Objectives:
determine the characteristic colors that metal salts emit; and relate the colors emitted by metal salts to the structure of the atom.
DRAFT
March 31, 2014
6
Materials:
0.50 grams of each of the following metal salts: Calcium chloride 6 pcs watch glass
Sodium chloride 1 pc 10-ml graduated cylinder Copper(II) sulfate 1 pc dropper
Potassium chloride safety matches Boric acid
100 mL 95% Ethanol (or ethyl alcohol) 100 mL 3 M hydrochloric acid
Procedure:
1. Place each metal salt on a watch glass and add 2 to 3 drops of 3 M hydrochloric acid.
2. Pour about 3 - 5 mL or enough ethyl alcohol to cover the size of a 1 peso-coin in the first watch glass. Light with a match and observe the color of the flame. (This will serve as reference for comparison of the flame color). Wait for the flame to be extinguished or put out on its own.
3. Repeat procedure No. 2 for each salt. Observe the color of the flame. Precautions:
1. Wear goggles, gloves and a safety apron while performing the activity. 2. Do this activity in a well-ventilated area.
3. Handle hydrochloric acid with care because it is corrosive. 4. Ethyl alcohol is flammable.
DRAFT
March 31, 2014
7 4. Write your observation in a table similar to the one below.
Table 1. Color of flame of metal salts Metal salt tested Element
producing color Color of the flame Boric acid boron
Calcium chloride calcium Sodium chloride sodium Potassium chloride potassium Copper(II) sulfate copper
Q1. Why do you think are there different colors emitted?
Q2. What particles in the heated compounds are responsible for the production of the colored light?
Q3. How did the scientists explain the relationship between the colors observed and the structure of the atom?
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You have observed that each of the substances you tested showed a specific color of the flame. Why do certain elements give off light of specific color when heat is applied? These colors given off by the vapors of elements can be analyzed with an instrument called spectroscope. See Figure1.
Figure 1. An atomic spectroscope
DRAFT
March 31, 2014
8 A glass prism separates the light given off into its component wavelength. The spectrum produced appears as a series of sharp bright lines with characteristic colors and wavelength on a dark background instead of being continuous like the rainbow. We call this series of lines the atomic spectrum of the element. The color, number and position of lines produced is called the “fingerprint” of an element. These are all constant for a given element. See Fig. 2.
Figure 2. Atomic spectra of H, Na, and Ne
How did Bohr explain what you observed in Activity 1 and the findings about the elements in a spectroscope? Individual lines in the atomic spectra of elements indicate definite energy transformations within the atom. Bohr considered the electrons as particles moving around the nucleus in fixed circular orbits. These orbits are found at definite distances from the nucleus. The orbits are known as the energy levels, n where n is a whole number 1, 2, 3…and so forth.
Electrons in each orbit have a definite energy, which increases as the distance of the orbit from the nucleus increases. As long as the electron stays in its orbit, there is no absorption or emission of energy. As shown in Figure 3, when an electron of an element absorbed extra energy (from a flame or electric arc), this electron moves to a higher energy level. At this point the electron is at its excited state. Once excited, the atom is unstable. The same electron can return to any of the lower energy levels releasing energy in the form of light with a particular color and a definite energy or wavelength. Bohr’s model explained the appearance of the bright line spectrum of the hydrogen atom but could not explain for atoms that has more than one electron.
DRAFT
March 31, 2014
9 Q4. Explain how your observation in Activity 1 relates to Bohr’s model of the atom.
You can explain using an illustration.
Q5. Which illustration below represents the energy of the electron as described by Bohr? Explain your answer.
a. b
The energy levels of electrons are like the steps of a ladder. The lowest step of the ladder corresponds to the lowest energy level. A person can climb up and down by going from step to step. Similarly, the electrons can move from one energy level to another by absorbing or releasing energy. Energy levels in an atom are not equally spaced which means that the amount of energy are not the same. The higher energy levels are closer together. If an electron occupies a higher energy level, it will take less energy for it to move to the next higher energy level. As a result of the Bohr model, electrons are described as occupying fixed energy levels at a certain distance from the nucleus of an atom.
However, Bohr’s model of the atom was not sufficient to describe atoms with more than one electron.
The way around the problem with the Bohr’s model is to know the arrangement of electrons in atoms in terms of the probability of finding an electron in certain locations within the atom. In the next activity, you will use an analogy to understand the probability of finding an electron in an atom.
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