CHM 101 Atoms & Flame Test Lab Questions

Atoms, Elements:Identification of Metals by Flame tests
Objectives
1. Watch videos of flame tests of metal cations in order to observe their characteristic
colors for use in identifying said metals,
2. Match the flame colors observed to an appropriate wavelength of visible light, and then
perform calculations to determine the frequency and energy of the emitted photons,
3. Relate these results to the types of electronic transitions occurring in these elements,
4. Practice writing electron configurations for these (and other) elements.
Discussion/background information
A chemical element is a pure chemical substance consisting of one type of atom
distinguished by its atomic number, which is the number of protons in its nucleus.
Examples of elements are iron, copper, silver, gold, hydrogen, carbon, nitrogen, and
oxygen. In total, 119 elements have been observed as of 2013, of which 94 occur
naturally on Earth.
Elements have characteristic properties that allow them to be distinguished from one
another. Many elements produce a distinct color when burned in a flame. Chemists
began studying colored flames long before the 18th century but in the 18 th century “flame
tests” begun to be used to distinguish between some elements. The color is produced
when electrons in the outer energy levels of an atom are removed by the intense heat,
then return to their original energy level. The distinct color seen actually are spectral lines
which are the result of energy changes in the electrons of the atoms. When an atom from
a metal element is heated, its electrons become excited, that is, their energy is increased.
Excited electrons soon lose the energy they have gained. This energy is lost in the form
of light, called photons. A photon is an individual packet of light having a specific
wavelength. These photons will have different colors depending on the element and its
discrete energy levels. That is, different wavelengths of light (colors) will be emitted
when the electrons of different elements go down the step(s) between their energy
level(s). Each element will have its own set of steps; therefore each will have its own
color or set of colors. Photons with wavelengths between 390 and 750 nm are visible to
us as distinct colors. In fact, compounds of metals provide the beautiful colors in a
fireworks display.
Compounds containing elements in Group I and II of the Periodic Table can be identified
by the colors they emit. The spectrum of the flame of each element consists of certain
colors— wavelengths—characteristic of that element. These lines are a characteristic of
an element as fingerprints are of an individual person. No two elements give the same
bright line spectrum.
Electromagnetic radiation is energy in the form of waves. Waves are characterized by their
wavelength () and frequency (). Wavelength is defined as the distance between
successive crests (or troughs) on a wave, and is measured in meters. Frequency is defined
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as the number of waves that pass a given point every second, and is measured in 1/seconds,
or Hertz (Hz).
All electromagnetic waves travel at the speed of light (c), or 2.998 x 108 m/s. The
relationship between the wavelength, frequency and speed of an electromagnetic wave is
given by the equation:
Electromagnetic radiation also occurs as discreet “packets” (or particles) called
photons. The energy of a photon (in Joules) is given by the equation:
Here, h is Planck’s constant, which has a value of 6.626 x 10 -34 Js.
Visible light is the most familiar example of electromagnetic radiation. Differences in
the wavelengths of visible light are manifested as different colors, shown in the Color
Spectrum below (colors can be seen in the PDF document on-line). Other examples of
electromagnetic radiation include X-rays, ultraviolet light, infrared light, microwaves and
radio waves.
:
400 nm
500 nm
UV Violet
IR
600 nm
700 nm
Red 
So, how does electromagnetic radiation relate to flame tests? Well, when an atom (or ion)
absorbs energy, its electrons can make transitions from lower energy levels to higher
energy levels. The energy absorbed could be in the form of heat (as in flame tests), or
electrical energy, or electromagnetic radiation. However, when electrons subsequently
return from higher energy levels to lower energy levels, energy is released predominantly
in the form of electromagnetic radiation.
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The spacing between energy levels in an atom determines the sizes of the transitions that
occur, and thus the energy and wavelengths of the collection of photons emitted.
Larger transition – higher energy
photon released (shorter
wavelength)
Small transition – lower energy
photon released
(longer wavlength)
1
2
3
4 56
If emitted photons are in the visible region of the spectrum, they may be perceived as lines
of different colors (note that photons outside the visible spectrum may also be emitted, but
cannot be seen). The result is called a line emission spectrum, and can serve as a
‘fingerprint’ of the element to which the atoms belong. For example, the line spectra
shown below for the elements helium and carbon are clearly quite different (colors can be
seen in the PDF document on-line).
Helium
Carbon
Unfortunately, techniques more sophisticated than those used in this lab are required to
obtain such line spectra. To the naked eye, when an element is vaporized in a flame (or an
electrical discharge) the emission spectrum will appear to be just one color. For example,
helium gas when excited by an electrical discharge emits light that appears an orange-peach
color. This one color results from a combination of all lines of the emission spectrum, in
proportion to their intensities. As many elements will still produce distinctive colors under
such conditions, simple flame tests can be used to identify these elements. In fact, flame
tests were used to identify elements long before the invention of modern techniques, such
as emission spectroscopy.
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By observing the color of light emitted from heating a metal solution, its visible light
spectrum can be seen, and thus, its identity can be determined. For Group IA and IIA
element compounds, flame tests are usually the easiest way of identifying which metal is
present in the compound.
However, there are some limitations to the test:
 The test cannot detect low concentrations of most ions.
 The brightness of the signal varies from one sample to another. For example, the
yellow from sodium is much brighter than the red from the same amount of
lithium.
 The test cannot differentiate between all elements. Several metals appear to
produce the same flame color. However, their light can be resolved (separated)
with a prism (which we do not have here at Santa Fe) into distinctly different
bands of colors on the electromagnetic spectrum (ROYGBIV). These bands of
colors are called atomic line spectra, and they are UNIQUE to each element.
 Some compounds do not change the color of the flame at all.
 If more than one metal is present or impurities or contaminants are present, the
test can be hard to interpret and inconclusive, for one color obscures another.
Sodium, in particular, is present in most compounds and will color the flame.
Sometimes a blue cobalt glass is used to filter out the yellow of sodium.
 The flame tends to turn orange at the end of the test.
There are other more precise methods of identifying certain elements – but the flame test
can give a useful hint as to where to look for those ones.
In this lab, we will study qualitatively the colors produced by Group 1A and Group 2A
metals, along with some transition metals species.
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Name: _______________________________Section # _______Date: _______________
Pre-Laboratory Assignment
1. What inaccuracies may be involved in using flame tests for identification purposes?
2. The characteristic bright-line spectrum (color) of an element is produced when
A. electrons are emitted by the nucleus as beta particles
B. electrons move to higher energy levels
C. electrons are gained by an atom
D. electrons fall back to lower energy levels
3. Define the following terms:
A. Ground state
B. Excited state
C. Quanta
Procedure: Watch the following Videos of Flame tests of Metal Ions. Each
video is less than 5 minutes.
1. https://www.youtube.com/watch?v=GPkUVOzlLys
2. https://videoportal.virtualsc.org/media/Flame+Test+/1_phaaj8wo/169417681
3. https://www.youtube.com/watch?v=chHUHW2TdKw (Unknown 2)
4. https://www.youtube.com/watch?v=TMz_XR3o5mg (Fe at 1:05)
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Data Collection & Analysis of Lab: From the videos watched, you will decipher the
color for each of the metal ions below. For each metal, obtain the wavelength of light
corresponding to the observed flame color from the table below or you may look up the
wavelength per color online as well. No two elements may be assigned the exact same
color/wavelength. Then using this wavelength, calculate the energy of the photons
emitted during the flame tests. Finally, answer the questions as indicated on postlaboratory assignment.
Dominant Color
Red
Red-Orange
Orange
Orange-Yellow
Yellow
Yellow-Green
Green
Green-Blue
Blue
Blue-Violet
Violet
Approximate
Wavelength (in nm)*
701
622
609
597
587
577
535
492
474
455
423
No metal should be just
orange
Show sample Calculation for Energy for Sr:
Sr = red = 701 nm = 701 x 10-9 m = λ
c = λν
h = 6.626 x 10-34 Js
n = c/λ
c = 2.998 x 108 m/s
E = hν
λ = 701 x 10-9 m
E = hc/λ
(6.626 x 10-34 Js)(2.998 x 108 m/s) = 2.83 x 10-19 J
701 x 10-9 m
Data Table:
Compound
Sodium Chloride
(video 1)
Potassium iodide
(video 1)
Calcium Chloride
(video 2)
Iron (III) chloride
(video 4)
Metallic
ion in
Compound
Color of
flame
Approximate
Wavelength,
/nm
Energy of
photon/J
Na+
Ca2+
Fe3+
Barium chloride
(video 2)
Copper (II) sulfate
(video 1)
Lithium chloride
(video 1)
Caesium Chloride
(video 1)
Unknown 1
(video 2)
Unknown 2
(video 3)
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Name: _________________________
Section # _______
Date ____________
Post Laboratory Assignment – each question is worth (1) point
1. How do the tests for the solutions containing known elements make it possible for
you to identify the element in an unknown substance?
2. Why did each element produce a different color?
3. Why was it important to use a different wood splint or platinum wire for each flame
test?
4. A fireworks display is produced by packing different chemicals and gunpowder in a
rocket shell and exploding the mixture in the air. Based on your observations from the
videos, which elements could produce the following colors of fireworks?
a) yellowish-green: ________________
b) pink/purple: __________________
c) bright red: ____________
d) orange-yellow: ________________
5. What is the group number for the following families of elements?
a) Sr, Ra, Be : __________________________
b) N, Sb, P: ____________________________
6. What is the family name for the group in which Ar and Rn are members?
7. Give a reasonable hypothesis as to why either Cu(NO 3)2 or CuSO4 could have been
used during the flame experiment. (1.5 pts)
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Electron Arrangement of the elements: Using Condensed, Noble gas and Lewis Dot structures
1. Scientists use Lewis Dot Structures to show the valance electrons of an element as dots. Since bonding involves the valence shell electrons
only, it is only necessary to illustrate those outer electrons.
2. Remember that metals tend to lose their electrons, falling back to their inner octet, becoming smaller, forming positive “cations”. Nonmetals
tend to gain electrons, filling up their current energy levels, becoming larger, forming negative “anions”. Complete the chart below.
Element
Atomic
#
Electronic Configuration of atom
neon
10
1s2 2s2 2p6
2
8
sodium
phosphorus
11
15
1s2 2s2 2p6 3s1
2
8
1
1s2 2s22p6 3s23p3
2
8
5
Noble gas configuration of
atom
Lewis Dot
Structure of
atom
..
[Ne]
Loss or Gain of
Electrons
Ion type/
charge
Ion
Symbol
0/ octet satisfied
none
none
lose 1 e-
cation
+
Na+
gain 3 e-
anion –
P3-
: Ne :
..
1
[Ne] 3s
·Na
[Ne] 3s23p3
.
:P·
.
magnesium
bromine
gallium
sulfur
potassium
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