The Periodic Table
Section: Chemistry | Syllabus: Cambridge IGCSE Combined Science (0653)
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 |
Identifying Trends in Groups
Elements in the same group share similar chemical properties, and often show a clear trend (a steady increase or decrease) in physical properties down the group. Exam questions may give data about an unfamiliar group and ask you to spot the trend.
Worked Example: Spotting a Trend from Data
- Question: the melting points of four elements in the same group are: element W = 98°C, element X = 64°C, element Y = 39°C, element Z = 28°C, listed in order down the group. Describe the trend in melting point down this group, and predict whether the next element down will have a higher or lower melting point. [2]
- Answer: melting point decreases down the group [1]; the next element down the group is predicted to have an even lower melting point than Z [1]
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
- ❌ Describing a trend without saying whether it increases or decreases
- ✓ always state the direction of the trend clearly (e.g. "increases down the group"), and use it to predict the next value in the pattern
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