Topic S1.3 · SL and HL

Electron configurations: notes and practice questions

9 min read Jump to 20 practice questions

Summary

This topic models electron energy states in atoms, covering emission spectra and electron configurations.
Emission spectra show discrete lines as electrons return to lower energy levels, providing evidence for discrete energy levels.
The main energy level $n$ can hold a maximum of $2n^2$ electrons.
Main energy levels are divided into s, p, d, and f sublevels, each containing a fixed number of orbitals.
Each orbital holds two electrons with opposite spin, following the Aufbau principle, Hund's rule, and Pauli exclusion principle.
Deduce full and condensed electron configurations for atoms and ions up to $Z = 36$ , including exceptions like Cr and Cu.
Qualitatively describe the relationship between colour, wavelength, frequency, and energy across the electromagnetic spectrum.

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.

Given in the booklet

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.

Key ideas

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.

Not assessed

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.

At HL

1.3.6 In an emission spectrum, the limit of convergence at higher frequency corresponds to ionization. Students explain trends and discontinuities in first ionization energy (IE) across a period and down a group, and calculate the first IE from spectral data giving the wavelength or frequency of the convergence limit.
1.3.7 Successive ionization energy data for an element give information about its electron configuration. Students deduce the group of an element from its successive ionization data.

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

23 questions · 8 easy · 14 medium · 1 hard

Showing 20 of 20

Question 1

EasyPaper 1A · calculator1 mark

What is the total number of orbitals in the principal energy level where $n = 4$ ?

A. 4

B. 8

C. 16

D. 32

Question 2

MediumPaper 1A · calculator1 mark

Which pair of species has the same number of electrons in the outermost principal energy level?

The letters do not represent symbols of elements.

$_{20}^{40}P^{2+}$

$_{16}^{32}Q$

$_{17}^{35}R^{-}$

$_{7}^{14}S$

A. P and Q

B. Q and S

C. P and R

D. R and S

Question 3

HardPaper 2 · calculator23 marks

(a)

A sample of chlorine consists of two isotopes, $^{35}Cl$ and $^{37}Cl$ .

(a) Contrast the sub-atomic structure of these two isotopes.

[1]

(b)(i)

(b) (i) The sample of chlorine is analysed in a mass spectrometer, producing a spectrum for the $Cl_2^+$ 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 $^{35}Cl$ is approximately three times that of $^{37}Cl$ .

[3]

(b)(ii)

(ii) A more precise measurement finds the composition by mass to be: $^{35}Cl$ : 75.76%, $^{37}Cl$ : 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).

[2]

(c)(i)

Magnesium chloride, $MgCl_2$ , and manganese(II) chloride, $MnCl_2$ , are two ionic compounds.

(c) (i) Deduce the type of bonding in magnesium chloride, $MgCl_2$ , using electronegativity values from section 9 of the data booklet.

[2]

(c)(ii)

(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 = $-641 \text{ kJ mol}^{-1}$

[3]

(c)(iii)

(iii) Explain, with reference to electron configurations, why the ionic radii of $Mg^{2+}$ , $Mn^{2+}$ and $Cl^-$ are different. Use section 10 of the data booklet.

[3]

(c)(iv)

(iv) Predict, with a reason, which has the stronger ionic bonding, manganese(II) chloride, $MnCl_2$ , or magnesium chloride.

[2]

(d)(i)

Magnesium chloride is white, but manganese(II) chloride is pale pink.

(d) (i) State the condensed electron configuration of a manganese atom.

[1]

(d)(ii)

(ii) State the reason, in terms of electron configuration, why manganese(II) chloride is coloured.

[1]

(d)(iii)

(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.

[2]

(e)(i)

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).

[2]

(e)(ii)

(ii) Deduce a balanced chemical equation for the reaction of fluorine gas with the aqueous chloride ions in the electrolyte.

[1]

Question 4

EasyPaper 1A · calculator1 mark

What is the charge of the monatomic ion usually formed by the element with the following electron configuration?

$[Ar] 3d^{10} 4s^2 4p^1$

A. $1+$

B. $3-$

C. $3+$

D. $13+$

Question 5

MediumPaper 1A · calculator1 mark

Which option shows the correct formula and electron configuration of the species in aluminium sulfide?

A. Formula: $Al_2S_3$ ; Al species: $[Ne]$ ; S species: $[Ar]$

B. Formula: $Al_2S_3$ ; Al species: [Ne] $3s^2 3p^1$ ; S species: [Ne] $3s^2 3p^4$

C. Formula: $Al_3S_2$ ; Al species: $[Ne]$ ; S species: $[Ar]$

D. Formula: $Al_3S_2$ ; Al species: [Ne] $3s^2 3p^1$ ; S species: [Ne] $3s^2 3p^4$

Question 6

EasyPaper 2 · calculator1 mark

Which of the following ions has the greatest number of valence electrons?

A. $PO_4^{3-}$

B. $SiO_3^{2-}$

C. $ClO_3^{-}$

D. $NH_4^{+}$

Question 7

MediumPaper 2 · calculator4 marks

(a)

The successive ionization energies for an unknown element, Z, are shown in the table below.

Ionization	Ionization Energy / kJ mol⁻¹
1st	1012
2nd	1907
3rd	2914
4th	4964
5th	6274
6th	21269

(a) Deduce the group number of element Z in the periodic table.

[1]

(b)

(b) Explain your reasoning for the answer in part (a).

[2]

(c)

[1]

Question 8

EasyPaper 1A · calculator1 mark

What is the total number of orbitals in the third main energy level ( $n=3$ )?

A. $5$

B. $9$

C. $10$

D. $18$

Question 9

MediumPaper 2 · calculator5 marks

(a)

The first six successive ionization energies for an element, Q, are given in the table below.

Ionization Number	Ionization Energy / kJ mol⁻¹
1st	787
2nd	1577
3rd	3232
4th	4356
5th	16091
6th	19805

(a) Deduce the group number of element Q in the periodic table.

[1]

(b)

(b) Explain your answer to part (a).

[2]

(c)

[1]

(d)

(d) Write the equation, including state symbols, that represents the third ionization energy of element Q.

[1]

Question 10

EasyPaper 1A · calculator1 mark

What is the maximum number of electrons that can be accommodated in the fourth principal energy level ( $n=4$ )?

A. 8

B. 16

C. 18

D. 32

Question 11

MediumPaper 2 · calculator1 mark

The line emission spectrum of hydrogen provides evidence for the existence of discrete energy levels in the atom. Which statement is correct?

A. A line corresponding to a wavelength of 103 nm has a lower energy than a line with a wavelength of 122 nm.

B. The spectral lines converge at longer wavelengths.

C. Electron transitions to the $n = 3$ level are responsible for lines in the visible region.

D. The series of lines produced by transitions to the $n = 1$ level are in the ultraviolet region.

Question 12

EasyPaper 1A · calculator1 mark

The line emission spectrum of hydrogen consists of a series of discrete lines. Which statement provides the reason for this observation?

A. Electrons in the hydrogen atom can occupy any energy state.

B. Electrons transition between discrete, quantized energy levels.

C. The nucleus of a hydrogen atom contains no neutrons.

D. The energy of emitted photons is continuous.

Question 13

MediumPaper 2 · calculator1 mark

Which of the following atoms has the greatest third ionisation energy?

A. Magnesium

B. Aluminium

C. Silicon

D. Phosphorus

Question 14

EasyPaper 1A · calculator1 mark

What is the charge of the monatomic ion usually formed by the element with the following electron configuration?

$[Ne] 3s^23p^4$

A. 2+

B. 2-

C. 4-

D. 6+

Question 15

MediumPaper 2 · calculator1 mark

Which electronic transition in the hydrogen atom results in the emission of electromagnetic radiation with the shortest wavelength?

A. $n=5 \rightarrow n=2$

B. $n=2 \rightarrow n=1$

C. $n=6 \rightarrow n=3$

D. $n=4 \rightarrow n=2$

Question 16

EasyPaper 1A · calculator1 mark

What is the total number of orbitals in a d sub-level?

A. 3

B. 5

C. 7

D. 10

Question 17

MediumPaper 2 · calculator3 marks

(a)

Magnesium and calcium are two alkaline earth metals in the periodic table.

(a) State the full electron configuration of a magnesium atom.

[1]

(b)

(b) Explain why the first ionization energy of calcium is lower than that of magnesium.

[2]

Question 18

MediumPaper 2 · calculator6 marks

(a)

The first four elements of Period 3 are sodium (Na), magnesium (Mg), aluminium (Al), and silicon (Si).

(a) State the full electron configuration of a magnesium atom and an aluminium atom.

[2]

(b)

(b) Explain why the first ionization energy of aluminium is lower than that of magnesium, which is an exception to the general trend across a period.

[2]

(c)

[2]

Question 19

MediumPaper 1A · calculator1 mark

Which of the following species have the same number of outer electrons? The letters do not represent symbols of elements.

${^{9}_{4}M}$

${^{11}_{5}N^{+}}$

${^{16}_{8}P^{2-}}$

${^{12}_{6}Q}$

A. M and P

B. N and Q

C. M and N

D. P and Q

Question 20

MediumPaper 1A · calculator1 mark

Which row shows the correct formula and electron configuration of the species in potassium sulfide?

	Formula	Electron configuration of the K species	Electron configuration of the S species
A.	$KS_2$	$[Ar]$	$[Ar]$
B.	$K_2S$	$[Ar]$	$[Ar]$
C.	$K_2S$	$[Ar] 4s^1$	$[Ne] 3s^2 3p^4$
D.	$KS_2$	$[Ar] 4s^1$	$[Ne] 3s^2 3p^4$

3 more Electron configurations questions in the app

Every answer is marked mark by mark, IB-style, and the AI tutor helps when you are stuck.

Where marks are lost

Reaching for "human error" or "only one trial." A source of error has to be a specific step in the method, not a general apology for the result.
Joining the dots instead of drawing a curve.
Naming a chemical instead of the property that distinguishes it, or vice versa. Answering with the nearest fact that comes to mind rather than the fact the command term and stem jointly ask for is a recurring way to answer a question that was not, quite, the one asked.

FAQ

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What does Electron configurations cover in IB Chemistry?

This topic models electron energy states in atoms, covering emission spectra and electron configurations. Emission spectra show discrete lines as electrons return to lower energy levels, providing evidence for discrete energy levels. The main energy level n can hold a maximum of 2n^2 electrons.

Is Electron configurations SL or HL?

Both. SL and HL students study Electron configurations, and HL goes further: 1.3.6 In an emission spectrum, the limit of convergence at higher frequency corresponds to ionization. Students explain trends and discontinuities in first ionization energy (IE) across a period and down a group, and calculate the first IE from spectral data giving the wavelength or frequency of the convergence limit.

How do I revise Electron configurations for IB Chemistry?

Start from the core idea: this topic models electron energy states in atoms, covering emission spectra and electron configurations. In the exam: 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. Then practise exam-style questions, easiest first, writing out every step of your working before you check it.

How does FourtyFive help me practise Electron configurations?

FourtyFive has 23 Electron configurations questions. Every answer you write is marked mark by mark, IB-style, and you see where each mark was won or lost. Every part has a hint, the AI tutor helps you through the step you are stuck on, and your Study Profile picks what to practise next.

Is FourtyFive free for Electron configurations practice?

Yes. A free account gives you 50 marked answers a month, and you do not need a card to sign up.

Can I handwrite Electron configurations answers on an iPad?

Yes. In the FourtyFive iPad app you write your working by hand with Apple Pencil, the way you would on paper, and it is marked the same way.

Electron configurations: notes and practice questions

How it is examined

Guiding questions

Linking questions

Practice questions

Question 1

Question 2

Question 3

Question 4

Question 5

Question 6

Question 7

Question 8

Question 9

Question 10

Question 11

Question 12

Question 13

Question 14

Question 15

Question 16

Question 17

Question 18

Question 19

Question 20

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Where marks are lost

Practise this topic as a session

Questions,
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How it is examined

Guiding questions

Linking questions

Practice questions

Question 1

Question 2

Question 3

Question 4

Question 5

Question 6

Question 7

Question 8

Question 9

Question 10

Question 11

Question 12

Question 13

Question 14

Question 15

Question 16

Question 17

Question 18

Question 19

Question 20

3 more Electron configurations questions in the app

Where marks are lost

Questions,answered.

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