Periodic Trends in Reactivity
This topic explains how an element's position in a vertical column (Group) of the Periodic Table determines its chemical properties and reactivity, allowing you to predict its behaviour relative to other elements in the same group.
Part of the ESAT Chemistry syllabus — revision for the Engineering and Science Admissions Test (ESAT), the UAT-UK admissions test for Cambridge, Imperial, Oxford and UCL.
Key points
- Elements within the same Group (vertical column) have similar chemical properties.
- This similarity arises because they have the same number of electrons in their outermost shell.
- For metal groups (e.g., Group 1, Group 2), reactivity increases as you descend the group.
- For non-metal groups (e.g., Group 16, Group 17), reactivity decreases as you descend the group.
- The most reactive metals are at the bottom-left of the table, and the most reactive non-metals are at the top-right.
Diagram
Definitions
- Group
- A vertical column in the Periodic Table. Elements in a group share the same number of outer-shell electrons and thus have similar chemical characteristics.
- Period
- A horizontal row in the Periodic Table. The period number indicates the highest principal energy level (shell) occupied by electrons in an atom of that element.
- Reactivity
- A measure of how readily a substance undergoes a chemical reaction. For metals, this typically relates to the ease of losing electrons; for non-metals, it relates to the ease of gaining electrons.
Worked example
Element Q is in Group 17 and Period 4. Element R is in Group 2 and Period 5. Element S is in Group 17 and Period 3. Arrange these three elements in order of increasing reactivity.
- 1
Identify the groups and trends.
Q and S are in Group 17 (halogens, non-metals), where reactivity decreases down the group.
R is in Group 2 (alkaline earth metals), where reactivity increases down the group.
- 2
Compare the two non-metals, Q and S.
They are in the same group.
S (Period 3) is above Q (Period 4).
For non-metals, reactivity is highest at the top, so S is more reactive than Q.
- 3
Consider the metal, R.
Reactivity trends are not directly comparable between different groups (metal vs non-metal) without more information.
However, the question asks to order the given elements.
Both Group 2 and Group 17 contain highly reactive elements.
- 4
Re-evaluate the comparison.
S is Chlorine (Cl).
Q is Bromine (Br).
R is Strontium (Sr).
Halogen reactivity decreases, so Cl > Br.
Strontium is a reactive metal.
The least reactive of the three is Bromine (Q), as it is a less reactive halogen.
The most reactive is likely Strontium (R), as alkaline earth metals are very reactive, especially further down the group.
Chlorine (S) is a very reactive non-metal.
- 5
Based on general chemical principles, the reactivity order is Br < Cl < Sr.
Therefore, the order of increasing reactivity is Q, S, R.
Answer: Q, S, R
Common mistakes
- ×Confusing the reactivity trends for metals and non-metals. Remember: Metal reactivity increases down a group, while non-metal reactivity decreases.
- ×Mixing up Groups (vertical) and Periods (horizontal) when applying the rules.
- ×Assuming reactivity increases down the group for all elements. This is only true for metals.
No-calculator tips
- ✓Sketch a small, blank Periodic Table grid to map out the positions of elements given in a question. This makes it easier to see which is 'above' or 'below' another.
- ✓Use atomic structure to remember the trends. For metals, the outermost electron is further from the nucleus and more easily lost down a group (more reactive). For non-metals, the attraction for a new electron is weaker further down a group due to increased distance and shielding (less reactive).
- ✓Mnemonic: 'Metals at the Bottom are Best' (most reactive). 'Non-metals on Top are Terrific' (most reactive).