Journey Inside the Atom · Lesson 5 of 14
Bohr's Model of the Atom
“Allowed energy levels address the stability problem and introduce shells and stationary states”
• Explain why Bohr proposed a new model. • Define stationary state and energy level. • Identify K, L, M and N shells. • Explain how shell energy changes outward. • Describe energy absorption and release in shell transitions. • Explain how Bohr addresses stability.
Rutherford's nucleus was a breakthrough, but the model left one basic question unanswered: why does an electron moving around a positive nucleus not continuously lose energy and fall inward? Niels Bohr kept the nucleus but changed the rules governing electron motion.
Bohr's Model of the Atom
An allowed state in which an electron has a definite energy and does not continuously lose energy while moving around the nucleus.
A permitted shell around the nucleus in which an electron has a definite amount of energy.
Bohr proposed that electrons can occupy only certain fixed circular paths called stationary states, orbits or shells. Because each allowed shell corresponds to a definite energy, the shells are also called energy levels.
Names and Order of Shells
The shells are represented by K, L, M, N and so on, corresponding to n = 1, 2, 3, 4 and so on. K is closest to the nucleus and has the least energy. Energy increases as the shell lies farther from the nucleus.
| Shell | Shell number | Energy trend |
|---|---|---|
| K | n = 1 | Lowest among these shells |
| L | n = 2 | Higher than K |
| M | n = 3 | Higher than L |
| N | n = 4 | Higher than M |
Moving Between Energy Levels
An electron can move from a lower-energy shell to a higher-energy shell by absorbing a fixed amount of energy equal to the difference between the two levels. When it moves from a higher level to a lower one, it releases that energy difference.
How Bohr's Model Explains Stability
The central postulate is that an electron in an allowed stationary state does not continuously lose energy. Its path therefore does not shrink toward the nucleus. This directly addresses the difficulty in Rutherford's planetary model.
Rutherford and Bohr both place electrons around a positive nucleus. The crucial difference is that Bohr restricts electrons to stationary energy levels in which they do not continuously lose energy.
Why the Shells Begin With K
The letters came from early X-ray terminology associated with Charles Barkla, who named an observed X-ray series K while leaving earlier letters available in case earlier series were found. Bohr adopted the same notation for shells.
The Model Continues to Evolve
Bohr's model explained many observations but was not the final description. Later work led to the quantum mechanical model, in which electrons are not pictured as travelling on fixed circular tracks. That deeper picture belongs to later study.
Quiz
Which description best matches Stationary State?
Which description best matches Energy Level?
Which term matches this description: An allowed state in which an electron has a definite energy and does not continuously lose energy while moving around the nucleus.
Which term matches this description: A permitted shell around the nucleus in which an electron has a definite amount of energy.
Which statement is a key takeaway from this lesson?
Practice Problems
- Why was a new model needed after Rutherford's model?
- What is a stationary state?
- Arrange K, L, M and N in order of increasing energy.
- What must an electron do to move to a higher-energy shell?
- Explain why Bohr's model can account for stability within its postulates.
Key Takeaways
• Bohr proposed that electrons occupy specific permitted energy levels around the nucleus. • Electrons in these stationary energy levels do not continuously lose energy. • Electrons can move between levels by absorbing or releasing definite amounts of energy. • The main shells are commonly represented by K, L, M and N.