2.6-2.8
Your guide: Miss SanaCounts atoms the way you count rupees: carefully.
An atom is like a block of flats. Electrons fill the lowest floors first.
Three rules tell us the order.
Smart Syllabus 2026 leaves out 2.7 and 2.8 (configuration and the periodic table, ions and free radicals) for the 2026 exam. Section 2.6 stays. Learn all three, because later chapters use them.
Shells hold at most 2, 8, 18, 32 electrons for K, L, M, N.
In shells, Na is 2, 8, 1 and Cl is 2, 8, 7.
In subshells, the symbol has a small number on top. That is the electron count.
Example: Na is 1s2 2s2 2p6 3s1.
Aufbau: fill subshells in order of rising energy.
The energy order comes from n + l. A lower n + l fills first.
Book example: 4s has 4 + 0 = 4 and 3d has 3 + 2 = 5. So 4s fills first.
If n + l is equal, the lower n goes first. 3d and 4p both have 5, so 3d goes first.
Book order: 1s < 2s < 2p < 3s < 3p < 4s < 3d. Then: 4p < 5s < 4d < 5p < 6s < 4f < 5d < 6p < 7s.
Pauli: no two electrons have all four quantum numbers equal. Two in one orbital have opposite spins.
Hund: put electrons in separate equal-energy orbitals with the same spin first.
Si (Z = 14): 1s2 2s2 2p6 3s2 3p2.
S (Z = 16): 1s2 2s2 2p6 3s2 3p4.
K (Z = 19): 1s2 2s2 2p6 3s2 3p6 4s1. The last electron goes to 4s.
The book lists Cr as 3d5 4s1 and Cu as 3d10 4s1. Half-filled and full subshells are stable.
Groups 1 and 2 have ns1 and ns2 outer shells. Group 13 has ns2 np1.
The block on the right has six columns where p fills. The middle has ten columns where d fills.
The two rows below have fourteen columns where f fills. These are 2, 6, 10, 14.
Valence electrons are those in the outermost shell. They take part in reactions.
A positive ion forms when electrons leave. Na+ has 10 electrons: 1s2 2s2 2p6, the same as neon.
A negative ion forms when atoms gain electrons. S2− has 18 electrons: 1s2 2s2 2p6 3s2 3p6, the same as argon.
A d-block atom loses its 4s electrons first, before 3d.
Book: Ti2+ is 1s2 2s2 2p6 3s2 3p6 3d2 and Cr3+ is 1s2 2s2 2p6 3s2 3p6 3d3.
A free radical has one or more unpaired electrons. Example: the chlorine atom Cl·, 3p5 with one unpaired electron.
Step 1 / 7
An atom is like a block of flats. Electrons fill the lowest floors first.
Three rules tell us the order.
Smart Syllabus 2026 leaves out 2.7 and 2.8 (configuration and the periodic table, ions and free radicals) for the 2026 exam. Section 2.6 stays. Learn all three, because later chapters use them.
Shells hold at most 2, 8, 18, 32 electrons for K, L, M, N.
In shells, Na is 2, 8, 1 and Cl is 2, 8, 7.
In subshells, the symbol has a small number on top. That is the electron count.
Example: Na is 1s2 2s2 2p6 3s1.
Aufbau: fill subshells in order of rising energy.
The energy order comes from n + l. A lower n + l fills first.
Book example: 4s has 4 + 0 = 4 and 3d has 3 + 2 = 5. So 4s fills first.
If n + l is equal, the lower n goes first. 3d and 4p both have 5, so 3d goes first.
Book order: 1s < 2s < 2p < 3s < 3p < 4s < 3d. Then: 4p < 5s < 4d < 5p < 6s < 4f < 5d < 6p < 7s.
Pauli: no two electrons have all four quantum numbers equal. Two in one orbital have opposite spins.
Hund: put electrons in separate equal-energy orbitals with the same spin first.
Si (Z = 14): 1s2 2s2 2p6 3s2 3p2.
S (Z = 16): 1s2 2s2 2p6 3s2 3p4.
K (Z = 19): 1s2 2s2 2p6 3s2 3p6 4s1. The last electron goes to 4s.
The book lists Cr as 3d5 4s1 and Cu as 3d10 4s1. Half-filled and full subshells are stable.
Groups 1 and 2 have ns1 and ns2 outer shells. Group 13 has ns2 np1.
The block on the right has six columns where p fills. The middle has ten columns where d fills.
The two rows below have fourteen columns where f fills. These are 2, 6, 10, 14.
Valence electrons are those in the outermost shell. They take part in reactions.
A positive ion forms when electrons leave. Na+ has 10 electrons: 1s2 2s2 2p6, the same as neon.
A negative ion forms when atoms gain electrons. S2− has 18 electrons: 1s2 2s2 2p6 3s2 3p6, the same as argon.
A d-block atom loses its 4s electrons first, before 3d.
Book: Ti2+ is 1s2 2s2 2p6 3s2 3p6 3d2 and Cr3+ is 1s2 2s2 2p6 3s2 3p6 3d3.
A free radical has one or more unpaired electrons. Example: the chlorine atom Cl·, 3p5 with one unpaired electron.
Q1. Quick Check 2.7: write the configuration of O2− (Z = 8).
O2− has 8 + 2 = 10 electrons. So 1s2 2s2 2p6.
Q2. Exercise: why does the 4s subshell fill before 3d?
n + l of 4s is 4 + 0 = 4. For 3d it is 3 + 2 = 5. Lower value means lower energy, so 4s fills first.
✗ “3d fills before 4s because 3 is less than 4.”
✓ Compare n + l. 4s is 4. 3d is 5.
✗ “A d-block atom loses 3d electrons first.”
✓ It loses 4s electrons first. Cr3+ ends at 3d3.
1. Which subshell fills right after 3p?
(b) Book order: 3p < 4s < 3d.
2. How many electrons does Na+ have? (Z = 11)
(c) 11 − 1 = 10. Same as neon.
3. What does Hund's rule say?
(a) Book definition.
4. The valence configuration of group 13 is:
(c) The book says so in 2.7.
5. Which has the same configuration as argon?
(a) S2− has 18 electrons.
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