuration, so their chemical properties are similar. Their physical properties can differ because their masses differ.
Unified Atomic Mass Unit
A very small unit represented by u and used to express atomic masses conveniently.
Atomic masses are too small for kilograms or grams to be convenient. The unit u provides a suitable scale. Older notation may use amu.
Applications of Isotopes
| Isotope | Use described in the chapter |
|---|---|
| Uranium-235 | Fuel in a nuclear reactor to generate electricity. |
| Cobalt-60 | Radiation treatment for cancer. |
| Iodine-131 | Treatment related to goitre and thyroid cancer. |
| Carbon-14 | Dating ancient fossils and artefacts in archaeology and geology. |
Element identity is controlled by proton number. Changing neutron number changes the isotope and mass number but not the element.
Problem
Two atoms each have 11 protons. One has 12 neutrons and the other 13. Compare them.
- 1.Both have atomic number 11.
- 2.They are the same element because proton number is the same.
- 3.Their mass numbers are 23 and 24.
- 4.They are isotopes because Z is the same and A differs.
Quiz
Which description best matches Isotopes?
Which description best matches Unified Atomic Mass Unit?
Which term matches this description: Atoms of the same element that have the same atomic number but different mass numbers.
Which term matches this description: A very small unit represented by u and used to express atomic masses conveniently.
Which statement is a key takeaway from this lesson?
Practice Problems
- Why are protium, deuterium and tritium all hydrogen?
- Find neutron counts in carbon-12, carbon-13 and carbon-14.
- Why are isotope chemical properties similar?
- Two atoms have the same mass number but different proton numbers. Are they isotopes? Explain.
Key Takeaways
• Isotopes are atoms of the same element with the same atomic number but different mass numbers. • They contain the same number of protons but different numbers of neutrons. • Because element identity depends on proton number, isotopes belong to the same element. • Different isotopes may have different nuclear properties and useful applications.