Lesson 2 of 4
Orbitals and their energies
What s, p and d orbitals mean, how many there are, and their shapes.
A-Level 9476 (2026-2027)
I can describe orbital numbers and relative energies.Syllabus
Syllabus 9476, 1(f). Describe orbital numbers and relative energies.
I can recognise s, p and d orbital shapes.Syllabus
Syllabus 9476, 1(g). Recognise s, p and d orbital shapes.
A shell contains subshells; a subshell contains orbitals
An orbital holds at most two electrons. A p subshell has three orbitals, not one.
The principal quantum number n labels a shell. Within a shell, subshells are labelled s, p and d. An orbital describes a region where an electron is likely to be found; it is not a circular track. Each orbital can hold at most two electrons with opposite spins.
| Subshells required here | Orbitals in each subshell | Maximum electrons | Relative energies in a many-electron atom |
|---|---|---|---|
| 1s | 1 | 2 | Lowest occupied subshell |
| 2s, 2p | 1 and 3 | 2 and 6 | 2s below 2p |
| 3s, 3p, 3d | 1, 3 and 5 | 2, 6 and 10 | 3s below 3p below 3d within n = 3 |
| 4s, 4p | 1 and 3 | 2 and 6 | 4s below 4p; the position of 3d relative to 4s depends on occupancy |
Orbitals within the same subshell have equal energy in an isolated atom. In many-electron atoms, penetration and shielding split the energies of different subshells. This differs from hydrogen, where orbitals with the same n have the same energy in the simple atomic model.
- Fill the inner subshells
1s, then 2s, 2p, 3s and 3p.
- Start period 4
K and Ca add electrons to 4s.
- Build the transition series
3d occupancy increases, with the Cr and Cu exceptions. Once 3d is occupied, do not assume that 4s electrons are more tightly held.
- Continue to 4p
After the 3d subshell fills, the period continues into 4p.
Watch out for this
A student says: "The third shell holds 18 electrons, so potassium's 19th electron goes into 3d before 4s." What is wrong with this?
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The third shell can hold 18 electrons, but potassium is [Ar]4s1, not [Ne]3s2 3p6 3d1. Filling follows the energies of the atom, not a rule that every shell must fill completely first.
Check your understanding
How many orbitals does a 3d subshell contain, and how many electrons can it hold?
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5 orbitals, holding 10 electrons
A d subshell has five orbitals. Each orbital holds two electrons of opposite spin, so 10.
Read the shape and orientation of an orbital
s is spherical; p has two lobes; the five d orbitals have distinct orientations.
s and p orbitals
A spherical s orbital is represented by a circle centred on a nucleus. A p orbital has two lobes on opposite sides of its nucleus. The lobe boundaries indicate a probability region, not a solid surface or electron path.
| Orbital | Where its electron-density lobes lie |
|---|---|
| dxy | Four lobes between the x and y axes in the xy plane. |
| dxz | Four lobes between the x and z axes in the xz plane. |
| dyz | Four lobes between the y and z axes in the yz plane. |
| dx2-y2 | Four lobes pointing along the x and y axes. |
| dz2 | Two main lobes along z and a ring of electron density around the middle in the xy plane. |
Two important d-orbital patterns
The dxy sketch has four lobes between perpendicular axes. The dz2 sketch has two vertical lobes and an equatorial ring. These distinguish between-axis orientation from the special dz2 shape; the shapes are schematic.
A drawing of a two-lobed p orbital is one orbital, not two orbitals. Similarly, the four lobes of most d orbitals belong to one orbital. An orbital shape is a probability description; it is not the shape of an electron. Mathematical wave functions are outside this syllabus.
Check your understanding
Which d orbital has two main lobes along the z axis and a ring of electron density around the middle?
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dz2
dz2 has two lobes along z with a ring in the xy plane. The other four d orbitals each have four lobes.