3.1.2Additional lessons
Electron Configurations and the Periodic Table — IB Chemistry HL Revision Notes & Topical Questions
5 min read 5 parts Lesson 49 of 172
Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
Part 1 of 5Reading the Table as a Map of Orbitals
Period number
- Period number The period equals the highest occupied principal energy level, so sulfur, with its outer electrons in n = 3, is in period 3.
s-block
- s-block Groups 1 and 2 plus helium are filling an s sub-level, so their outer electrons are ns¹ or ns².
p-block
- p-block Groups 13 to 18 are filling a p sub-level, giving outer configurations from ns²np¹ to ns²np⁶.
d-block
- d-block The transition elements fill the (n−1)d sub-level, so scandium to zinc fill 3d while 4s also holds electrons.
f-block
- f-block The lanthanoids and actinoids fill 4f and 5f sub-levels and sit in two rows beneath the main table.
Part 2 of 5Writing Configurations
1
Use the Aufbau order 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p and fill each sub-level to its maximum capacity.
2
Condensed notation replaces the inner electrons with the previous noble gas symbol, so sodium is [Ne] 3s¹.
3
Chromium is [Ar] 3d⁵4s¹ and copper is [Ar] 3d¹⁰4s¹ because half-filled and full 3d sub-levels are especially stable.
4
To form a cation from a d-block atom, remove the 4s electrons first and then 3d electrons if more are needed.
5
Anions add electrons to the outermost sub-level, so S²⁻ has the same configuration as argon.
| Species | Configuration |
|---|---|
| Mg | 1s² 2s² 2p⁶ 3s² |
| Fe | [Ar] 3d⁶ 4s² |
| Fe³⁺ | [Ar] 3d⁵ |
| Cl⁻ | [Ne] 3s² 3p⁶ |
Part 3 of 5Patterns Within Groups
Patterns Within Groups
Same outer shape
- Same outer shape Elements in one group share the same outer configuration, which is why they show similar chemistry.
Valence electrons
- Valence electrons For main-group elements the group number (units digit for groups 13 to 18) tells how many valence electrons there are.
Noble gases
- Noble gases Full outer shells, ns²np⁶, explain their low reactivity, with helium having a full 1s².
Isoelectronic ions
- Isoelectronic ions Ions such as N³⁻, O²⁻, F⁻, Na⁺ and Mg²⁺ all have the neon configuration but different nuclear charges.
Part 4 of 5Worked Example
Question Write the full electron configuration of iron and of the Fe³⁺ ion, and state its block.
- Iron has atomic number 26, so 26 electrons fill 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d⁶.
- In condensed form this is [Ar] 3d⁶ 4s², a d-block element in period 4.
- To make Fe³⁺, remove the two 4s electrons first, then one 3d electron.
- Fe³⁺ therefore has 23 electrons: [Ar] 3d⁵.
Answer
- Fe: [Ar] 3d⁶4s²;
- Fe³⁺: [Ar] 3d⁵;
- d-block
Part 5 of 5Exam Checklist & Tips
3.1.2 – Key Points for the Exam
- Valence electrons Electrons in the outermost occupied energy level of an atom.
- Aufbau principle Electrons occupy the lowest available energy sub-level first.
- Isoelectronic Species with the same number of electrons and the same configuration.
- d-block element An element whose highest-energy electrons are in a d sub-level.
- Condensed configuration A shorthand giving the previous noble gas in brackets followed by outer electrons.
Common Mistakes
- Removing 3d electrons before 4s electrons when forming transition metal ions.
- Writing chromium as [Ar] 3d⁴4s² and copper as [Ar] 3d⁹4s².
- Using the group number as the valence electron count for transition elements.
- Assuming isoelectronic ions have the same radius, when the one with more protons is smaller.
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