When a salt is placed in the flame of a Bunsen burner, the flame changes color. What feature of the metal ions in the salt determines the color of the flame in this experiment? (A) Reactivity to oxygen, (B) atomization energy, (C) bond energy, (D) spacing of nuclear energy levels, or (E) spacing of electron energy levels.
The process of heating a metal ion in a Bunsen burner is called a flame test. And by observing the color of the flame, we can identify the metal ion present in the sample as different ions produce different flame colors. When we heat the metal ions, they absorb energy and then release it in the form of colored light. But what specifically is going on in the atom that causes the energy to be released?
An ordinary lithium ion’s electrons are arranged like so, with its two electrons on the same energy level. When an ion is heated, its electrons can absorb the heat energy and jump to a higher energy level. Finally, upon returning to their original energy levels, the electrons release a specific amount of energy in the form of a photon with a specific wavelength. Since the electrons are jumping and falling between the same energy levels, whenever that particular ion is heated, photons with the same specific wavelengths are released each time and the same color flame is produced.
Note that not just one wavelength of light is released; a characteristic pattern of multiple different wavelengths are released. This pattern is visible when viewing the flame through a spectroscope.
So, which of the answers below best fits this explanation? The correct answer is (E) the spacing of electron energy levels. Since the light energy is released when an electron returns from a higher to a lower energy level, the difference in energy of the electron energy levels determines the color. More spaced-out energy levels will result in higher-energy photons with short wavelengths that will appear blue or purple. While less spaced-out energy levels will result in lower-energy photons with long wavelengths that will appear red.
Let’s confirm our answer by looking at the remaining choices. Since we’re dealing with the movement of electrons and not the movement of protons and neutrons, it’s incorrect to say that this phenomenon has to do with nuclear energy levels.
Bond energy is defined as the energy required to break the bonds of one mole of a substance. We can eliminate bond energy as a possible answer because in flame tests, our metal ion can be dissolved in hydrochloric acid before burning. Since the ion burns the same color whether it is bonded in a salt or dissolved in an acid, the energy of its bond with another element is irrelevant to the formation of the flame and its resulting color.
Atomization energy is defined as the energy required to form one mole of gas of a substance. Again, we can eliminate this answer as well. While our flame is quite hot, it’s not hot enough to turn metals into gases. The flame color is due to the movement of electrons and not due to a change of state of the metal ion.
Lastly, reactivity to oxygen is incorrect as well. Many metals do react with oxygen to form oxides, such as iron reacting with the air to form rust. But this is a separate process that does not occur during the flame test.
All of these answers are related to energy in some way, but only one is specifically relevant to flame tests. The feature that determines the color of the flame is (E) the spacing of the electron energy levels.