Shells & orbitals
Shells & orbitals
The first four shells
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A shell is a principal energy level. We number the shells n = 1, 2, 3 and 4, starting nearest the nucleus. Each shell contains subshells, which contain orbitals.
The expression 2n² gives the maximum capacity of a shell: 2, 8, 18 and 32 electrons for the first four shells. It does not tell you the order in which all the subshells fill.
That distinction matters when you reach 4s and 3d. Use shell capacities for maximum numbers, and the filling-order diagram for actual configurations.
n the principal energy level (electron shell).
Third-shell capacity
Calculate the maximum number of electrons that can occupy the third shell, using 2n².
Worked answer
n = 3, so 2 × 3² = 18.
🧪 TEST IT
Work on paper. Attempt each question before opening its answer. These are original CrackingChem questions, written in OCR A style.
- State the maximum number of electrons in each of the first four shells. [4]
- Use 2n² to calculate the fourth-shell capacity. [1]
Total: 5 marks
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- Shells 1–4: 2, 8, 18, 32 respectively. [4]
- 2 × 4² = 32 electrons. [1]
Atomic orbitals and shapes
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An orbital describes a region with a high probability of finding an electron. It is not a little circular track followed by the electron.
An s orbital is spherical. A p orbital has two lobes, often called a dumb-bell shape. The nucleus is at the centre.
A p subshell has three orbitals of the same energy. They point in three mutually perpendicular directions. Each orbital can hold up to two electrons, with opposite spins.
When showing where electrons go in a diagram, orbitals are usually drawn as boxes. One box represents one orbital; one arrow represents one electron.
- Atomic orbital
- A region around the nucleus that can hold up to two electrons with opposite spins.
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Work on paper. Attempt each question before opening its answer. These are original CrackingChem questions, written in OCR A style.
- Define atomic orbital. [2]
- Identify the type of orbital shown by A and by B–D. [2]
- Describe the shape and relative arrangement of the three p orbitals. [2]
Total: 6 marks
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- A region around the nucleus [1] that can hold up to two electrons with opposite spins [1].
- A is an s orbital; B–D are p orbitals. [2]
- Each p orbital is dumb-bell shaped [1]; the three have the same shape and energy and point in directions at right angles [1].
Subshells, orbitals and capacities
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Count the orbitals first, then double the number to find the maximum electrons. Each orbital holds up to two electrons.
- An s subshell has one orbital, so it holds up to 2 electrons.
- A p subshell has three orbitals, so it holds up to 6 electrons.
- A d subshell has five orbitals, so it holds up to 10 electrons.
An orbital and a subshell are different levels of organisation. Only the s subshell happens to contain a single orbital.
| Subshell | Orbitals | Electrons per orbital | Maximum electrons |
|---|---|---|---|
| s | 1 | 2 | 2 |
| p | 3 | 2 | 6 |
| d | 5 | 2 | 10 |
Why does d hold ten?
Explain the capacity of a d subshell.
Worked answer
- There are five d orbitals.
- Each holds at most two electrons with opposite spins.
- 5 × 2 = 10.
🧪 TEST IT
Work on paper. Attempt each question before opening its answer. These are original CrackingChem questions, written in OCR A style.
Complete the table. [6]
| Subshell | Number of orbitals | Maximum electrons |
|---|---|---|
| s | … | … |
| p | … | … |
| d | … | … |
Total: 6 marks
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| Subshell | Orbitals | Maximum electrons |
|---|---|---|
| s | 1 | 2 |
| p | 3 | 6 |
| d | 5 | 10 |
One mark for each correct entry: 6 marks.
Electrons-in-box diagrams
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In an electrons-in-boxes diagram, each box is one orbital and each arrow is one electron. The arrow direction represents spin.
- For equal-energy orbitals, place one electron in each box before adding a second electron to any box.
- Draw the singly occupied orbitals with parallel spins in these ground-state arrangements.
- When two electrons share a box, draw opposite arrows. One orbital holds no more than two electrons.
A p subshell therefore uses three boxes; a d subshell uses five. Check both the total electron count and the way they are distributed.
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Work on paper. Attempt each question before opening its answer. These are original CrackingChem questions, written in OCR A style.
Complete an electrons-in-box diagram for each arrangement. Draw three p-orbital boxes on paper and show spins clearly.
- Carbon: 2p². [2]
- Fluorine: 2p⁵. [2]
Total: 4 marks
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- Carbon: separate orbitals [1] with parallel spins [1].
Subshell Orbital 1 Orbital 2 Orbital 3 2p² ↑ ↑ Empty - Fluorine: two pairs and one single electron [1], with opposite spins within each pair [1].
Subshell Orbital 1 Orbital 2 Orbital 3 2p⁵ ↑↓ ↑↓ ↑
Nitrogen and oxygen have 2p³ and 2p⁴ arrangements.
- Draw their 2p electrons-in-box diagrams. [2]
- Use the diagrams to explain the singly-before-pairing rule. [2]
- State the spin requirement for two electrons in the same orbital. [1]
Total: 5 marks
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Subshell Orbital 1 Orbital 2 Orbital 3 N, 2p³ ↑ ↑ ↑ One mark for each diagram. [2]Subshell Orbital 1 Orbital 2 Orbital 3 O, 2p⁴ ↑↓ ↑ ↑ - Equal-energy orbitals are occupied singly before pairing [1]. Nitrogen has one electron in each; oxygen pairs only after all three are occupied [1].
- Opposite spins. [1]
Three proposed 2p⁴ arrangements are shown. Two arrows in the same cell occupy one orbital.
| Diagram | Orbital 1 | Orbital 2 | Orbital 3 |
|---|---|---|---|
| A | ↑↓ | ↑ | ↑ |
| B | ↑↓ | ↑↓ | Empty |
| C | ↑↑ | ↑ | ↑ |
- Identify the correct diagram. [1]
- Explain the error in each other diagram. [4]
Total: 5 marks
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- A. [1]
- B pairs electrons while one equal-energy orbital is empty [1], breaking singly-before-pairing [1]. C places same-spin electrons in one orbital [1]; a pair must have opposite spins [1].