To describe the distribution of electrons around the atomic nucleus, physicists use a simple historical notation: the electron shells K, L, M, N, O, P, and Q. This notation was introduced in the early 20th century by physicist Charles Barkla (1877-1944) during the study of X-rays. This notation allows for a quick visualization of how electrons are distributed by increasing energy levels, from the shell closest to the nucleus (K) to the outer shells.
Each letter corresponds to a principal quantum number n:
K Shell: n = 1 (first shell, closest to the nucleus)
L Shell: n = 2 (second shell)
M Shell: n = 3 (third shell)
N Shell: n = 4 (fourth shell)
O Shell: n = 5 (fifth shell)
P Shell: n = 6 (sixth shell)
Q Shell: n = 7 (seventh shell)
Each shell can contain a maximum number of electrons defined by the formula 2n²:
K Shell (n=1): maximum 2 electrons (2 × 1² = 2)
L Shell (n=2): maximum 8 electrons (2 × 2² = 8)
M Shell (n=3): maximum 18 electrons (2 × 3² = 18)
N Shell (n=4): maximum 32 electrons (2 × 4² = 32) → Uranium reaches this: K(2) L(8) M(18) N(32)
O Shell (n=5): maximum 50 electrons (2 × 5² = 50) → Never reached (uranium has only 21 electrons in O)
P Shell (n=6): maximum 72 electrons (2 × 6² = 72) → Never reached
Q Shell (n=7): maximum 98 electrons (2 × 7² = 98) → Never reached
N.B.:
In practice, no known element completely fills the shells beyond N. The heaviest natural element, uranium (Z=92), has the configuration K(2) L(8) M(18) N(32) O(21) P(9) Q(2). The heaviest confirmed synthetic element, oganesson (Z=118), has the configuration K(2) L(8) M(18) N(32) O(32) P(18) Q(8).
Each shell is divided into subshells designated by the letters s, p, d, f:
s Subshell: can contain up to 2 electrons (1 orbital)
p Subshell: can contain up to 6 electrons (3 orbitals)
d Subshell: can contain up to 10 electrons (5 orbitals)
f Subshell: can contain up to 14 electrons (7 orbitals)
K Shell (n=1): contains only 1s (2 electrons max)
L Shell (n=2): contains 2s and 2p (2 + 6 = 8 electrons max)
M Shell (n=3): contains 3s, 3p, and 3d (2 + 6 + 10 = 18 electrons max)
N Shell (n=4): contains 4s, 4p, 4d, and 4f (2 + 6 + 10 + 14 = 32 electrons max)
O Shell (n=5): contains 5s, 5p, 5d, and 5f (2 + 6 + 10 + 14 = 32 electrons max theoretical, although the theoretical 5g subshell does not exist in known elements)
P Shell (n=6): contains 6s, 6p, 6d, and 6f (2 + 6 + 10 + 14 = 32 electrons max for known subshells)
Q Shell (n=7): contains 7s, 7p, and potentially 7d (only 7s and 7p electrons are observed in known elements)
This notation indicates the total number of electrons present in each shell, without detailing the subshells. It is particularly useful for quickly visualizing the overall electronic distribution of an atom.
Helium (2 electrons): 1s² → K(2)
The K shell is complete and saturated.
Neon (10 electrons): 1s² 2s² 2p⁶ → K(2) L(8)
The K and L shells are complete and saturated.
Sodium (11 electrons): 1s² 2s² 2p⁶ 3s¹ → K(2) L(8) M(1)
The K and L shells are complete, the M shell contains only 1 electron out of 18 possible.
Argon (18 electrons): 1s² 2s² 2p⁶ 3s² 3p⁶ → K(2) L(8) M(8)
The K and L shells are complete. The M shell contains 8 electrons but is not complete (the 3s and 3p subshells are saturated, but 3d remains empty).
Calcium (20 electrons): 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² → K(2) L(8) M(8) N(2)
Note that the 4s subshell fills before the 3d, which is why the M shell remains at 8 electrons.
Titanium (22 electrons): 1s² 2s² 2p⁶ 3s² 3p⁶ 3d² 4s² → K(2) L(8) M(10) N(2)
The M shell begins to fill with 3d electrons.
The filling order does not strictly follow the order of K, L, M, N shells... due to the energy levels of the subshells. The general order is:
1s → 2s → 2p → 3s → 3p → 4s → 3d → 4p → 5s → 4d → 5p → 6s → 4f → 5d → 6p → 7s → 5f → 6d...
This principle explains why, for example, potassium (19 electrons) has the configuration K(2) L(8) M(8) N(1): the 19th electron goes into 4s rather than 3d because the 4s subshell is of lower energy than 3d.
The K, L, M, N, O, P, Q notation allows:
• Quick visualization of the overall electronic structure of an atom
• Easy identification of the valence shell (outer shell)
• Understanding of chemical properties related to valence electrons
• Explanation of the classification of elements in the periodic table
• Prediction of oxidation states and chemical reactivity of elements
