Ch 3 · Electrons, energy levels and atomic orbitals · AS & A Level Chemistry 9701
Topic 3 of 4 in Atomic structure — AS & A Level Chemistry 9701 — 58 lessons.
Introduction
Electrons do not orbit the nucleus in fixed paths; they occupy regions of space, called orbitals, grouped into sub-shells and shells of defined energy. This lesson builds the vocabulary and rules needed to deduce and represent the electronic configuration of any atom or ion up to krypton.
Key ideas — Shells, sub-shells, orbitals and the ground state
A shell is a group of electrons with the same principal quantum number, n (n = 1, 2, 3, dots), which broadly determines their energy and distance from the nucleus; higher n means higher energy and greater average distance. Each shell is divided into sub-shells, labelled s, p, d (and f), which have slightly different energies within a shell. Each sub-shell is composed of one or more orbitals, a region of space around the nucleus in which there is a high probability of finding an electron, with each orbital holding a maximum of two electrons of opposite spin. The ground state of an atom is the electronic configuration of lowest total energy, in which electrons occupy the lowest-energy orbitals available.
🧑🏫 Teacher's explanation
Think of an atom like a massive multi-story hotel. The shells are the floors of the hotel. The sub-shells are the different types of suites on each floor (e.g., standard rooms, deluxe suites, penthouses). The orbitals are the individual beds within those suites. Each bed can hold exactly two people, but they must sleep head-to-toe (opposite spin) to avoid bumping into each other.
📖 Definitions
🔍 Worked example — Easy
Write the full electronic configuration for a magnesium atom (Z=12).
🔍 Worked example — Standard
Write the shorthand (noble gas) electronic configuration for a Calcium ion (Ca^2+).
🔍 Worked example — Exam-style
Calculate the total number of electrons in an atom with the configuration 1s^2 2s^2 2p^6 3s^2 3p^6 3d^10.
⚠️ Common errors
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🎯 Exam question
Key ideas — Orbitals per sub-shell and electron capacity
Each orbital holds at most two electrons, so an s sub-shell (1 orbital) holds up to 2 electrons, a p sub-shell (3 orbitals) holds up to 6, and a d sub-shell (5 orbitals) holds up to 10.
🧑🏫 Teacher's explanation
Think of an atom like a massive hotel. The 'shells' are the floors, the 'sub-shells' are the types of suites (like studio, deluxe, or penthouse), and the 'orbitals' are the individual rooms within those suites. Each room (orbital) is small and can only accommodate a maximum of two people (electrons).
📖 Definitions
🔍 Worked example — Easy
Write the full electronic configuration for a Magnesium atom (Z=12).
🔍 Worked example — Standard
Write the electronic configuration for the Chloride ion, Cl^- (Z=17).
🔍 Worked example — Exam-style
Write the shorthand (noble gas) electronic configuration for Iron (Z=26).
⚠️ Common errors
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🎯 Exam question
(a) State the number of unpaired electrons in this atom. [1] (b) Explain, in terms of electron configuration, why this atom is a free radical. [1] [2 marks]
Key ideas — Orbital shapes
An s orbital is spherically symmetrical about the nucleus: the electron density is the same in every direction at a given distance. A p orbital has a dumbbell (figure-of-eight) shape, consisting of two lobes on either side of the nucleus with a node at the nucleus; the three p orbitals in a sub-shell (p_x, p_y, p_z) are identical in shape and energy but point along three mutually perpendicular axes.
🧑🏫 Teacher's explanation
Imagine you are looking for a lost ball in a dark room. You don't know exactly where it is, but you know it is likely somewhere within a certain distance from the center. An orbital is like a 'probability zone'—it is not a physical path like a planet's orbit, but a region of space where you are most likely to find an electron.
📖 Definitions
🔍 Worked example — Easy
Describe the shape and orientation of the three orbitals in a 2p sub-shell.
🔍 Worked example — Standard
How many electrons can the 3p sub-shell hold in total, and how many orbitals make it up?
🔍 Worked example — Exam-style
An atom has the electronic configuration 1s^2 2s^2 2p^6 3s^2 3p^3. How many unpaired electrons does it have?
⚠️ Common errors
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🎯 Exam question
Key ideas — Order of filling and electronic configurations
Within the first three shells plus 4s and 4p, sub-shells fill in order of increasing energy: 1s < 2s < 2p < 3s < 3p < 4s < 3d < 4p. Note that 4s fills before 3d because it is lower in energy, despite belonging to a higher principal shell; once occupied, however, 3d electrons are removed before 4s electrons when a transition metal atom is ionised, since the relative energies change once the 3d sub-shell contains electrons.
🧑🏫 Teacher's explanation
Think of an atom like a multi-story hotel where guests (electrons) want to stay in the cheapest rooms (lowest energy) first. The ground floor is the 1s sub-shell, and as you go up, the rooms get more expensive. However, there is a weird rule in this hotel: sometimes a room on the 4th floor is actually cheaper than a room on the 3rd floor. This is why the 4s sub-shell fills before the 3d sub-shell.\n\nWhen guests leave the hotel (ionization), they don't always leave from the floor they arrived in. Once the 3d rooms are occupied, the 4s rooms actually become slightly more expensive than the 3d ones, so the 4s guests are the first to be kicked out. This is a common trap in exams!\n\nAlso, remember that electrons are antisocial. They want to be as far apart as possible to minimize 'inter-electron repulsion'. This is why, when filling sub-shells like 2p, electrons will occupy separate orbitals before they start pairing up in the same one.
📖 Definitions
🔍 Worked example — Easy
Write the full electronic configuration for a neutral Phosphorus atom (Z=15).
🔍 Worked example — Standard
Write the shorthand electronic configuration for a neutral Calcium atom (Z=20) using the noble gas notation.
🔍 Worked example — Exam-style
Write the full electronic configuration for the Cu^2+ ion (Z=29).
⚠️ Common errors
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🎯 Exam question
Key ideas — Electrons-in-boxes and Hund's rule; free radicals
In electrons-in-boxes notation, each orbital is drawn as a box and each electron as an arrow, with opposite spins shown as opposing arrows (uparrowdownarrow). Electrons fill orbitals to minimise energy: within a sub-shell of degenerate (equal-energy) orbitals, such as the three p orbitals, each orbital receives one electron, with parallel spins, before any orbital receives a second, because electrons in separate orbitals experience less inter-electron (electrostatic) repulsion than two electrons forced into the same orbital. Only once every orbital in the sub-shell holds one electron does pairing begin.
🧑🏫 Teacher's explanation
Think of electrons like students choosing seats in a lecture hall. If there is a row of single seats (degenerate orbitals) available, students will naturally spread out to have their own space to avoid bumping into each other. Only when every single seat in that row is taken will students start sitting next to each other in pairs. This 'social distancing' minimizes the electrostatic repulsion between the negative charges of the electrons.
📖 Definitions
🔍 Worked example — Easy
Draw the electrons-in-boxes notation for the ground state electronic configuration of Nitrogen (Z=7).
🔍 Worked example — Standard
Write the full electronic configuration for the Oxygen ion, O^2-.
🔍 Worked example — Exam-style
An element has the electronic configuration [Ne] 3s^2 3p^4. Is this species a free radical? Explain using electron pairing.
⚠️ Common errors
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🎯 Exam question
Worked example
Question. (a) Give the full electronic configuration of a P^3- ion (Z = 15 for phosphorus). [2] (b) Sketch, using electrons-in-boxes notation, the arrangement of the 2p electrons of an oxygen atom (Z = 8), and explain why they are arranged this way. [3]
🔍 Worked example — Easy
Write the full electronic configuration of a Mg^2+ ion (Z = 12 for Magnesium).
🔍 Worked example — Standard
Using electrons-in-boxes notation, represent the 3p sub-shell of a Phosphorus atom (Z = 15).
🔍 Worked example — Exam-style
Explain why the electronic configuration of Chromium (Z = 24) is 1s^2 2s^2 2p^6 3s^2 3p^6 4s^1 3d^5 instead of 1s^2 2s^2 2p^6 3s^2 3p^6 4s^2 3d^4.
⚠️ Common errors
Common mistakes
Practice
Answers