Learning goals Subatomic Particles, Isotopes, Ions Atomic Orbitals: Energies and Shapes Orbitals and Electron Configuration First Ionisation Energy Trends Successive Ionisation Energies Atomic Structure learning outcomes Atomic structure links particle evidence to electron arrangements and ionisation-energy data. Learn it in this order: particles → orbitals → configurations → ionisation energies.
Before you start Beginner path: follow the Lessons (Recommended Order) from top to bottom.
Revision: Jump to: Quick Reference · What You Must Memorise · Common Exam Traps · Hub Quiz .
Prerequisites: skim the Prerequisites list first if you’re rusty.
Useful links: A Level portal · Exam Skills
What You’ll Learn
Compare the relative masses and charges of protons, neutrons and electrons, and predict how their beams behave in an electric field.
Deduce proton, neutron and electron counts for atoms and ions, and describe how mass and charge are distributed in an atom.
Distinguish isotopes using proton and neutron numbers.
Describe the number, relative energies and shapes of s, p and d orbitals, including the distinct d_z²
Write electron configurations of atoms and ions from proton number and charge.
Explain the factors that affect ionisation energy.
Use successive ionisation-energy data to deduce electron configurations and position in the Periodic Table.
Lessons (Recommended Order)
Subatomic Particles, Isotopes, Ions Particle properties, electric-field deflection, nuclide symbols, isotopes, ions, and atomic mass/charge distribution.
Atomic Orbitals: Energies and Shapes s/p/d orbital counts, energies and complete syllabus shapes, plus the 4s vs 3d distinction.
Orbitals and Electron Configuration Orbital filling and exam-safe configurations for atoms and ions, including Cr, Cu and transition-metal cations.
First Ionisation Energy Trends Drivers, anomalies, and mark-scheme wording for trend questions.
Successive Ionisation Energies Interpret jumps to deduce valence electrons and group.
Quick Reference
What You Must Memorise
Proton number, Z : number of protons.Nucleon number, A : protons + neutrons.Isotope : same Z A First ionisation energy : energy to remove 1 mole of electrons from 1 mole of gaseous atoms to form 1 mole of gaseous 1+ ions.Successive ionisation energies : energies to remove 1 mole of electrons one-at-a-time from gaseous atoms/ions.“Big jump” in successive IE : start removing an inner-shell electron (after valence electrons are removed). Common Exam Traps
First ionisation energy definition missing per mole , gaseous , or 1+ ions .
“Explain the trend” answers that don’t name drivers (nuclear charge, shielding, distance) and link to attraction.
Mixing up the Period 3 anomalies: Mg → Al (3s vs 3p) vs P → S (pairing/repulsion).
For ions, changing protons/neutrons (ions form by electron gain/loss).
Successive IE: using the jump as “number of shells” instead of valence vs inner-shell removal.
Electron configuration: removing electrons from the wrong subshell (4s before 3d for transition ions).
Electron configuration: writing a neutral atom configuration when the question is for an ion (make the ion first, then write its configuration).
Hub Quiz
Use practice in two passes: first without notes, then return to the lesson covering the idea you found difficult.
Atomic Structure Hub Quiz Check particle definitions, orbital requirements, electron configurations and ionisation-energy reasoning.
Atomic Structure Knowledge Check Find the idea that needs more work, use the feedback, then try a fresh question independently.
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Which statement correctly describes how mass and charge are distributed in an atom?
Nearly all the mass and all the positive charge are in a very small nucleus of protons (relative mass 1) and neutrons (relative mass 1); electrons (relative mass 1/1836) occupy the much larger surrounding space. Mass and positive charge are spread evenly through the atom, with electrons embedded in it. Electrons provide about half of the mass because there are as many electrons as protons. The nucleus occupies most of the volume of the atom and contains the electrons. A proton beam and an electron beam enter the same electric field at the same speed. How do their small deflections compare?
They are deflected in opposite directions, and the electron's deflection is about 1840 times larger. They are deflected in opposite directions through equal angles, because their charges are equal in size. The proton's deflection is about 1840 times larger, because it has the larger mass. They are deflected in the same direction, and the electron's deflection is larger. Which statement about ²⁴Mg²⁺ and ²⁶Mg (magnesium, proton number 12) is correct?
They are isotopes: ²⁴Mg²⁺ has 12 neutrons and 10 electrons, while ²⁶Mg has 14 neutrons and 12 electrons. ²⁴Mg²⁺ has 14 electrons, because the charge is added to the proton number. They are different elements, because their nucleon numbers are different. ²⁶Mg has 26 protons, because the nucleon number equals the proton number. Which statement about the 3p, 4s, 3d and 4p subshells in the conventional filling order is correct?
They fill in the order 3p, 4s, 3d, 4p, and every p orbital is dumbbell-shaped along one axis. They fill in the order 3p, 3d, 4s, 4p, because all of shell 3 fills before shell 4. They fill in the order 4s, 3p, 3d, 4p, and p orbitals are spherical. They fill in the order 3d, 4s, 3p, 4p, because d orbitals are lowest in energy. What is the ground-state electron configuration of Fe³⁺ (iron, proton number 26)?
[Ar] 3d⁵ [Ar] 3d³ 4s² [Ar] 3d⁶ [Ar] 3d⁴ 4s¹ Why is the first ionisation energy of oxygen lower than that of nitrogen?
In oxygen one 2p orbital holds a pair of electrons, and repulsion between them makes one easier to remove. Oxygen has a smaller nuclear charge than nitrogen. The electron removed from oxygen is in a higher-energy subshell than nitrogen's. The electron removed from nitrogen is paired, so it is harder to remove. The first four successive ionisation energies of a Period 3 element are 578, 1817, 2745 and 11577 kJ mol⁻¹. Which conclusion is correct?
The element is in Group 13 and its outer electron configuration is 3s² 3p¹. The element is in Group 2, because the first value roughly triples at the second ionisation. The element is in Group 14, because four values are listed. The element is in Group 13 and its outer electron configuration is 3s³.
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This check needs JavaScript to record answers. The reviewed questions are listed below for study.
Which statement correctly describes how mass and charge are distributed in an atom?
Nearly all the mass and all the positive charge are in a very small nucleus of protons (relative mass 1) and neutrons (relative mass 1); electrons (relative mass 1/1836) occupy the much larger surrounding space. Mass and positive charge are spread evenly through the atom, with electrons embedded in it. Electrons provide about half of the mass because there are as many electrons as protons. The nucleus occupies most of the volume of the atom and contains the electrons. A proton beam and an electron beam enter the same electric field at the same speed. How do their small deflections compare?
They are deflected in opposite directions, and the electron's deflection is about 1840 times larger. They are deflected in opposite directions through equal angles, because their charges are equal in size. The proton's deflection is about 1840 times larger, because it has the larger mass. They are deflected in the same direction, and the electron's deflection is larger. Which statement about ²⁴Mg²⁺ and ²⁶Mg (magnesium, proton number 12) is correct?
They are isotopes: ²⁴Mg²⁺ has 12 neutrons and 10 electrons, while ²⁶Mg has 14 neutrons and 12 electrons. ²⁴Mg²⁺ has 14 electrons, because the charge is added to the proton number. They are different elements, because their nucleon numbers are different. ²⁶Mg has 26 protons, because the nucleon number equals the proton number. Which statement about the 3p, 4s, 3d and 4p subshells in the conventional filling order is correct?
They fill in the order 3p, 4s, 3d, 4p, and every p orbital is dumbbell-shaped along one axis. They fill in the order 3p, 3d, 4s, 4p, because all of shell 3 fills before shell 4. They fill in the order 4s, 3p, 3d, 4p, and p orbitals are spherical. They fill in the order 3d, 4s, 3p, 4p, because d orbitals are lowest in energy. What is the ground-state electron configuration of Fe³⁺ (iron, proton number 26)?
[Ar] 3d⁵ [Ar] 3d³ 4s² [Ar] 3d⁶ [Ar] 3d⁴ 4s¹ Why is the first ionisation energy of oxygen lower than that of nitrogen?
In oxygen one 2p orbital holds a pair of electrons, and repulsion between them makes one easier to remove. Oxygen has a smaller nuclear charge than nitrogen. The electron removed from oxygen is in a higher-energy subshell than nitrogen's. The electron removed from nitrogen is paired, so it is harder to remove. The first four successive ionisation energies of a Period 3 element are 578, 1817, 2745 and 11577 kJ mol⁻¹. Which conclusion is correct?
The element is in Group 13 and its outer electron configuration is 3s² 3p¹. The element is in Group 2, because the first value roughly triples at the second ionisation. The element is in Group 14, because four values are listed. The element is in Group 13 and its outer electron configuration is 3s³.
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Which description of the neutron gives its relative charge, relative mass and location in the atom correctly?
Relative charge 0; relative mass 1; in the nucleus Relative charge 0; relative mass 1/1836; outside the nucleus Relative charge +1; relative mass 1; in the nucleus Relative charge 0; relative mass 0; in the nucleus Separate beams of protons, neutrons and electrons, all at the same speed, pass between two oppositely charged parallel plates. Which description of their paths is correct?
Electrons curve strongly towards the positive plate, protons curve slightly towards the negative plate and neutrons go straight on. Electrons and protons are deflected through equal angles in opposite directions and neutrons go straight on. Electrons curve towards the negative plate, protons towards the positive plate and neutrons go straight on. Protons curve strongly towards the negative plate, electrons slightly towards the positive plate and neutrons curve slightly towards the negative plate. Which statement about the species ³²S²⁻ and ³⁴S (sulfur, proton number 16) is correct?
They are isotopes: ³²S²⁻ has 16 neutrons and 18 electrons, while ³⁴S has 18 neutrons and 16 electrons. They are not isotopes, because they contain different numbers of electrons. ³²S²⁻ has 14 electrons, because a 2− charge means two electrons are removed. ³⁴S has 34 neutrons, because the nucleon number counts the neutrons. Which statement about the orbitals with principal quantum number 3 is correct?
There is one 3s, three 3p and five 3d orbitals; the 3s orbital is spherical and each 3p orbital is dumbbell-shaped. The 3s, 3p and 3d orbitals all have the same energy in a many-electron atom. There are three orbitals in the third shell, one for each subshell. The 3s orbital is dumbbell-shaped and the 3p orbitals are spherical. What is the ground-state electron configuration of a chromium atom (proton number 24)?
[Ar] 3d⁵ 4s¹ [Ar] 3d⁴ 4s² [Ar] 3d⁶ [Ar] 3d³ 4s² 4p¹ Why is the first ionisation energy of aluminium lower than that of magnesium?
The electron removed from aluminium is in a 3p orbital, which is higher in energy and partly shielded by the 3s electrons. Aluminium has a smaller nuclear charge than magnesium. The electron removed from aluminium is paired, so repulsion makes it easier to remove. The aluminium atom has one more electron shell than magnesium. The first four successive ionisation energies of an element are 738, 1451, 7733 and 10543 kJ mol⁻¹. Which conclusion is correct?
The element is in Group 2 and its outer electron configuration is ns². The element is in Group 4, because four ionisation energies are given. The element is in Group 2 and its outer electron configuration is np². The element is in Group 3, because the jump occurs at the third ionisation energy.
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