Periodicity - Chemical Formulas
In this activity we will explore how variations in chemical formulas relate to the periodic table.
1. Each student is assigned one or more elements.
2. Use the Handbook of Chemistry and Physics or WebElements to find the formulas of the element and chlorine. If a compound of the element and chlorine is not found, then look for any other halide (ie. F, Br or I).
3. Prepare a card (or Post-It note) with the symbol of the element, the atomic number, the atomic weight, and the formulas you found.
4. Arrange the cards in order of increasing atomic weight in one line.
5. Rearrange the cards so that elements with similar chemical properties will form columns. You will see Mendeleev's periodic table emerge.
Questions
1. Do you see a relationship between the Roman numeral at the top of the column and the formulas of the compounds?
2. Despite the fact that we see NaCl and AgCl (both with one chlorine) in column I, sodium and silver have very different properties when it comes to other reactions. How do sodium and silver behave in the presence of oxygen or water? See how the alkali metals react with water.
3. How does the modern periodic table resolve the differences in some of the chemical properties of elements like sodium and silver?
4. What information can we get from the Roman numerals at the tops of the columns?
Valence and Oxidation number
The Roman numerals tell us the valence of the elements in that column. The term valence refers to the "combining power" of the element and tells the number of bonds the element could form with a halide. In addition, the Roman numeral tells us the "oxidation number" of the element. Compute the oxidation number of the elements. The oxidation number of chlorine is -1. The sum of the oxidation numbers for a compound is always zero.
A / KCl / CaCl2 / GaCl3 / GeCl4 / AsCl5 / SeF6 / BrF5 / Period 4 representative elements
B / CuCl / ZnCl2 / ScCl3 / TiCl4 / VF5 / CrF6 / MnF4 / Period 4 transition metals
+1 / +2 / +3 / +4 / +5 / +6 / Oxidation numbers
· The greatest number of fluorine atoms that a single chlorine atom will bond with is 5, BrF5. But iodine, also in column VII will bond with seven fluorine atoms, IF7.
· In MnF4 manganese doesn't have a combining power of 7, but it does in Mn2O7. Also, rhenium forms ReF7.
Now, repeat using the formulas of the oxides. Compute the oxidation number of the elements. The oxidation number of oxygen is -2. The sum of the oxidation numbers for a compound is always zero.
I / II / III / IV / V / VI / VIIA / K2O / CaO / Ga2O3 / GeO2 / As2O5 / SeO3 / Br2O5 / Period 4 representative elements
B / Cu2O / ZnO / Sc2O3 / TiO2 / V2O5 / CrO3 / Mn2O7 / Period 4 transition metals
+1 / +2 / +3 / +4 / +5 / +6 / +7 / Oxidation numbers
While the word "valence" still has many uses in chemistry, we seldom use it within the context of "combing power", or expressing the number of bonds an element can form.
More on valence and its replacement, "oxidation number".
Oxidation number periodic table.
Refer to the oxidation number periodic table. You will see the oxidation numbers that we have predicted, plus others. Most oxidation numbers can be predicted from the periodic table. Many elements exhibit more than one oxidation number. The metals which do not are found in groups IA, IIA and IIIB, as well as B, Al, Ga, In, Zn, Cd, and Ag.
Valences and oxidation numbers of the nonmetals.
While we use chlorine which is univalent to determine the valences of the metals and some nonmetals, we assume that chlorine, as well as the other halogens have valences of 1, since they form 1 bond with the univalent metals in group IA. We can say that the halogens have oxidation numbers of -1. Also, the chalcogens have oxidation numbers of -2 and the nitrogen family of nonmetals have oxidation numbers of -3. With the exceptions of carbon and nitrogen and the peroxides, each nonmetal will have only one negative oxidation number.