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Observing Periodic Properties of Alkaline Earth Elements and Halogens, Summaries of Chemistry

An experiment designed to study the properties of elements in group 2a (alkaline earth elements) and group 7a (halogens) of the periodic table. The experiment involves observing and comparing the properties of the ions mg2+, ca2+, sr2+, ba2+, f-, cl-, br-, and i- through various tests. The goal is to identify an unknown ionic compound by determining the presence of its group 2a and group 7a ions.

Typology: Summaries

2021/2022

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Experiment 17 - Periodic Properties of Elements: The Alkaline
Earth Elements and the Halogens
The periodic table arranges the elements in order of increasing atomic number
into horizontal rows whose lengths are such that elements with similar properties recur
periodically - that is, groups of similar elements fall into a single vertical column on the
table. The elements in any vertical column of the periodic table are similar in their
chemical properties, and are therefore considered to make up a chemical family.
In this experiment, some of the properties of elements in two chemical families
will be observed. These families are Group 2A, also known as the alkaline earth
elements; and Group 7A, also known as the halogens. In each group, the four most
important elements in the group will be studied. In Group 2A, these are magnesium (Mg),
calcium (Ca), strontium (Sr), and barium (Ba). In Group 7A, they are fluorine (F),
chlorine (Cl), bromine (Br), and iodine (I). These elements will be studied mostly in the
form of ions, not as the neutral elements. All Group 2A elements react readily to lose two
electrons and form ions with a +2 charge. All of the halogens readily gain one extra
electron to form ions with a -1 charge. In Part 1 of the experiment, some properties of the
Group 2A cations Mg2+, Ca2+, Sr2+, and Ba2+ will be observed and compared. In Part 2,
some properties of the halide ions F, Cl-, Br- and I- will be studied. In part 3, an unknown
ionic compound will be identified. The unknowns will be in the form of an aqueous
solution and will contain a Group 2A cation and a Group 7A anion.
In Parts 1 and 2, you will observe that the properties of the elements in a
particular group of the periodic table are generally similar to each other but they are not
identical to each other. The information that is obtained in parts 1 and 2 about the
differences in properties of elements in each group will be used in Part 3 to determine
which of these elements is present in the unknown. When the properties of elements in
the same group of the periodic table are different from each other, it is usually found that
the properties change in progression in the same order in which the elements are arranged
on the periodic table. The following chart illustrates this for some properties of the
halogens. The elements are listed on the chart in the same order in which they appear on
the periodic table.
Element Melting Boiling Density Ionization
Point Point (as liquid) Energy
F -218 °C -188 °C 1.11 g/mL 403 kcal/mol
Cl -101 °C - 34 °C 1.56 g/mL 302 kcal/mol
Br -7 °C + 59°C 3.12 g/mL 274 kcal/mol
I +114 °C + 184 °C 4.93 g/mL 243 kcal/mol
(as solid)
Safety Precautions:
Wear your safety goggles.
Silver nitrate (AgNO3) solutions will stain skin and clothes. If you suspect you may
have spilled AgNO3 on yourself, rinse it off immediately. The stains are dark brown
and they don’t show up right away. You’ll know the next day whether or not you
spilled AgNO3 on yourself.
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Experiment 17 - Periodic Properties of Elements: The Alkaline

Earth Elements and the Halogens

The periodic table arranges the elements in order of increasing atomic number into horizontal rows whose lengths are such that elements with similar properties recur periodically - that is, groups of similar elements fall into a single vertical column on the table. The elements in any vertical column of the periodic table are similar in their chemical properties, and are therefore considered to make up a chemical family. In this experiment, some of the properties of elements in two chemical families will be observed. These families are Group 2A, also known as the alkaline earth elements; and Group 7A, also known as the halogens. In each group, the four most important elements in the group will be studied. In Group 2A, these are magnesium (Mg), calcium (Ca), strontium (Sr), and barium (Ba). In Group 7A, they are fluorine (F), chlorine (Cl), bromine (Br), and iodine (I). These elements will be studied mostly in the form of ions, not as the neutral elements. All Group 2A elements react readily to lose two electrons and form ions with a +2 charge. All of the halogens readily gain one extra electron to form ions with a - 1 charge. In Part 1 of the experiment, some properties of the Group 2A cations Mg2+, Ca2+, Sr2+, and Ba2+^ will be observed and compared. In Part 2, some properties of the halide ions F, Cl-, Br-^ and I-^ will be studied. In part 3, an unknown ionic compound will be identified. The unknowns will be in the form of an aqueous solution and will contain a Group 2A cation and a Group 7A anion. In Parts 1 and 2, you will observe that the properties of the elements in a particular group of the periodic table are generally similar to each other but they are not identical to each other. The information that is obtained in parts 1 and 2 about the differences in properties of elements in each group will be used in Part 3 to determine which of these elements is present in the unknown. When the properties of elements in the same group of the periodic table are different from each other, it is usually found that the properties change in progression in the same order in which the elements are arranged on the periodic table. The following chart illustrates this for some properties of the halogens. The elements are listed on the chart in the same order in which they appear on the periodic table. Element Melting Boiling Density Ionization Point Point (as liquid) Energy F - 218 °C - 188 °C 1.11 g/mL 403 kcal/mol Cl - 101 °C - 34 °C 1.56 g/mL 302 kcal/mol Br - 7 °C + 59°C 3.12 g/mL 274 kcal/mol I +114 °C + 184 °C 4.93 g/mL 243 kcal/mol (as solid) Safety Precautions:

  • Wear your safety goggles.
  • Silver nitrate (AgNO 3 ) solutions will stain skin and clothes. If you suspect you may have spilled AgNO 3 on yourself, rinse it off immediately. The stains are dark brown and they don’t show up right away. You’ll know the next day whether or not you spilled AgNO 3 on yourself.

Waste Disposal:

  • All of the waste from this lab may be dumped in the inorganic waste bottles (which have a blue label) in the fume hood. Procedure - Part 1 - Properties of Group 2A Cations To study the Group 2A cations, four tests will be performed on solutions containing these ions. These solutions are labeled Mg2+, Ca2+, Sr2+, and Ba2+. The solutions actually contain the nitrates of the Group 2A elements: Mg(NO 3 ) 2 , Ca(NO 3 ) 2 , Sr(NO 3 ) 2 , and Ba(NO 3 ) 2. Nitrate is a spectator ion and will not interfere with any of the tests. Between tests, the test tubes should be washed, rinsed with tap water, and then rinsed twice with small amounts of deionized water. The tubes do not have to be dried. Test 1: Add about 10 drops of each of the four solutions into separate test tubes. To each of the test tubes, add about 10 drops of 1 M H 2 SO 4. Swirl the test tube vigorously to mix the contents. Observe whether or not an insoluble precipitate forms. If a precipitate does form, it the sulfate of the Group 2A ion. For example, adding H 2 SO 4 to Ba2+^ gives a precipitate of barium sulfate, BaSO 4. Record the results of these tests, including the colors of any precipitates formed. Test 2: Again add about 10 drops of each of the four Group 2A cation solutions to each of four separate test tubes. To each sample, add about 10 drops of 1 M Na 2 CO3 (aq) and mix. The precipitates that form are the carbonates of the Group 2A ions, such as calcium carbonate (CaCO 3 ). Test 3: Again add about 10 drops of each of the four Group 2A cation solutions to each of four separate test tubes. To each sample, add about 10 drops of 0.25 M (NH 4 ) 2 C 2 O4 (aq). The precipitates that form are the oxalates of the Group 2A ions, such as barium oxalate (BaC 2 O 4 ). Test 4: Again add about 10 drops of each of the four Group 2A cation solutions to each of four separate test tubes. To each sample, add about 10 drops of 1 M (NH 4 ) 2 MoO 4 (aq). The precipitates that form are the molybdates of the Group 2A ions, such as barium molybdate (BaMoO 4 ). The data that you record for these four test should show that none of the Group 2A cations reacts in exactly the same way in all four of the tests as any other Group 2A cation. The differences in behavior of the cations will be used in Part 3 to determine which one is present in an unknown. Procedure - Part 2 - Properties of Group 7A Anions Obtain solutions containing each of the four Group 7A anions. These solutions are labeled F-, Cl-, Br-^ and I-. The solutions actually contain the sodium salts of these ions: NaF, NaCl, NaBr, and NaI. The sodium ion (Na+) is a spectator ion and will not interfere with any of the tests. Test 1: Add about 10 drops of each of the four halide ion solutions into separate test tubes. Add about 5 drops of 0.1 M AgNO 3 to each of the test tubes. Caution: AgNO 3 can cause dark brown stains on skin or clothes! Rinse it off immediately if you come into contact with this solution. Swirl the test tube vigorously to mix the contents. Observe whether or not an insoluble precipitate forms. The precipitates that