The periodic table: Classification of elements: notes and practice questions
- This topic covers advanced aspects of periodic trends and transition element chemistry.
- Discontinuities in first ionization energy across a period provide evidence for energy sublevels, explained by the energy of the electron removed.
- Transition elements exhibit variable oxidation states, high melting points, magnetic and catalytic properties, and form coloured compounds and complex ions due to incomplete d-sublevels.
- Variable oxidation states are explained by successive ionization energies being close in value.
- Deduce electron configurations of first-row transition element ions.
- Coloured transition element complexes result from electron promotion between split d-sublevels.
- Apply the colour wheel and to relate absorbed and observed light for complexes.
How it is examined
Trend explanations are the workhorse, usually 2 to 3 marks, and they want the causal chain: nuclear charge, shielding, atomic radius, then the property. A trend stated without a cause scores one of two. Oxidation state deduction is a 1-mark part and appears in both papers. May 2025 HL Paper 2 TZ1 asked candidates to deduce the oxidation state of S in `[Fe(S₂O₃)₂(H₂O)₂]⁻` for [1], and to explain why a transition element complex is coloured for [3], which needs d-sublevel splitting, absorption of a specific wavelength, and the complementary colour being seen.
The periodic table with atomic numbers and relative atomic masses. Ionic and atomic radii, first ionization energies, electron affinities, electronegativities, and melting points. HL: the colour wheel and c = λf. What is recall: group and period names, the direction and reason for every trend, the oxidation state rules.
- 3.1.1 The periodic table consists of periods, groups and blocks. Students identify the positions of metals, metalloids and non-metals.
- 3.1.2 The period number shows the outer energy level that is occupied by electrons. Elements in a group have a common number of valence electrons. Students deduce the electron configuration of an atom up to Z = 36 from the element's position, and vice versa.
- 3.1.3 Periodicity refers to trends in properties of elements across a period and down a group. Students explain the periodicity of atomic radius, ionic radius, ionization energy, electron affinity and electronegativity.
- 3.1.4 Trends down a group include the increasing metallic character of group 1 elements and the decreasing non-metallic character of group 17 elements. Students describe and explain the reactions of group 1 metals with water, and of group 17 elements with halide ions.
- HL: knowledge of different types of magnetism will not be assessed.
- HL: students are not expected to know the different d-orbital splitting patterns and their relation to the coordination number.
Guiding questions
- How does the periodic table help us to predict patterns and trends in the properties of elements?
Linking questions
- Nature of science, Structure 1.2 How has the organization of elements in the periodic table facilitated the discovery of new elements?
- Structure 2.1, 2.2 How do differences in bonding explain the differences in the properties of metal and non-metal oxides?
- Reactivity 3.2 How can oxidation states be used to analyse redox reactions?
- Inquiry 2, Tool 2 Why are simulations often used in exploring the trends in chemical reactivity of group 1 and group 17 elements?
- Nature of science, Structure 2.3 (HL) What are the arguments for and against including scandium as a transition element?
- Reactivity 3.4 (HL) What is the nature of the reaction between transition element ions and ligands in forming complex ions?
- Tool 1, Inquiry 2 (HL) How can colorimetry or spectrophotometry be used to calculate the concentration of a solution of coloured ions?
Practice questions
13 questions · 3 easy · 9 medium · 1 hardQuestion 1
EasyPaper 2 · calculator1 markWhich of the following species can act as ligands in the formation of a complex ion?
I.
II.
III.
A. I and II only
B. I and III only
C. II and III only
D. I, II and III
A ligand must be able to donate a lone pair of electrons to a central metal ion. Draw the Lewis structures for each species to identify which ones have available lone pairs.
Question 2
MediumPaper 2 · calculator1 markAqueous solutions containing the hexaaquacopper(II) ion, , are blue, whereas aqueous solutions containing the hexaaquazinc(II) ion, , are colourless. What is the reason for this difference?
A. The energy gap between the split d-orbitals is larger for than for .
B. The ion has a partially filled d-subshell, while the ion has a completely filled d-subshell.
C. Water acts as a ligand for but not for .
D. The complex ion absorbs blue light from the visible spectrum.
Consider the electron configurations of the central metal ions in each complex. What is required for an electron to be promoted between split d-orbitals?
Question 3
HardPaper 2 · calculator23 marksA sample of chlorine consists of two isotopes, and .
(a) Contrast the sub-atomic structure of these two isotopes.
(b) (i) The sample of chlorine is analysed in a mass spectrometer, producing a spectrum for the ion. The spectrum shows three peaks at m/z values of 70, 72 and 74. Explain the origin and relative heights of these three peaks, given that the abundance of is approximately three times that of .
(ii) A more precise measurement finds the composition by mass to be: : 75.76%, : 24.24%. Calculate the relative atomic mass of chlorine from this sample, giving your answer to two decimal places. (Use isotopic masses of 35.0 and 37.0 for this calculation).
Magnesium chloride, , and manganese(II) chloride, , are two ionic compounds.
(c) (i) Deduce the type of bonding in magnesium chloride, , using electronegativity values from section 9 of the data booklet.
(ii) Determine the lattice enthalpy of magnesium chloride, assuming the bonding is purely ionic. Use sections 9, 10 and 12 of the data booklet and the following data:
Enthalpy of formation of magnesium chloride =
(iii) Explain, with reference to electron configurations, why the ionic radii of , and are different. Use section 10 of the data booklet.
(iv) Predict, with a reason, which has the stronger ionic bonding, manganese(II) chloride, , or magnesium chloride.
Magnesium chloride is white, but manganese(II) chloride is pale pink.
(d) (i) State the condensed electron configuration of a manganese atom.
(ii) State the reason, in terms of electron configuration, why manganese(II) chloride is coloured.
(iii) Manganese(II) chloride absorbs light with a wavelength of approximately 530 nm. Describe why this is consistent with the observed colour of the compound. Use sections 2 and 15 of the data booklet.
A copper key is to be electroplated with manganese using an aqueous solution of manganese(II) chloride as the electrolyte.
(e) (i) Deduce the half-equations for the reactions occurring at the anode (made of pure manganese) and the cathode (the copper key).
(ii) Deduce a balanced chemical equation for the reaction of fluorine gas with the aqueous chloride ions in the electrolyte.
Isotopes of an element have the same number of protons but a different number of another sub-atomic particle. What is this particle and how does its number differ between and ?
The peaks correspond to different combinations of the two chlorine isotopes in a diatomic molecule. The height of each peak is related to the probability of that specific combination occurring. Consider the relative abundances of the isotopes.
The relative atomic mass is the weighted average of the isotopic masses. Multiply each isotopic mass by its fractional abundance and sum the results.
Find the electronegativity values for magnesium and chlorine. The difference in their electronegativity values will indicate the type of bonding.
Construct a Born-Haber cycle for the formation of from and . Use Hess's Law to find the unknown lattice enthalpy. Remember to account for the stoichiometry, especially for chlorine.
Compare the number of electron shells and the nuclear charge (number of protons) for each ion.
The strength of ionic bonding depends on the charge of the ions and the distance between them (ionic radii). Compare these factors for and .
Manganese is in the first row of the d-block. Remember the filling order of the 4s and 3d sub-levels.
The colour of transition metal compounds is related to the electronic structure of the transition metal ion. What is special about the d-sublevel in coloured ions?
The colour we see is the complementary colour to the one that is absorbed. Use the colour wheel in the data booklet to find the complementary colour of the absorbed light.
In electroplating, the object to be plated is the cathode, and the metal used for plating is the anode. Oxidation occurs at the anode and reduction occurs at the cathode.
Consider the relative oxidizing strengths of the halogens. A more reactive halogen will displace a less reactive halide from its salt solution.
Question 4
EasyPaper 1A · calculator1 markWhich are characteristic properties of transition metals?
I. High catalytic activity
II. Formation of complex ions
III. Low density
A. I and II only
B. I and III only
C. II and III only
D. I, II and III
Consider the defining chemical and physical properties of the d-block elements. Think about their electron configurations, bonding, and common industrial uses. Which of the listed properties are generally true for metals like iron, copper, and vanadium, and which are not?
Question 5
MediumPaper 2 · calculator1 markWhich of the following compounds forms a colourless aqueous solution?
A.
B.
C.
D.
Consider the electron configuration of the central metal ion in each compound. Colour in transition metal complexes is usually due to electrons transitioning between split d-orbitals. This is only possible if the d-subshell is partially filled.
Question 6
EasyPaper 1A · calculator1 markWhich first-row d-block element does not exhibit an oxidation state of +3 in its compounds?
A. Cr
B. Fe
C. Co
D. Zn
Consider the common oxidation states for the first-row transition metals. Think about which element has a very stable electron configuration that makes it reluctant to form a +3 ion.
Question 7
MediumPaper 2 · calculator1 markWhich statement about manganese and its compounds is incorrect?
A. The maximum oxidation state of manganese is +7.
B. The electron configuration of the ion is .
C. An aqueous solution of potassium permanganate, , is colourless.
D. Manganese(IV) oxide, , can act as a catalyst.
Review the characteristic properties of transition metals, including variable oxidation states, electron configurations of their ions, the colour of their complexes, and their catalytic activity. Consider the specific properties of manganese.
Question 8
MediumPaper 1A · calculator1 markWhat is the primary reason for the variable oxidation states observed in transition elements?
A. The d-orbitals are degenerate.
B. The successive ionization energies involving the and electrons are similar.
C. They form coloured compounds.
D. The sublevel is always filled before the sublevel.
Think about which electrons are lost when transition metals form ions. How does the energy required to remove successive electrons compare for the outermost sub-levels?
Question 9
MediumPaper 1A · calculator1 markSolutions containing the hexaaquacobalt(II) ion, , appear pink. Which statement correctly explains this observation?
A. Pink light is absorbed as electrons are promoted from a lower to a higher energy d-orbital.
B. Pink light is released as electrons fall from a higher to a lower energy d-orbital.
C. Green light is released as electrons fall from a higher to a lower energy d-orbital.
D. Green light is absorbed as electrons are promoted from a lower to a higher energy d-orbital.
The colour we see is the light that is transmitted or reflected, not absorbed. The absorbed light has the energy needed to excite an electron. What is the relationship between the absorbed colour and the observed colour?
Question 10
MediumPaper 1A · calculator1 markWhich ion would be expected to form a colourless aqueous solution?
A.
B.
C.
D.
The colour of aqueous transition metal ions is due to d-orbital splitting and electron transitions. This phenomenon requires a partially filled d-subshell. Determine the electron configuration for each ion.
Question 11
MediumPaper 1A · calculator1 markWhich statements are correct for the complex ion ?
I. Cyanide ions are behaving as Lewis bases.
II. The oxidation state of iron is .
III. The coordination number of the iron ion is 6.
A. I and II only
B. I and III only
C. II and III only
D. I, II and III
Consider the definitions of a Lewis base, how to calculate the oxidation state of the central metal ion in a complex, and the definition of coordination number. The charge on a cyanide ion () is .
Question 12
MediumPaper 1A · calculator1 markThe hexaaquairon(III) ion, , appears yellow in aqueous solution. Which statements correctly explain this observation?
I. The complex ion absorbs violet light.
II. The energy difference between the split d-orbitals corresponds to the energy of yellow light.
III. An electron is excited from a lower energy d-orbital to a higher energy d-orbital.
A. I and II only
B. I and III only
C. II and III only
D. I, II and III
Consider the relationship between the colour of a substance and the colours of light it absorbs. Also, recall the electronic transitions that are responsible for colour in transition metal complexes.
Question 13
MediumPaper 1A · calculator1 markAqueous solutions containing the hexaaquairon(III) ion, , are yellow. What is the reason for this colour?
A. Violet light is absorbed when electrons are promoted between the orbitals in the split d-sublevels.
B. Yellow light is emitted when electrons fall between the orbitals in the split d-sublevels.
C. Violet light is absorbed when electrons fall between the orbitals in the split d-sublevels.
D. Yellow light is absorbed when electrons are promoted between the orbitals in the split d-sublevels.
Consider the relationship between the colour of light absorbed and the colour observed. Also, think about what happens to electrons when a substance absorbs energy from light.
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