8.1 Arrangement of elements
- Syllabus
- 0620–2026–2027
- Topic
- 8.1
- Level
- —
Learning objectives
8.1.1Periodic Table as an arrangement of• Describe Periodic Table as an arrangement of elements in periods and groups and in order of increasing proton number/atomic number8.1.2Change from metallic to non-metallic• Describe change from metallic to non-metallic character across a period8.1.3Relationship between group number and• Describe relationship between group number and the charge of the ions formed from elements in that group8.1.4Similarities in the chemical• Explain similarities in the chemical properties of elements in the same group of the Periodic Table in terms of their electronic configuration8.1.5How the position of an element in the• Explain how the position of an element in the Periodic Table can be used to predict its properties8.1.6Trends in groups, given information• Identify trends in groups, given information about the elements
Read the arrangement of the Periodic Table
The Periodic Table arranges elements in order of increasing proton number, also called atomic number. Each element therefore has one fixed position.
| Feature | Direction | What it tells you |
|---|---|---|
| period | horizontal row | atoms have the same number of occupied electron shells |
| group | vertical column | main-group atoms have the same number of outer-shell electrons |
| atomic number | increases from one element to the next | equals the number of protons in every atom of that element |
To identify a position, give both coordinates: for example, selenium is in Period 4 and Group VI. To compare two elements, first decide whether they share a row (period) or a column (group).
The table is ordered by proton number, not by relative atomic mass. Period number and group number describe different electron features and must not be interchanged.
Track metallic character across a period
Across a period from left to right, metallic character decreases and non-metallic character increases.
| Across a period | Left-hand elements | Right-hand elements |
|---|---|---|
| character | more metallic | more non-metallic |
| electron tendency | more likely to lose electrons and form positive ions | more likely to gain or share electrons |
| typical oxide character | more basic | more acidic |
If two elements are in the same period, the one farther left is normally more metallic. Thus sodium is more metallic than silicon, and silicon is more metallic than chlorine.
Do not use this left-to-right rule to compare elements in different periods without additional information. It describes a trend across one horizontal row.
Predict main-group ion charges
Main-group atoms form ions by losing or gaining outer-shell electrons to reach a full outer shell. The group number gives the number of outer electrons and therefore predicts the ion charge.
| Group | Outer electrons | Usual electron change | Typical ion charge |
|---|---|---|---|
| I | 1 | loses 1 | +1 |
| II | 2 | loses 2 | +2 |
| III | 3 | loses 3 | +3 |
| V | 5 | gains 3 | −3 |
| VI | 6 | gains 2 | −2 |
| VII | 7 | gains 1 | −1 |
Rubidium is in Group I, so it forms Rb⁺. Magnesium is in Group II, so it forms Mg²⁺. Sulfur is in Group VI, so it forms S²⁻.
The sign records electron movement: losing negative electrons gives a positive ion; gaining electrons gives a negative ion. Group IV elements do not normally form simple ±4 ions, and Group VIII noble gases do not usually form ions.
Explain similarities within a group
Elements in the same group have the same number of electrons in their outer shell. Because chemical reactions involve outer electrons, they have similar chemical properties.
| Same down a group | Changes down a group |
|---|---|
| number of outer-shell electrons | number of occupied electron shells |
| typical ion charge | atomic/proton number |
| broad reaction pattern and compound formula pattern | size, density, melting/boiling point and often reactivity |
Oxygen and sulfur are both in Group VI: each has six outer-shell electrons and commonly forms 2− ions. Calcium and strontium are both in Group II: each has two outer-shell electrons and commonly forms 2+ ions.
Similar chemical properties do not mean identical physical properties. Elements lower in a group have more electron shells, so their sizes and many measured properties change.
Predict properties from Periodic Table position
| Position clue | Prediction |
|---|---|
| period number | number of occupied electron shells |
| main-group number | number of outer-shell electrons |
| same group as a known element | similar reactions and typical compound formulae |
| left side | metallic behaviour, positive ions and often basic oxides |
| right side (except noble gases) | non-metallic behaviour and often acidic oxides |
| Group VIII | full outer shell and very low reactivity |
Locate the element, extract period and group information, then combine only justified clues. For an unknown below magnesium in Group II, predict a metal with two outer electrons, a 2+ ion, broadly similar reactions and a basic oxide.
Use predicted ion charge or valency to construct formulae. A Group II metal M and a Group VII non-metal X form MX₂; a Group I metal and a Group VI non-metal form M₂X.
Position supports broad predictions, not an exact melting point, colour or reaction rate unless a group trend or numerical data is supplied. State the periodic clue behind each prediction.
Identify a trend from group data
A group trend is a systematic change in a property as atomic number and position down the group increase.
| Step | Question to ask |
|---|---|
| order | Are the elements arranged from top to bottom of the group? |
| compare | Does the value increase, decrease or remain approximately constant? |
| check | Does the pattern hold across all supplied entries, allowing for stated anomalies? |
| predict | Should an unknown value lie above, below or between the known values? |
| justify | Which two or more data points establish the direction? |
If density rises 0.18 → 0.90 → 1.78 → 3.71 down a noble-gas group, identify increasing density. If melting points fall down a supplied group, predict a lower—not an invented exact—value for the next element unless interpolation is requested.
Do not import a memorised trend from another group. The objective asks you to identify the pattern in the information given; different properties and groups can change in different directions.