3.1.2Additional lessons
Electron Configurations and the Periodic Table — IB Chemistry SL Revision Notes & Topical Questions
5 min read 6 parts Lesson 38 of 98
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 6Structure of the Table
The layout of the periodic table is a map of electron configurations.
Period
- Period The period number equals the number of the highest occupied electron shell, so sodium with 3s¹ is in Period 3.
Group (s and p)
- Group (s and p) For Groups 1 and 2 the group number is the number of valence electrons;
- for Groups 13 to 18 it is the number of valence electrons plus 10.
Blocks
- Blocks The table divides into s, p and d blocks according to the sublevel that is being filled.
Similar properties
- Similar properties Elements in a group have the same number of valence electrons, so they react in similar ways.
Part 2 of 6Writing Configurations
1
Find the atomic number to know how many electrons to place.
2
Fill sublevels in order of increasing energy: 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p.
3
Use the noble gas shorthand for inner shells, for example chlorine is [Ne]3s²3p⁵.
4
For the first-row d block, 4s fills before 3d, giving iron as [Ar]3d⁶4s².
5
Remember the two exceptions: chromium is [Ar]3d⁵4s¹ and copper is [Ar]3d¹⁰4s¹ because half-filled and filled d sublevels are particularly stable.
| Element | Configuration | Period | Group | Block |
|---|---|---|---|---|
| Mg | 1s²2s²2p⁶3s² | 3 | 2 | s |
| Cl | [Ne]3s²3p⁵ | 3 | 17 | p |
| Fe | [Ar]3d⁶4s² | 4 | 8 | d |
| Ca | [Ar]4s² | 4 | 2 | s |
Part 3 of 6Reading a Configuration
Reading a Configuration
Highest shell
- Highest shell The largest principal quantum number n in the configuration gives the period.
Valence electrons
- Valence electrons Count the electrons in the outer shell to find the group for s and p block elements.
Last sublevel
- Last sublevel The sublevel that finishes the configuration gives the block.
Ions
- Ions To write the configuration of a cation, remove electrons from the highest shell first;
- for transition metals remove 4s before 3d.
Part 4 of 6Links to Properties
Group 1
- Group 1 Each has one outer s electron, which is easily lost, making them highly reactive metals.
Group 17
- Group 17 Each has seven valence electrons and gains one electron to complete the octet, forming 1− ions.
Group 18
- Group 18 Full outer shells make the noble gases very unreactive.
Transition metals
- Transition metals Partly filled d sublevels explain variable oxidation states and coloured compounds.
Part 5 of 6Worked Example
Question
- An element has the configuration 1s²2s²2p⁶3s²3p³.
- Identify its period, group and block.
- The highest shell is n = 3, so it is in Period 3.
- It has 2 + 3 = 5 electrons in the outer shell, so the group number is 5 + 10 = 15.
- The last electrons enter a p sublevel, so it is in the p block; the element is phosphorus.
Answer Period 3, Group 15, p block (phosphorus).
Part 6 of 6Exam Checklist & Tips
3.1.2 – Key Points for the Exam
- Period A horizontal row of the periodic table, equal to the highest occupied shell.
- Group A vertical column of elements with the same number of valence electrons.
- Block A region of the table identified by the sublevel being filled: s, p or d.
- Valence shell The outermost occupied electron shell.
Common Mistakes
- Writing 3d before 4s when filling electrons.
- Giving the group of a p block element as the number of valence electrons without adding 10.
- Forgetting that Cr and Cu are exceptions.
- Removing 3d electrons before 4s when forming transition metal ions.
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