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Chemical misconceptions 121
Chemical stability
Target level
This probe may be used at post-16 level, or with students in the 14–16 age range who have studied atomic structure.
Topics
Relative stability of atoms and ions.
Rationale
Research has shown that students entering post-16 chemistry courses may have acquired the notion that species with octet configurations or full outer shells are always more stable than species with other configurations. These judgements are sometimes made regardless of the context, and any other factors that might be relevant. These ideas are discussed in Chapter 6 of the Teachers’ notes. This set of probes asks students to compare the stability of various triads of related species. There are seven probes in the set. The first four compare an atom with two of its ions, and are suitable for able students in the 14–16 year age range as well as for more advanced students. The other three probes introduce more subtle points, and are suitable for students on post-16 courses. When these materials were piloted teachers found the probes useful and the student responses interesting. It was found that for many students ‘the complete shell of electrons was regarded as automatically conferring stability’. One teacher noted that ‘it is clear that the complete shell of electrons dominates their thinking. We have spent some time looking at stability in terms of energy changes and forces between charged particles, so it shows me how easily people revert to simple and familiar ideas’.
Instructions
The probes may be used in a number of ways. It is suggested that it is most useful to distribute copies of the various probes in such a way that each student has a different task to their neighbours, and then – after allowing time to complete the sheet - moving to a discussion of the responses (either in groups or in the whole class). Each student in a 14–16 year old group will require a copy of one of the worksheets: Chemical stability (1) – Na+^ / Na / Na7– Chemical stability (2) – Cl7+^ / Cl / Cl– Chemical stability (3) – C 4+^ / C / C4– Chemical stability (4) – Be2+^ / Be / Be6– Each student in a post-16 group will require a copy of one of the worksheets above, or one of Chemical stability (5) – Cl (1.8.8) / Cl (2.7.8) / Cl (2.8.7) Chemical stability (6) – Cl / Cl–^ / Cl 11– Chemical stability (7) – O / O –^ / O 2–
122 Chemical stability
Resources
Student worksheets
Feedback for students
A sheet of answers and feedback points for discussion is provided for teachers.
Chemical stability (4) – Be2+^ / Be / Be6–
10. Energy is required to ionise the atom to form the cation, so the neutral atom should be considered more stable. (However, see the comments on Question 1 about the context in which students might consider the question.) 11. The highly charged anion is clearly unstable compared the neutral atom, although some students may feel that the octet structure on the anion makes it more stable. 12. The highly charged metal anion is clearly unstable compared the moderately charged metal cation. Some students may feel both are stable because they have full shells, whilst others may suggest that the anion is more stable as it has an octet, or because it has more full shells.
Chemical Stability (5) – Cl (1.8.8) / Cl (2.7.8) / Cl (2.8.7)
13. Neither of these configurations are stable, and both would be expected to undergo spontaneous transition to 2.8.7. The 1.8.8 structure will emit a greater energy quanta as the electron drops from the n=3 to the n=1 level (cf the quantum jump from n=3 to n=2 in the 2.7.8 species). Some students may feel that both are stable as they have octets in the outer shells, and 1.8.8 may be considered more stable as it has two octets. 14. 2.8.7 is the ground state of the chlorine atom and is more stable. The excited state (2.7.8) will decay to the ground state. Some students may feel that the excited state is more stable as it has an octet in the outer shell. 15. 2.8.7 is the ground state of the chlorine atom and is more stable. The excited state (1.8.8) will decay to the ground state. Some students may feel that the excited state is more stable as it has an octet in the outer shell.
Chemical stability (6) – Cl / Cl
-
/ Cl
11–
16. Chlorine has an exothermic electron affinity, so the chloride ion may be considered more stable than the atom, despite the atom’s neutrality. 17. The common Cl–^ anion is clearly more stable than the highly charged Cl11–^ ion. Cl– has an outer octet of electrons (but not a full shell), and Cl11–^ has a full outer shell (but more than an octet). 18. The neutral atom is clearly more stable than the highly charged Cl11–^ ion. However, Cl 11–^ has a full outer shell and may be considered more stable by some students.
Chemical stability (7) – O / O–^ / O2–
19. Oxygen has an exothermic electron affinity, so the O–^ ion may be considered more stable than the atom, despite the atom’s neutrality. 20. Although the oxide is O2–^ and is common, the O–^ anion will actually repel away another electron, so is more stable than O2–^. (See the comments about chemical context in Q1). The electron affinity for the oxygen atom is -142 kJ mol–1^ , but the electron ‘affinity’ [sic] of the O–^ ion is endothermic: +844 kJ mol–^. 21. The O2–^ ion, although common is, in isolation, less stable than the neutral atom. (The sum of the two electron affinity values is endothermic – see Question 20.)
124 Chemical stability
Chemical stability (1)
The diagrams below represent three chemical species:-
1. Tick ✔ one of the four statements: Why did you think this was the answer?
❑ Na+^ is more stable than Na __________________________________________________________
❑ Na+^ and Na are equally stable __________________________________________________________
❑ Na+^ is less stable than Na __________________________________________________________
❑ I do not know __________________________________________________________
2. Tick ✔ one of the four statements: Why did you think this was the answer?
❑ Na is more stable than Na7–^ __________________________________________________________
❑ Na and Na7–^ are equally stable __________________________________________________________
❑ Na is less stable than Na7–^ __________________________________________________________
❑ I do not know __________________________________________________________
3. Tick ✔ one of the four statements: Why did you think this was the answer?
❑ Na7–^ is more stable than Na+^ __________________________________________________________
❑ Na7–^ and Na+^ are equally stable __________________________________________________________
❑ Na7–^ is less stable than Na+^ __________________________________________________________
❑ I do not know __________________________________________________________
Chemical stability (1) – page 1 of 1 H P
OT OCO PY
P
Na+
11+ 11+
Na
11+
Na7–
Sodium ion with electronic configuration of 2.
Sodium atom with electronic configuration of 2.8.
Sodium ion with electronic configuration of 2.8.
Chemical stability (3) – page 1 of 1 H P
OT OCO PY
P
Chemical stability (3)
The diagrams below represent three chemical species:-
7. Tick ✔ one of the four statements: Why did you think this was the answer?
❑ C4+^ is more stable than C __________________________________________________________
❑ C4+^ and C are equally stable __________________________________________________________
❑ C4+^ is less stable than C __________________________________________________________
❑ I do not know __________________________________________________________
8. Tick ✔ one of the four statements: Why did you think this was the answer?
❑ C is more stable than C4–^ __________________________________________________________
❑ C and C 4–^ are equally stable __________________________________________________________
❑ C is less stable than C4–^ __________________________________________________________
❑ I do not know __________________________________________________________
9. Tick ✔ one of the four statements: Why did you think this was the answer?
❑ C4–^ is more stable than C4+^ __________________________________________________________
❑ C4–^ and C 4+^ are equally stable __________________________________________________________
❑ C4–^ is less stable than C4+^ __________________________________________________________
❑ I do not know __________________________________________________________
C4+
6+ 6+
C
6+
C4–
Carbon ion with electronic configuration of 2
Carbon atom with electronic configuration of 2.
Carbon ion with electronic configuration of 2.
Chemical stability (4)
The diagrams below represent three chemical species:-
10. Tick ✔ one of the four statements: Why did you think this was the answer?
❑ Be2+^ is more stable than Be __________________________________________________________
❑ Be2+^ and Be are equally stable __________________________________________________________
❑ Be2+^ is less stable than Be __________________________________________________________
❑ I do not know __________________________________________________________
11. Tick ✔ one of the four statements: Why did you think this was the answer?
❑ Be is more stable than Be6–^ __________________________________________________________
❑ Be and Be6–^ are equally stable __________________________________________________________
❑ Be is less stable than Be6–^ __________________________________________________________
❑ I do not know __________________________________________________________
12. Tick ✔ one of the four statements: Why did you think this was the answer?
❑ Be6–^ is more stable than Be2+^ __________________________________________________________
❑ Be6–^ and Be2+^ are equally stable __________________________________________________________
❑ Be6–^ is less stable than Be2+^ __________________________________________________________
❑ I do not know __________________________________________________________
H P Chemical stability (4) – page 1 of 1
OT OCO PY
P
Be2+
4+ 4+
Be
4+
Be6–
Beryllium ion with electronic configuration of 2
Beryllium atom with electronic configuration of 2.
Beryllium ion with electronic configuration of 2.
Chemical stability (6)
The diagrams below represent three chemical species:
16. Tick ✔ one of the four statements: Why did you think this was the answer?
❑ Cl is more stable than Cl–^ __________________________________________________________
❑ Cl and Cl–^ are equally stable __________________________________________________________
❑ Cl is less stable than Cl–^ __________________________________________________________
❑ I do not know __________________________________________________________
17. Tick ✔ one of the four statements: Why did you think this was the answer?
❑ Cl –^ is more stable than Cl11–^ __________________________________________________________
❑ Cl –^ and Cl11–^ are equally stable __________________________________________________________
❑ Cl –^ is less stable than Cl11–^ __________________________________________________________
❑ I do not know __________________________________________________________
18. Tick ✔ one of the four statements: Why did you think this was the answer?
❑ Cl 11–^ is more stable than Cl __________________________________________________________
❑ Cl 11–^ and Cl are equally stable __________________________________________________________
❑ Cl 11–^ is less stable than Cl __________________________________________________________
❑ I do not know __________________________________________________________
H P Chemical stability (6) – page 1 of 1
OT OCO PY
P
Cl
17+ 17+
Cl–
17+
Cl11–
Chlorine atom with electronic configuration of 2.8.
Chloride ion with electronic configuration of 2.8.
Chloride ion with electronic configuration of 2.8.
Chemical stability (7)
The diagrams below represent three chemical species:
19. Tick ✔ one of the four statements: Why did you think this was the answer?
❑ O is more stable than O–^ __________________________________________________________
❑ O and O –^ are equally stable __________________________________________________________
❑ O is less stable than O–^ __________________________________________________________
❑ I do not know __________________________________________________________
20. Tick ✔ one of the four statements: Why did you think this was the answer?
❑ O–^ is more stable than O2–^ __________________________________________________________
❑ O–^ and O2–^ are equally stable __________________________________________________________
❑ O–^ is less stable than O2–^ __________________________________________________________
❑ I do not know __________________________________________________________
21. Tick ✔ one of the four statements: Why did you think this was the answer?
❑ O2–^ is more stable than O __________________________________________________________
❑ O2–^ and O are equally stable __________________________________________________________
❑ O2–^ is less stable than O __________________________________________________________
❑ I do not know __________________________________________________________
Chemical stability (7) – page 1 of 1 H P
OT OCO PY
P
O
8+
O –
8+
O2–
Oxygen atom with electronic configuration of 2.
Oxygen ion with electronic configuration of 2.
Oxide ion with electronic configuration of 2.
8+
