Periodicity and Predictions of Properties

Periodicity and Predictions of Properties Objectives 1 1. To become familiar with the organization of the periodic table. 2. To study periodic properties. 3. To make predictions based on periodic properties. 4. To use the periodic table to assist in writing chemical formulas.

Discussion The periodic law states that many properties of the elements are periodic functions of their atomic numbers. A periodic function is one which goes through cycles, with maximum and minimum values at regular intervals. The atomic radius, melting point, and boiling point are periodic properties of the elements. The periodic table arranges the elements in order of increasing atomic number in such a way that the periodic nature of properties is made clear. This is done by placing in the same vertical column of the table those elements that appear at corresponding positions in the cycle of properties. For example, since the atomic radii of Li, Na, K, Rb, and Cs appear at maxima in the cycles for atomic radii as a function of atomic number, we would expect that these elements would be in the same column in the periodic table. These elements belong to a family, or Group, and have many other properties which would also indicate that they should be classified together. In general, properties of the elements in a Group tend to change gradually as one goes from the lightest to the heaviest atom in the group. Within the framework of the periodic table properties vary according to a pattern as you move across the table in a cycle, or period, or up and down one of the Groups. If the pattern for the variation is known, it is often possible to predict a property of one element from the properties of elements which lie nearest that element in the table. Depending on the position of the element in the table, the best prediction will either be based on elements in the same row, or on elements in the same column, as the element for which the prediction is to be made. In this experiment you will make some graphs of periodic properties versus atomic number in order to discover the patterns the properties follow. You will then use those patterns to predict some properties of elements. In the last part of the experiment you will be asked to assign elements to their proper positions in the table on basis of their characteristic properties.

Procedure 1. Using the data in Table 1, plot the atomic radius of each element on the y-axis against its atomic number on the x-axis. Make the x-axis on the long side of the paper and choose the scales so that all data will fit on the sheet. Connect all points to make a line graph. Label each peak with the symbol of the element.

2. Ionization Energy – If you want to remove an electron from an atom a certain amount of energy is needed. The amount of energy needed varies among atoms and is called ionization energy. This energy is needed to overcome the attraction between the electron and the nucleus of the atom. It is called ionization energy because the result is an ion or positively charged atom. It is typically measured in kJ/mol.

Organize the data in Table 2 Ionization Energies, and then graph the atomic number of the element (on the horizontal axis) versus the ionization energy (on the vertical axis). Connect all points to make a line graph. Think about the scale you will use.

3. Electronegativity – Atoms have different abilities to attract electrons to themselves when they are bonded in a compound or molecule. The measure of this ability to pull an electron to an atom within a compound or molecule is referred to as electronegativity. A bonded atom with a higher electronegativity can pull stronger on an electron with a weaker electronegativity.

1 Adapted from Periodicity and Predictions of Properties in Chemistry Masterton, Slowinski and Walford. Holt, Rinehart and Winston publishers (1980) by Ema Gluckmann 10/04; modified 11/10

1 Organize the data in Table 3, and then graph the atomic number of the element (on the horizontal axis) versus the electronegativity (on the vertical axis). Connect all points to make a line graph. Think about the scale you will use.2

4. Fill in the blanks in the periodic table provided you by your teacher with the letter symbol for each of the mystery elements whose properties are described. The idea is to identify the mystery element and to put its symbol in the proper place in the blank table. In doing this part of the experiment you may use the periodic table in your text and any information in the text. (a) A, B, C, and D belong to a family, the members of which are all gases. A is commonly used in advertising signs. B was first discovered on the sun and is used in weather balloons. C was used to make the first compounds of an element of this group. D is the family member that is present in the largest amount in air. (b) E and F are members of a family containing both gaseous and solid elements. E forms a diatomic molecule and is the major constituent of the atmosphere. Element F is a dangerous poison and a metalloid. (c) G, H, and I belong to family of very active metals, all of which react with chlorine to produce salts with the general formula XCl. G is a member of the first period to contain 18 elements. H has the highest ionization energy of the family and I has the lowest. (d) J, K, and L belong the same family and all are metals. A compound of J is a major component of bones and teeth. K is commonly used in flash bulbs for producing light. L is a radioactive element discovered by Marie Curie. (e) M is a gas and has some properties similar to the elements in both Group 1 and Group 7. It is a unique element in this respect. (f) N, O, and P commonly form -1 ions when they combine with metals. N is a liquid and O is a non- radioactive solid. P is the most chemically reactive of all the non-metals. (g) Q, R, S, and T are in different families but in the same period. Q is a gas used for water purification. R is a yellow non-metallic solid. S is a metalloid used in transistors. T is a metal of low density used in aircraft construction. (h) U, V, W, and X are all transition elements. U is an excellent conductor of heat and electricity and is commonly used in wiring and cookware. V is the only metal that is a liquid at room temperature. W is the metal which is produced in the largest quantity. Although once used in coins, X is now used mostly in expensive jewelry. (i) Y is an actinide fuel used in nuclear reactors. Z is the actinide named for the “father” of the Periodic Table.

Calculations and Questions 1. (a) Describe any regularity that is present in the graph of atomic radius versus atomic number. Which elements occupy the peaks in the cycles? Are the periods, or cycles, of the same length? (b) In Table 1 the atomic radii of Mg and Fe were left blank intentionally. The values are actually known, but you should be able to predict them based on the pattern of properties on the graph. Note that the part of cycle from Li (3) to B (5) is repeated in the part of the cycle from Na (11) to Al (13). We would expect, then, that the radius of Mg would have a value that would maintain the pattern observed between Li and B. On that basis, predict the radius of the Mg atom. (c) As part of the last cycle on the graph, which starts at K (19), we have the first series of transition metals. By assuming that the Fe (26) atom would maintain the pattern set by atoms near it in atomic number, predict the radius of the Fe atom.

2. For the graphs you constructed in Procedures 2 and 3 describe any patterns you see in the graphs. Are these properties periodic properties? Explain any patterns you observe; i.e. explain the reason behind them.

2 Sacramento Area Science Project; An Education Partnership; UC Davis and CSU Sacramento; http://sasp.ucdavis.edu

2 3. Using the periodic table prepared in Procedure 4, predict the formulas for compounds of the following elements: (Use these mystery letters in the formulas.) (a) M and P (c) E and M (e) A and G (b) T and Q (d) I and R (f) J and N In predicting the formulas you may find the following formulas of known substances to be useful:

AlBr3 HCl PH3 Li2O BaCl2 NaI

4. Ionic radius. Atoms become ions when electrons are gained or lost. Table 4 compares some parent ions with their daughter cations or anions. a. What do you see is the general trend for when an atom loses electrons (becomes a cation)? What is the explanation for this phenomenon? b. What do you see is the general trend for when an atom gains electrons (becomes an anion)? What is the explanation for this phenomenon?

Table 1 Atomic radii of the elements in nm (10-9 m) Element Atomic Atomic Element Atomic Atomic number radius number radius H 1 0.037 K 19 0.231 He 2 0.050 Ca 20 0.197 Li 3 0.152 Sc 21 0.160 Be 4 0.111 Ti 22 0.146 B 5 0.088 V 23 0.131 C 6 0.077 Cr 24 0.125 N 7 0.070 Mn 25 0.129 O 8 0.066 Fe 26 --- F 9 0.064 Co 27 0.125 Ne 10 0.070 Ni 28 0.124 Na 11 0.186 Cu 29 0.128 Mg 12 --- Zn 30 0.133 Al 13 0.143 Ga 31 0.122 Si 14 0.117 Ge 32 0.122 P 15 0.110 As 33 0.121 S 16 0.104 Se 34 0.117 Cl 17 0.099 Br 35 0.114 Ar 18 0.094 Kr 36 0.109

Table 2 Ionization energy of representative elements (kJ/mol)

Atomic Ionization Atomic Ionization Atomic Ionization Number Energy Number Energy Number Energy 1 1312 26 762 15 1012 3 520 27 760 33 947 11 496 28 737 8 1314 19 419 29 746 16 1000 4 900 30 906 34 941 12 738 5 801 9 1681 20 590 13 578 17 1251 21 633 31 579 35 1140 22 659 6 1086 2 2372 23 651 14 787 10 2081 24 653 32 762 18 1521 25 717 7 1402 36 1351

3 Table 3 Electronegativity values of representative elements

Atomic Electronegativity Atomic Electronegativity Atomic Electronegativity Number Number Number 1 2.1 26 1.8 15 2.1 3 1.0 27 1.8 33 2.0 11 0.9 28 1.8 8 3.5 19 0.8 29 1.9 16 2.5 4 1.5 30 1.6 34 2.4 12 1.2 5 2.0 9 4.0 20 1.0 13 1.5 17 3.0 21 1.3 31 1.6 35 2.8 22 1.5 6 2.5 2 - 23 1.6 14 1.8 10 - 24 1.6 32 1.8 18 - 25 1.5 7 3.0 36 3.0

Table 4 Atomic and ionic radii of various representative elements in nm (10-9 m)

Element Atomic Atomic Ion Ionic number radius radius H 1 0.037 H- 0.154 Li 3 0.152 Li+ 0.078 Be 4 0.111 Be2+ 0.034 N 7 0.070 N3- 0.130 O 8 0.066 O2- 0.140 F 9 0.064 F- 0.133 Na 11 0.186 Na+ 0.102 Mg 12 0.160 Mg2+ 0.072 P 15 0.110 P3- 0.212 Cl 17 0.099 Cl- 0.181

Conclusion: Write out a conclusion for this lab describing what you learned about periodic trends of the various properties studied.

This is a blank periodic table that you may fill in, cut out, and attach to your lab notebook for procedure 4a-i