Journey Inside the Atom · Lesson 6 of 14
What Components Contribute to the Mass of an Atom?
“The neutron completes the basic particle picture and explains why proton count alone cannot account for mass”
• Explain why proton number alone cannot account for atomic mass. • Describe Chadwick's discovery of the neutron. • Compare charge and location of the three subatomic particles. • Explain why mass comes mainly from the nucleus. • Describe the chapter's account of neutrons in heavier nuclei.
Once protons and electrons were known, a puzzle remained. Hydrogen has one proton while helium has two, yet helium has a mass about four times that of hydrogen rather than only twice as much. Another massive particle had to be present in the nucleus without adding positive charge.
What Components Contribute to the Mass of an Atom?
Discovery of the Neutron
James Chadwick identified a subatomic particle with mass nearly equal to that of a proton but no electrical charge. It was named the neutron. Neutrons are found in nuclei of atoms except ordinary hydrogen.
A neutral subatomic particle located in the nucleus with mass nearly equal to that of a proton.
| Particle | Symbol | Relative charge | Location |
|---|---|---|---|
| Electron | e⁻ | −1 | Outside the nucleus |
| Proton | p⁺ | +1 | Nucleus |
| Neutron | n⁰ | 0 | Nucleus |
Electrons are so light compared with protons and neutrons that their mass is ignored in the mass-number calculations used in this chapter. Atomic mass therefore comes mainly from nuclear protons and neutrons.
Neutrons in Lighter and Heavier Atoms
Lighter atoms often have similar numbers of protons and neutrons. Carbon can have six of each and oxygen eight of each. In heavier nuclei the neutron count can be larger. The chapter gives iron with twenty-six protons and thirty neutrons and uranium with ninety-two protons and one hundred forty-six neutrons.
Why the Nucleus Remains Bound
Protons all carry positive charge and electrically repel one another. The chapter explains that neutrons help reduce the effect of this repulsion and are associated with the nuclear force that binds particles in the nucleus. Heavier nuclei therefore contain many neutrons to help maintain a tightly bound nucleus.
The strong binding force described in the chapter as helping hold particles together inside the nucleus.
Scientific Contributions
The chapter highlights neutron-scattering research at the Bhabha Atomic Research Centre in Mumbai, including work using the Dhruva reactor to investigate superconductors, battery electrodes and drug molecules. These studies show how knowledge of neutrons contributes to materials research.
Quiz
Which description best matches Neutron?
Which description best matches Nuclear Force?
Which term matches this description: A neutral subatomic particle located in the nucleus with mass nearly equal to that of a proton.
Which term matches this description: The strong binding force described in the chapter as helping hold particles together inside the nucleus.
Which statement is a key takeaway from this lesson?
Practice Problems
- Why did helium's mass suggest another nuclear particle must exist?
- Compare the three subatomic particles by charge and location.
- Why is electron mass ignored in these calculations?
- Why do heavier nuclei often contain more neutrons than protons?
- How did neutron discovery strengthen the atomic model?
Key Takeaways
• Protons and neutrons are present in the nucleus, while electrons are found outside it. • Protons carry positive charge, neutrons have no charge and electrons carry negative charge. • Protons and neutrons contribute almost all of the mass of an atom. • Neutrons help explain why atomic mass cannot be determined from proton number alone.