The Periodic Table
Section: 8. The Periodic Table | Syllabus: Cambridge IGCSE Chemistry (0620)
Periods and Groups
The Periodic Table arranges all known elements in order of increasing proton number (atomic number), in horizontal periods and vertical groups.
| Periods | Groups | |
|---|---|---|
| Direction | horizontal rows | vertical columns |
| What the number tells you | number of electron shells occupied | number of electrons in the outer shell |
Worked Example: Finding Period from Electron Structure
- Question: titanium has electronic structure 2,8,10,2. State its period number. [1]
- Answer: titanium occupies 4 electron shells, so it is in Period 4
Metallic and Non-Metallic Character Across a Period
Metals are found on the left of the Periodic Table, non-metals on the right. Moving left to right across a period, character changes from metallic to non-metallic.
| Metals | Non-metals | |
|---|---|---|
| Position | left / centre | right |
| Ion formed | positive (lose electrons) | negative (gain electrons) |
| Oxide formed | basic | acidic |
Group Number and Ion Charge
For main-group elements, the group number tells you the charge of the ion the element normally forms:
| Group | Outer electrons | Ion formed | Example |
|---|---|---|---|
| I | 1 | 1+ | Na⁺ |
| II | 2 | 2+ | Mg²⁺ |
| VI | 6 | 2− | O²⁻ |
| VII | 7 | 1− | Cl⁻ |
Groups I-III form positive ions by losing their outer electrons; Groups V-VII form negative ions by gaining electrons to complete the outer shell.
Worked Example: Predicting an Ion's Charge
- Question: an element is in Group VI of the Periodic Table. State the charge on the ion it is most likely to form. [1]
- Answer: Group VI atoms have 6 outer electrons, so they gain 2 electrons to complete the outer shell → charge = 2−
A Brief History
The modern table developed from Dmitri Mendeleev's 1869 table, which arranged elements by atomic mass, grouped elements with similar properties together, and left gaps for undiscovered elements, correctly predicting their properties. The modern table instead orders elements by atomic (proton) number, which better reflects electron configuration and chemical behaviour.
Common Mistakes
- ❌ Confusing period number with group number
- ✓ period = number of electron shells (row); group = number of outer-shell electrons (column) - they answer different questions
- ❌ Assuming every element forms an ion with charge equal to its group number
- ✓ this rule works reliably for main groups I, II, VI and VII, but transition metals (the central block) commonly form ions with variable charges not tied simply to a group number
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