Question: How do we use the model of the electronic structure of the atom to understand periodic trends of the elements? I. Data Collection Using a web browser access the following address: http://www.chem.iastate.edu/group/greenbowe/sections/projectfolder/flashfiles/matters/periodictbl2.html When the animation starts you will see Figure I below: Figure I. A. Look at the atomic sizes shown in the table and describe all the trends that you see in the space below. Based on the elements that are populating the periodic table it appears that the atomic radius decreases going from let to right across periods, and increases from top to bottom in rows. 11/25/10 1 Periodicity Activity
Using the mouse, click on an atom in the row of atoms above the periodic table and drag them to their respective location in the periodic table. Complete the same information by listing the atoms and their atomic radii in the Periodic Table in Figure I above. II. Data Analysis and Interpretation A. Select one of the Groups (1, 2, 16 or 17) in Atom View and complete the following the Table below: (a) Element symbol (b) Atomic Radii (c) (# of protons) (d) Total # of 11/25/10 2 Periodicity Activity (e) (f) # of inner core (g) # of (h) Level the electron(s) occupy Electron configuration Li 1.34 Å 3 3 1s 2 2s 1 2 1 n = 2 Na 1.54 Å 11 11 1s 2 2s 1 2p 6 3s 1 10 1 n = 3 K 1.96 Å 19 19 1s 2 2s 1 2p 6 3s 1 3p 6 4s 1 18 1 n = 4 Rb 2.11 Å 37 37 1s 2 2s 1 2p 6 3s 1 3p 6 4s 1 3d 10 4p 6 5s 1 Table I. 36 1 n = 5 After completing Table I. work with several other students or participate in a class discussion and answer parts B - E below. B. Which columns (c h) in Table I. cannot explain the trend in atomic radii observed in your group and why? Column c does not by itself explain the trend in atomic radius going down a group. In Column c the total nuclear charge increases as the atomic radius increases. Since the atomic radius is getting larger as the nuclear charge gets larger it is difficult to relate the trend in atomic radius with total nuclear charge. Also considering Column d the total number of one must take care in connecting the increase in total with the increasing radius. The reason is that as we go across a period the total number of increases, yet the atomic radius decreases. So we would not want to believe that it is just the total number of that are responsible for the increasing atomic radius. Column g does not help because the number of is the same for each element. C. Which columns (c h) in Table I. might explain the trend in atomic radii observed in your group and why? Column e (and the related column h) is our best evidence for understanding the trend in atomic radii going down a group. When considering the electron configuration for each element we should note the location of the outer most electron. We focus on that electron because of all the in an atom, it is the outermost, or, that are responsible for determining the radius of an atom. So our best explanation for the observed increase in atomic radius going down a group in the periodic table is the location of the electron. Going down a group the electron is located in a different, higher shell (n value). Based on our knowledge of the hydrogen atom, as the e value increases the size of the shell increases, so the electron get further from the nucleus and the atomic radius increases. Also considering Column d the total number of one might think that it should be considered as a possible explanation for the trend. However, we must
take care in connecting the increase in total with the increasing radius. The reason is that as we go across a period the total number of increases, yet the atomic radius decreases. So we would not want to believe that it is just the total number of that are responsible for the increasing atomic radius. D. Which column (c h) in Table I. best explains the trend in atomic radii observed in your group and why? Column e (and the related column h) is our best evidence for understanding the trend in atomic radii going down a group. When considering the electron configuration for each element we should note the location of the outer most electron. We focus on that electron because of all the in an atom, it is the outermost, or, that are responsible for determining the radius of an atom. So our best explanation for the observed increase in atomic radius going down a group in the periodic table is the location of the electron. Going down a group the electron is located in a different, higher shell (n value). Based on our knowledge of the hydrogen atom, as the e value increases the size of the shell increases, so the electron get further from the nucleus and the atomic radius increases. E. Write a general statement that explains the trend in atomic radius that applies to any Group in the Periodic Table. When going down a group in the periodic table the electron(s) are located in a shell with a larger and larger n value. As the n value for the shell that contains the electron(s) increases the shell gets larger and larger, and therefore the electron gets further from the nucleus and the atomic radius increases. For a specific comparison of atoms in the same group the best explanation will include the electron configuration for each atom, the identification of the n value (level) where the are located, and then the statement that as the n value for the shell that contains the electron(s) increases the shell gets larger and larger, and therefore the electron gets further from the nucleus and the atomic radius increases. 11/25/10 3 Periodicity Activity
III. Data Analysis and Interpretation A. Select one of the periods in the atom view and complete the table below. (a) Element symbol (b) Atomic Radii (c) (# of protons) (d) Total # of (e) (f) # of inner core (g) # of (h) Level the electron(s) occupy Electron configuration Na 1.54 Å 11 11 1s 2 2s 2 2p 6 3s 1 10 1 n = 3 Mg 1.30 Å 12 12 1s 2 2s 2 2p 6 3s 2 10 2 n = 3 Si 1.11 Å 14 14 1s 2 2s 2 2p 6 3s 2 3p 2 10 4 n = 3 S 1.02 Å 16 16 1s 2 2s 2 2p 6 3s 2 3p 4 10 6 n = 3 Cl 0.98 Å 18 18 1s 2 2s 2 2p 6 3s 2 3p 5 10 7 n = 3 Table II. After completing Table II. work with several other students or participate in a class discussion and answer parts B - E below. B. Which column(s) (c h) in Table II. cannot explain the trend in atomic radii observed in your period and why? Column (d) cannot explain the trend in atomic radii going across the period. The increase in the total number of proceeding from left to right does not agree with a decrease in the atomic radius. Column (f) in and of itself cannot explain the trend in atomic radii, however, as will be discussed later the number of inner core can be used to understand the trend in atomic radii going across a period. Column (g) cannot explain the trend in atomic radii going across a period from left to right. The number of increases as we go across the period, yet the atomic radius is decreasing. So Column (g) would not be useful. Column (h) in and of itself can not explain the trend in atomic radius. However, it is used in combination with other columns to explain the trend. This will be explained below. C. Which column(s) (c h) in Table II. might explain the trend in atomic radii observed in your period and why? Column (c) might be used to explain the trend in atomic radii going across a period. As the total nuclear charge increases the atomic radius decreases, so there might be a connection. Column (e) might be important to help understand the trend in atomic radii going across a period. D. Which column(s) (c h) in Table II. best explains the trend in atomic radii observed in your group and why? To explain the trend in atomic radii going across the period Columns (e), (g), (f) and (c) must be combined to best explain the observed trend in atomic radii going across a period. 11/25/10 4 Periodicity Activity
Looking at Column (e), the electron configuration, the are in the same shell (in this example the shell with an n value of 3). So all of the (Column (g)) are shielded by the same number of (Column (f)). Since the total nuclear charge is increasing (Column (c)) the experience a greater attraction to the nucleus and the atomic radius decreases going across the period. E. Write a general statement that explains the trend in atomic radius that applies to any Group in the Periodic Table. Looking at Column (e), the electron configuration, the are in the same shell (in this example the shell with an n value of 3). So all of the (Column (g)) are shielded by the same number of (Column (f)). Since the total nuclear charge is increasing (Column (c)) the experience a greater attraction to the nucleus and the atomic radius decreases going across the period. For a specific comparison of the atomic radii of two elements in the same period, the best explanation will include the electron configuration for each atom, the identification of the n value (level) where the are located, and then the statement that as the nuclear charge increase is larger for one atom compared to the other and the number of that shield the from the nucleus are the same, that the in the atom further to the right experience a greater attraction to the nucleus compared to the atom to the left, and therefore the atom further to the right in the period has a smaller atomic radius compared to the atom further to the left in the period. 11/25/10 5 Periodicity Activity
IV. Data Analysis and Interpretation A. Effective nuclear charge (ENC) is a term used to describe the amount of nuclear charge experienced by a particular electron in an atom. charge is the total positive charge in the nucleus. For example the nuclear charge for a carbon atom is +6. The nuclear charge is equivalent to the atomic number for an element. Carbon has six protons so consider the following information; (a) Element symbol (b) (# of protons) (c) Total # of (d) Electron configuration (e) # of inner core (f) # of (g) Level the electron(s) occupy (h) Effective (ENC) for the C +6 6 1s 2 2s 2 2p 2 2 4 2 +4 If we focus on the electron configuration for carbon we see there are 4 in the second level and two in the first level. Electrons in the second level are further away from the nucleus. The 2 in the first level (inner core ) partially shield the from some of the nuclear charge. To a first approximation we will assume each inner core electron shields one proton from the in the outer most level ( ). The ENC is calculated by subtracting the number of inner core (IC) from the total nuclear charge (Z): ENC = Z IC. In the case of carbon, ENC = +6 2 = +4. Transfer columns (a h) from Table I and Table II into Table III below. (a) Group Element symbol (b) Atomic Radii (c) (# of protons) (d) Total # of (e) Electron configuration (f) # of inner core (g) # of (h) Level the electron(s) occupy (i) Effective (ENC) for the Be 0.90 Å 4 4 1s 2 2s 2 2 2 n = 2 +2 Mg 1.30 Å 12 12 1s 2 2s 1 2p 6 3s 2 10 2 n = 3 +2 Cs 1.74 Å 20 20 1s 2 2s 1 2p 6 3s 2 3p 6 18 2 n = 4 +2 4s 2 Sr 1.92 Å 38 38 1s 2 2s 1 2p 6 3s 2 3p 6 36 2 n = 5 +2 4s 2 3d 10 4p 6 5s 2 Period Element symbol Na 1.54 Å 11 11 1s 2 2s 2 2p 6 3s 1 10 1 n = 3 +1 Mg 1.30 Å 12 12 1s 2 2s 2 2p 6 3s 2 10 2 n = 3 +2 Si 1.11 Å 14 14 1s 2 2s 2 2p 6 3s 2 2p 2 10 4 n = 3 +4 11/25/10 6 Periodicity Activity
S 1.02 Å 16 16 1s 2 2s 2 2p 6 3s 2 2p 4 10 6 n = 3 +6 Cl 0.98 Å 17 17 1s 2 2s 2 2p 6 3s 2 2p 5 10 7 n = 3 +7 Table III. B. Complete column (i) in Table IV for each element. C. In the space below show the calculation for ENC for the for any two of the elements in Table III. The effective nuclear charge on a electron in Mg: ENC = Z IC = 12 10 = +2 The effective nuclear charge on a electron in S: ENC = Z IC = 16 10 = +6 After completing Table III. work with several other students or participate in a class discussion and answer parts D - F below. D. Is there a relationship between ENC experienced by the and the trend in atomic radius for the elements in the Group from Table I? Explain. The ENC for all of the elements in Group IA are the same (+1). The atomic radii increases going down a Group, so there does not appear to be any relationship between the ENC and the atomic radii in a Group in the Periodic Table. E. Is there a relationship between ENC and the trend in atomic radius for the elements in the period from Table II? Explain. The ENC for the elements in the second period increases from left to right. The atomic radii also increases going across a period from left to right, so there does appear to be a relationship between the ENC and the atomic radii in a Period in the Periodic Table. Atomic radius does decrease as the ENC increases however it is critical that we remember that in a particular row in a period all the are in the same shell. Under such circumstances the number of shielding the is a constant, so as the nuclear charge increases the experience a greater attraction to the nucleus and the atomic radius will decrease. NOTE: Students should never say a particular atom has an effective nuclear charge, or it has an effective nuclear charge. Students must identify what electron they are talking about and what its specific effective nuclear charge is. 11/25/10 7 Periodicity Activity
F. Hypothetical statement made by a student to another student: When I look at Figure I. it appears to me that as the nuclear charge increases the atomic size increases. Cite evidence that supports and/or refutes this student s statement. The statement is true, however it is critical that the student include in the statement that this is the case only when the atoms that are being compared, are in the same Group. Consider what happens to the nuclear charge when going across a period from left to right, it increases. However, the atomic radius decreases. So the student s statement only works going down a Group when the are in different shells. Going across a period the are in the same shell. 11/25/10 8 Periodicity Activity
V. Data Collection Switch to the Ion View. Figure II. Using the mouse, click on an ion in the row of ions above the periodic table and drag them to their respective location in the periodic table. Complete the same information by listing the ions and their ionic radii in the Periodic Table in Figure II above. 11/25/10 9 Periodicity Activity
VI. Data Analysis and Interpretation A. Using the atomic and ion size information from Figure I and Figure II. select one metal from Group 1 or 2 and select one nonmetal from Group 16 or 17 and complete Table IV. In Table IV. enter the metal you selected in the first row and complete the row of information. In the next row below enter the cation for that metal and complete the information in the row. Do the same for the nonmetal you selected. (a) (b) (c) (# of (d) Atomic Total # # of inner # of Element /ion of core symbol Radii protons) Electron configuration Mg 1.30 Å 12 12 1s 2 2s 2 2p 6 3s 2 10 2 n = 3 (e) (f) (g) (h) Level the electron(s) occupy Mg 2+ 0.66 Å 12 10 1s 2 2s 2 2p 6 2 0 (in the n = 2 Cl 0.98 Å 17 17 1s 2 2s 2 2p 6 3s 2 3p 5 10 3 rd shell) 7 n = 3 Cl 1 1.81 Å 17 18 1s 2 2s 2 2p 6 3s 2 3p 6 10 8 n = 3 Table IV. After completing Table IV. work with several other students or participate in a class discussion and answer parts B - F below. B. Which columns (c h) in Table III. cannot explain the trend in radii observed for the metal atom and its cation and why? Column (f) cannot explain the observed change in the radii of the metal and the metal ion. Column (g) does not help explain the observed trend in the radii of the metal and the metal ion. C Which columns (c h) in Table III. explain the trend in radii observed for the metal atom and its cation and why? When comparing the atomic radius of the metal with its ion, consider Column (e) the electron configuration and notice the metal has two in the n= 3 level (Column h). The electron configuration for the metal ion has no in the n = 3 level. The outer most in Mg 2+ are in the n = 2 level. Electrons in the n = 3 level are further from the nucleus compared to in the n = 2 level, so the metal is larger than the metal ion. Columns (c) and (d). 11/25/10 10 Periodicity Activity
D. In general what is the relationship between the radius for the neutral metal atom and its cation? After writing the electron configuration of both the metal and its ion, one can identify what level the are located for each. In the case of the metal ion the will typically be located in one shell lower compared to the of the metal. E. Which column (c h) in Table III. best explain the trend in radii observed for the metal atom and its cation and why? Columns (e) and (h) are the most important, while columns (c) and (d) are supporting. For metal ions the outer most are in a shell that is one less than the outer most shell in the metal atom. The smaller the n value occupied by the outer most the smaller the radius of the atom or ion. F. Write a general statement that explains the trend in radius between a metal atom and its ion that applies to any combination of metal and its cation. Based on the electron configuration the outer most in a metal ion will be one level less than the outer most in the metal. The smaller the value of n for the shell the closer the are to the nucleus and the smaller the radius compared to in a higher value of n. G. Which columns (c h) in Table III. cannot explain the trend in radii observed for the nonmetal atom and its anion and why? Column (f) cannot explain the observed change in the radii of the metal and the metal ion. Column (h) in and of itself does not explain the observed trend in the radii of the metal and the metal ion. 11/25/10 11 Periodicity Activity
H. Which columns (c h) in Table III. explain the trend in radii observed for the nonmetal atom and its anion and why? When comparing the atomic radius of the nonmetal with its ion, consider Column (e) the electron configuration and notice the nonmetal and its ion have in the n= 3 level (Column h). Since the for the nonmetal and its ion are in the same level (Column (h)), and both the nonmetal and its ion have the same number of protons (Column (c)) there will be greater electron-electron repulsions in the nonmetal ion compared to the nonmetal and the radius of the nonmetal ion will be greater than the radius of the nonmetal. I. In general what is the relationship between the radius for the neutral nonmetal atom and its anion? The radius of the nonmetal ion will be larger than the radius of the nonmetal. j. Which column (c h) in Table III. best explains the trend in radii observed for the nonmetal atom and its anion and why? Columns (e), (h), and (c) are most important, while Columns (d) and (g) play a supportive role. K. Write a general statement that explains the trend in radius between a nonmetal atom and its anion that applies to any combination of nonmetal atom and its anion. Based on the electron configuration of the nonmetal and its ion the occupy the same shell. Since the nonmetal and its ion have the same nuclear charge, the electron-electron repulsions are greater for the nonmetal ion compared to the nonmetal so the radius of the nonmetal ion will be larger than the radius of the nonmetal. 11/25/10 12 Periodicity Activity
Complete the Table below. Element symbol Ion Radii (c) (# of protons) (d) Total # of (e) Electron configuration (f) # of inner core (g) # of (h) Level the electron(s ) occupy (i) Effective (ENC) on the outermost S 2-1.84 Å 16 18 1s 2 2s 2 2p 6 3s 2 3p 6 10 8 n = 3 +6 Cl 1.81 Å 17 18 1s 2 2s 2 2p 6 3s 2 3p 6 10 8 n = 3 +7 K + 1.33 Å 19 18 1s 2 2s 2 2p 6 3s 2 3p 6 10 8 n = 3 +9 Ca 2+ 0.99 Å 20 18 1s 2 2s 2 2p 6 3s 2 3p 6 10 8 n = 3 +10 Table V. Which columns (c h) in Table V. cannot explains the trend in ion radii observed in this selection of ions and why? Columns (d), (f), (g) or (h) by themselves cannot explain the trend in the radius of the ions in this set. Which columns (c h) in Table V. explain the trend in ion radii observed in this selection of ions and why? Columns (e), (h), (i) and (c) can be used to explain the trend in the radii of this set of ions. After writing the electron configuration for each of the ions, the are all in the same (Column (h)) shell. So all of the (Column (g)) are shielded by the same number of inner core (Column (f)). So as the nuclear charge increases (Column (c)), the experience a greater attraction to the nucleus (Column (i)), and the radius gets smaller. Which column(s) (c h) in Table V. best explains the trend in ion radii observed in this selection of ions and why? Columns (e), (h), (i) and (c) can be used to explain the trend in the radii of this set of ions. After writing the electron configuration for each of the ions, the are all in the same (Column (h)) shell. So all of the (Column (g)) are shielded by the same number of inner core (Column (f)). So as the nuclear charge increases (Column (c)), the experience a greater attraction to the nucleus (Column (i)), and the radius gets smaller. 11/25/10 13 Periodicity Activity
Write a general statement that explains the trend in ion radius that applies to a related (isoelectronic) set of ions in the Periodic Table. Write the electron configuration for each of the ions, then identify shell where the are located. When the are in the same shell, and shielded by the same number of inner core, the greater the nuclear charge the greater the effective nuclear charge experienced by the and the smaller the radius of the ion. 11/25/10 14 Periodicity Activity