bohrs atomic model

on the level

As a young man, Bohr worked in Rutherfords lab in England. Because Rutherfords model was weak on the position of the electrons, Bohr focused on them. He hypothesized that electrons can move around the nucleus only at fixed distances from the nucleus based on the amount of energy they have. He called these fixed distances energy levels, or electron shells. He thought of them as concentric spheres, with the nucleus at the center of each sphere. In other words, the shells consisted of sphere within sphere within sphere. Furthermore, electrons with less energy would be found at lower energy levels, closer to the nucleus. Those with more energy would be found at higher energy levels, farther from the nucleus. Bohr also hypothesized that if an electron absorbed just the right amount of energy, it would jump to the next higher energy level. Conversely, if it lost the same amount of energy, it would jump back to its original energy level. However, an electron could never exist in between two energy levels. These ideas are illustrated in the Figure 1.2. Q: How is an atom like a ladder? A: Energy levels in an atom are like the rungs of a ladder. Just as you can stand only on the rungs and not in between them, electrons can orbit the nucleus only at fixed distances from the nucleus and not in between them.

modeling the atom

The existence of the atom was first demonstrated around 1800 by John Dalton. Then, close to a century went by before J.J. Thomson discovered the first subatomic particle, the negatively charged electron. Because atoms are neutral in charge, Thomson thought that they must consist of a sphere of positive charge with electrons scattered through it. In 1910, Ernest Rutherford showed that this idea was incorrect. He demonstrated that all of the positive charge of an atom is actually concentrated in a tiny central region called the nucleus. Rutherford surmised that electrons move around the nucleus like planets around the sun. Rutherfords idea of atomic structure was an improvement on Thomsons model, but it wasnt the last word. Rutherford focused on the nucleus and didnt really clarify where the electrons were in the empty space surrounding the nucleus. The next major advance in atomic history occurred in 1913, when the Danish scientist Niels Bohr published a description of a more detailed model of the atom. His model identified more clearly where electrons could be found. Although later scientists would develop more refined atomic models, Bohrs model was basically correct and much of it is still accepted today. It is also a very useful model because it explains the properties of different elements. Bohr received the 1922 Nobel prize in physics for his contribution to our understanding of the structure of the atom. You can see a picture of Bohr 1.1.


energy by the spoonful

Bohrs model of the atom is actually a combination of two different ideas: Rutherfords atomic model of electrons orbiting the nucleus and German scientist Max Plancks idea of a quantum, which Planck published in 1901. A quantum (plural, quanta) is the minimum amount of energy that can be absorbed or released by matter. It is a discrete, or distinct, amount of energy. If energy were water and you wanted to add it to matter in the form of a drinking glass, you couldnt simply pour the water continuously into the glass. Instead, you could add it only in small fixed quantities, for example, by the teaspoonful. Bohr reasoned that if electrons can absorb or lose only fixed quantities of energy, then they must vary in their energy by these fixed amounts. Thus, they can occupy only fixed energy levels around the nucleus that correspond to quantum increases in energy. This is a two-dimensional model of a three-dimensional atom. The concen- tric circles actually represent concentric spheres. Q: The idea that energy is transferred only in discrete units, or quanta, was revolutionary when Max Planck first proposed it in 1901. However, what scientists already knew about matter may have made it easier for them to accept the idea of energy quanta. Can you explain? A: Scientists already knew that matter exists in discrete units called atoms. This idea had been demonstrated by John Dalton around 1800. Knowing this may have made it easier for scientists to accept the idea that energy exists in discrete units as well.


instructional diagrams

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electrons with the most energy are located closest to the nucleus of the atom.

a. true

-->  b. false

bohr hypothesized that if an electron gained just the right amount of energy, it would

-->  a) jump to the next higher energy level.

b) drop down to the next lower energy level.

c) move halfway to the next higher energy level.

d) crash into the nucleus of the atom.

the idea of the quantum was first introduced by

a) bohr.

-->  b) planck.

c) thomson.

d) rutherford.

energy levels around the atomic nucleus correspond to quantum increases in energy.

-->  a. true

b. false

diagram questions

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