Electron configurations: notes and practice questions
- This topic covers the higher level aspects of electron configurations, focusing on ionization energy and spectral data.
- The limit of convergence in an emission spectrum corresponds to the first ionization energy.
- Explain trends and discontinuities in first ionization energy (IE) across a period and down a group.
- Calculate the first ionization energy from spectral data that gives the wavelength or frequency of the convergence limit.
- Deduce the group of an element from its successive ionization energy data.
How it is examined
Writing a configuration is a reliable 1-mark part: May 2025 HL Paper 2 TZ1 2(a) was "Deduce the electron configuration of the Co²⁺ ion" for [1], accepting either `1s²2s²2p⁶3s²3p⁶3d⁷` or `[Ar]3d⁷`. The HL convergence-limit calculation is a 3-mark chain: energy per mole to energy per atom, then f = E/h, then λ = c/f, and the mark scheme awards [3] for a correct final answer. Ions matter: students lose the mark by removing 4s electrons in the wrong order for transition metal cations.
The electromagnetic spectrum. The Planck constant h, the speed of light c, and the equations E = hf and c = λf (HL). First ionization energy values. The periodic table, which gives the block structure. Aufbau, Hund and Pauli are recall, as are the Cr and Cu exceptions and the orbital shapes.
- 1.3.1 Emission spectra are produced by atoms emitting photons when electrons in excited states return to lower energy levels. Students qualitatively describe the relationship between colour, wavelength, frequency and energy across the electromagnetic spectrum, and distinguish a continuous from a line spectrum.
- 1.3.2 The line emission spectrum of hydrogen provides evidence for electrons in discrete energy levels that converge at higher energies. Students describe the hydrogen emission spectrum, including the relationships between the lines and energy transitions to the first, second and third energy levels.
- 1.3.3 The main energy level is given an integer number n and can hold a maximum of 2n² electrons.
- 1.3.4 A more detailed model divides the main energy level into s, p, d and f sublevels of successively higher energies. Students recognize the shape and orientation of an s atomic orbital and the three p atomic orbitals.
The names of the different series in the hydrogen emission spectrum will not be assessed. Do not ask for Lyman, Balmer or Paschen by name.
Guiding questions
- How can we model the energy states of electrons in atoms?
Linking questions
- Structure 3.1 How does an element's highest main energy level relate to its period number? What is the relationship between energy sublevels and the block nature of the periodic table?
- Nature of science, Structure 1.2 How do emission spectra provide evidence for the existence of different elements?
- Inquiry 2 In the study of emission spectra from gaseous elements and of light, what qualitative and quantitative data can be collected from instruments such as gas discharge tubes and prisms?
- Structure 3.1 (HL) How does the trend in IE values across a period and down a group explain the trends in properties of metals and non-metals?
- Structure 3.1 (HL) How do patterns of successive IEs of transition elements help to explain the variable oxidation states of these elements?
Practice questions
14 questions · 12 medium · 2 hardQuestion 1
MediumPaper 2 · calculator1 markWhich statement about the element manganese (Mn) is correct?
A. A manganese atom has the electron configuration .
B. The highest occupied principal energy level in a manganese atom is .
C. A manganese atom has 5 unpaired electrons.
D. The 3d sub-level in a manganese atom contains 3 fully occupied orbitals.
First, write down the full electron configuration for a neutral manganese atom (Z=25). Then, analyze each statement based on this configuration, considering the definitions of principal energy level, sub-level, orbital, and Hund's rule for electron pairing.
Question 2
HardPaper 2 · calculator24 marksAntimony (Sb) and Bismuth (Bi) are elements in group 15 of the periodic table.
(a) Antimony has two stable isotopes. 57.21% of antimony atoms contain 70 neutrons and the remainder contain 72 neutrons.
(i) Deduce the nuclear symbol of the isotope of antimony containing 72 neutrons. Use section 6 of the data booklet.
(ii) Calculate, to two decimal places, the relative atomic mass of antimony.
Bismuth(III) nitrate, , is a common salt of bismuth.
(b) (i) The compound contains both ionic and covalent bonds. State which particles are joined by covalent bonds and which are joined by ionic bonds.
(ii) Distinguish between covalent and ionic bonding in terms of electron distribution.
(iii) State the enthalpy term that characterizes the strength of the bonding between the ions in an ionic solid.
(iv) Write an equation for the formation of aqueous bismuth(III) nitrate from solid bismuth(III) oxide and nitric acid.
(v) Calculate the volume, in , of nitric acid required to react completely with of solid bismuth(III) oxide.
(vi) Predict, with a reason, whether bismuth(III) oxide is expected to be primarily acidic, basic or amphoteric.
(vii) Discuss how the relative reactivity of zinc and bismuth could be established using the metals and aqueous solutions of their nitrates.
(viii) Discuss the products formed at the electrodes during the electrolysis of aqueous bismuth(III) nitrate. Use the standard electrode potential and section 24 of the data booklet.
Bismuth compounds are sometimes used in fireworks to produce special effects.
(c) (i) State the feature of the atomic emission spectrum of an element that corresponds to its first ionization energy.
(ii) Calculate the wavelength, in nm, that corresponds to the first ionization energy of bismuth. Use sections 1, 2 and 8 of the data booklet.
(iii) Explain why the first ionization energy of bismuth is lower than that of polonium (Po), in terms of nuclear charge and electron shielding.
First, find the atomic number (number of protons) for Antimony from the periodic table. The mass number is the sum of protons and neutrons. The nuclear symbol is written with the mass number as a superscript and the atomic number as a subscript to the left of the element symbol.
The relative atomic mass is the weighted average of the masses of its isotopes. You'll need the mass number and abundance of both isotopes. The abundance of the second isotope is 100% minus the abundance of the first.
Consider the structure of the polyatomic nitrate ion and how it interacts with the bismuth cation. Covalent bonds typically form between non-metal atoms, while ionic bonds form between metal cations and non-metal anions.
Think about what happens to the valence electrons in each type of bond. Are they shared or transferred?
This term refers to the enthalpy change when one mole of a solid ionic compound is formed from its gaseous ions.
Bismuth(III) oxide is a basic oxide. It will react with an acid in a neutralization reaction to form a salt and water. Remember to balance the equation and include state symbols.
First, calculate the moles of bismuth(III) oxide using its mass and molar mass. Then, use the stoichiometry from your balanced equation in (b)(iv) to find the moles of nitric acid required. Finally, use the concentration of the nitric acid to find the volume.
Consider the position of bismuth in the periodic table and the trend in metallic character down group 15. How does the acid-base character of oxides change with metallic character?
A more reactive metal can displace a less reactive metal from a solution of its salt. Describe a simple experiment to test this.
At the cathode (negative electrode), reduction occurs. Compare the standard electrode potentials for the reduction of and water. At the anode (positive electrode), oxidation occurs. Compare the oxidation of water and the nitrate ion.
Ionization corresponds to the removal of an electron, which means the electron transitions to the n=∞ energy level. What happens to the spectral lines as they approach this limit?
First, find the first ionization energy of Bismuth from the data booklet (in kJ mol⁻¹). Convert this to energy per atom (in J) using Avogadro's constant. Then use the Planck-Einstein relation () and the wave equation () to find the wavelength.
Bismuth and Polonium are in the same period. Consider how the number of protons and the location of the valence electrons change as you move from Bi to Po.
Question 3
MediumPaper 2 · calculator1 markHow many occupied orbitals are there in a ground-state gallium atom?
A. 1
B. 3
C. 7
D. 9
First, write out the full electron configuration for a neutral gallium atom (Ga). Then, consider each p subshell (, etc.) and determine how many orbitals in each are occupied by at least one electron. Sum these up.
Question 4
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 5
MediumPaper 2 · calculator1 markWhich species would be expected to have an emission spectrum consisting of a series of convergent lines, similar to that of a hydrogen atom?
I.
II.
III.
A. I only
B. II only
C. I and II only
D. I and III only
Emission spectra like hydrogen's are characteristic of species that have only one electron. Determine the number of electrons in each of the given species.
Question 6
MediumPaper 2 · calculator1 markWhich of the following atoms in their ground state possess unpaired electrons?
I. Manganese (Z = 25)
II. Zinc (Z = 30)
III. Chromium (Z = 24)
A. II only
B. I and II only
C. I and III only
D. I, II and III
Write out the full or condensed electron configuration for each atom. Remember to apply Hund's rule and consider any exceptions to the Aufbau principle for d-block elements.
Question 7
MediumPaper 2 · calculator1 markWhich of the following represents a ground-state electron configuration that is not permitted?
A.
B.
C.
D.
Consider the Aufbau principle, which states that electrons fill the lowest available energy levels before filling higher levels. Also, recall any exceptions to this principle for transition metals. Which configuration violates the fundamental order of filling orbitals for a ground state?
Question 8
MediumPaper 2 · calculator1 markWhich species possesses the largest number of unpaired electrons?
A.
B.
C.
D.
First, write the full or condensed electron configuration for each neutral atom. For the ions, remember to remove electrons from the highest principal energy level first (i.e., the 4s orbital before the 3d orbital). Then, use orbital box diagrams for the d-subshell and apply Hund's rule to determine the number of unpaired electrons in each species.
Question 9
MediumPaper 2 · calculator1 markManganese is a transition metal that can form ions with different oxidation states. Which of the following is the correct electron configuration of the ion?
A.
B.
C.
D.
First, write the electron configuration for a neutral manganese atom. Then, remember the rule for removing electrons to form a positive ion from a transition metal: electrons in the highest principal energy level are removed first.
Question 10
MediumPaper 2 · calculator1 markParamagnetism is a property of materials that are attracted to an external magnetic field. This property is due to the presence of unpaired electrons. Which of the following species is the most paramagnetic?
A.
B.
C.
D.
To determine which species is most paramagnetic, you need to find the one with the most unpaired electrons. Write out the condensed electron configuration for each species. Remember to account for any charges on ions and be aware of exceptions to the Aufbau principle for transition metals.
Question 11
MediumPaper 1A · calculator1 markWhat is the electron configuration of the ion?
A.
B.
C.
D.
First, write the full electron configuration for a neutral copper atom, remembering any exceptions to the Aufbau principle for transition metals. Then, consider which electrons are removed first when forming a positive ion.
Question 12
MediumPaper 1A · calculator1 markWhat is the electron configuration of a chromium(III) ion, ?
A.
B.
C.
D.
First, write the electron configuration for a neutral chromium atom, remembering it is an exception to the Aufbau principle. Then, determine the configuration of the ion by removing electrons from the orbital with the highest principal quantum number first.
Question 13
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 14
MediumPaper 1A · calculator1 markThe table lists the first six successive ionization energies of an element X.
| Ionization number | 1st | 2nd | 3rd | 4th | 5th | 6th |
|---|---|---|---|---|---|---|
| Ionization energy / | 738 | 1451 | 7733 | 10540 | 13630 | 17995 |
Which is the formula of the stable nitride of the element X?
A.
B.
C.
D.
Identify the largest proportional increase between consecutive ionization energies. This indicates the removal of an electron from a new, inner electron shell. The number of electrons removed before this jump corresponds to the number of valence electrons. Use this to determine the charge of the stable ion formed by element X and combine it with the nitride ion ().
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