Introductory Chemistry Lab 4: Electron Configuration Lab 4: Electron Configuration Objectives • Observe energy emitted from different energy levels when salt compounds are ignited • Obtain a general knowledge of what produces different colors in fireworks Introduction Have you ever wondered how fireworks burst into different col‐
ors? Atoms radiate light when a change in energy state of the electrons occurs. Various salt compounds can be added to produce a specific color. Fireworks and their awesome colors and brilliance are based around this concept. The main ingredi‐
ents of fireworks are potassium chlorate or perchlorate, charcoal, and sulfur. With each different ingredient or compound, a different color can be produced to give a beautiful multi‐colored light show. The energy of atoms is quantized, meaning there are specific energy levels. This is similar to climbing up a ladder: you can only step on the rungs, not in between. In a similar fashion, the changes in energy of electrons must be whole energy level changes, not between two levels. Picture your foot on the rung of a ladder closest to the ground. This is like an elec‐
tron in the first principle energy level (n = 1). Scientists invented a naming system for describing the location of a specific electron in an atom. Each principle energy level has a sublevel or sublevels (s, p, d, or f). The system for naming is called the electron configuration. For example, the electron configuration for helium is 1s2 because it is in the first principal energy level (1), the first sublevel (s), and has two electrons. Principle energy level He
1s 2
Number of electrons Sublevel The lowest energy configuration for an atom is called its ground state; as energy is added from the ground state, electrons tend to fill energy levels and sublevels in a specific order (see Figure 2). The first sublevel in any orbital is the “s” sub‐
level, which can hold up to two electrons. The next sublevel filled is the “p” sub‐
level, which can hold up to six electrons. Continuing, the “d” sublevel can hold up to 10 electrons and “f” sublevel can hold up to 14 electrons. Electrons will always occupy the lowest available energy level first, meaning that an electron will not exist in a higher energy level or sublevel when a lower one is vacant. When an atom receives the right amount of energy from an outside source, an electron will absorb that energy and move to a higher energy level corresponding to that energy. This electron will not usually stay at the higher energy for long; Figure 1: The flame of a burning lithium salt demonstrates the bright colors that can be achieved by burning different compounds. Table 1: Wavelength ranges for the visible spectrum Color Shades Wavelength Range (nm) Red 650 – 750 Orange 595 – 650 Yellow 580 – 595 Yellow ‐ Green 560 – 580 Green 500 ‐560 Green ‐ Blue 490 – 500 Blue ‐ Green 480 – 490 Blue 435 – 480 Violet 400 – 435 41 Lab 4: Electron Configuration Principal energy level
instead it will emit or release that extra energy in the form of light, and in the process return to its original energy level. This outside energy source might be in the form of heat or electromagnetic waves (light). As it turns out, the wavelength of light emitted de‐
pends directly on the quantity of energy absorbed and released. Higher energies correspond to smaller wavelengths, and lower en‐
ergies correspond to longer wavelengths of light. Table 1 summa‐
rizes several color shades and their wavelength ranges. 42 overlap
overlap
Increasing Energy
n=4
4p
3d
4s
n=3
3p
3s
n=2
2p
2s
n=1
1s
Figure 1: Energy level diagram for electron configuration. Energy levels are filled starting from the bottom and moving upwards. Note the positions that overlap. 2. How is the light emitted by an atom related to its electron configuration? overlap
{
1. What is an electron configuration? 4f
5p
4d
5s
{
Pre‐lab Questions n=5
{
Electrons of different elements absorb and emit different amounts of energy. This means that by observing the color (wavelength) of light emitted by a substance when its atoms absorb and release en‐
ergy, scientists can determine its chemical composition. This phe‐
nomena helps explain how fireworks produce different colors of light. When the atoms of different materials in a firework are ex‐
cited by heat, they will absorb and release energy in particular wavelengths, producing a particular variety of colors. Sublevel
Lab 4: Electron Configuration Experiment: Chemistry of Fireworks In this experiment the flame from a tea light candle is the outside energy source. The flame emits a broad range of energy, but the electrons of the atom being heated will only absorb specific amounts of energy. Materials • Safety Equipment: Eye goggles, gloves • Lithium chloride solution (LiCl) • Sodium chloride solution (NaCl) • Potassium chloride solution (KCl) • Calcium chloride solution (CaCl2) • Activated charcoal © • Butane lighter • Tea light candle • 5 paperclips • Modeling clay Procedure 1. Straighten out five large paper clips and make a small (approximately 2‐5 mm in diameter) loop in the end of each one by gently bending it. 2. Place a round piece of clay on the straight end of the paperclip as you did in Lab 7. This will act as a holder. 3. Place in order the LiCl, NaCl, KCl, and CaCl2 saturated solutions along with the activated charcoal. Set one of the paper clip wire wands next to each sample. 4. Light the candle using the butane lighter. CAUTION: Both lighter and candle can cause fire or burns to skin and/or clothing if the flame comes into contact with skin or clothes. Be sure you have your safety goggles on! 5. Hold the paper clip wire wand for the LiCl at the very end of the non‐looped end in order to avoid burns. Heat the looped end in the candle flame until its loop is faintly orange and any coating is burned off. 6. Dip the loop into the LiCl solution. CAUTION: The loop will remain extremely hot for several minutes fol‐
lowing being in the flame. Do not touch the loop! 7. Bring the loop of the paper clip into the flame. Make observations about what is happening, especially any color changes. HINT: The color change will be most apparent around the edges of the flame. You may have to try this a few times to see the color change. 8. Extinguish the candle and record your observations in Table 1. 9. Repeat the steps 4‐7 for each of the other solutions and the activated charcoal. Use a different paper clip for each one. 10. To clean‐up, you may throw away the paper clips after they have cooled to room temperature. 43 Lab 4: Electron Configuration Data Table 1: Results of firework material ignition Substance Observations Lithium chloride Sodium chloride Potassium chloride Calcium chloride Activated charcoal Post‐lab Questions 1. Write out the electron configurations of each of the metals of the salt compounds used and of carbon. Potassium is already done as an example for you. HINT: The periodic table is very helpful and can be used as guide. Element K Li Na Ca C 44 Electron Configuration 1s2 2s2 2p6 3s2 3p6 4s1 Lab 4: Electron Configuration 2. What is the approximate wavelength of light emitted by each of the salts? Salt LiCl NaCl KCl CaCl2 C Color Wavelength 3. Why does a salt compound give off light or a colored flame when burned? 4. Did sodium chloride and charcoal give off similar colors? Why or why not? 45