Question

Find the order of metallic radius of
Mg, Li, Be, B, Al

Hint:

While general trends work most of the time, the comparison between Group 1 elements of a lower period and Group 2/13 elements of a higher period (like Li vs Al or Mg) can be tricky. Remembering approximate values or established orders for the first 20 elements is very helpful.

Answer 1

Step 1: Understanding the Concept:
The atomic (or metallic/covalent) radius of elements follows specific periodic trends:
1. It decreases across a period from left to right due to increasing effective nuclear charge, which pulls the electron cloud closer to the nucleus.
2. It increases down a group from top to bottom due to the addition of new principal electron shells, which outweighs the increase in nuclear charge.

Step 2: Key Formula or Approach:
Locate the given elements in the periodic table to compare their radii based on periods and groups.

Step 3: Detailed Explanation:
Let's place the elements in their respective periods and groups:
- Period 2: Li (Group 1), Be (Group 2), B (Group 13)
- Period 3: Mg (Group 2), Al (Group 13)
First, apply the trend across Period 2:
$\text{Radius of Li}>\text{Radius of Be}>\text{Radius of B}$
Next, apply the trend across Period 3:
$\text{Radius of Mg}>\text{Radius of Al}$
Now, compare elements down the groups (Period 3 elements are generally larger than Period 2 elements of the same or nearby groups):
- Mg is below Be, so Mg $>$ Be.
- Al is below B, so Al $>$ B.
Comparing Li and Mg: This is a diagonal relationship, but generally, Group 1 elements are very large. The standard metallic radius of Mg ($\sim 160$ pm) is slightly larger than Li ($\sim 152$ pm).
Comparing Li and Al: Li ($\sim 152$ pm) is larger than Al ($\sim 143$ pm).
Combining these observations with standard values:
Mg (160 pm) $>$ Li (152 pm) $>$ Al (143 pm) $>$ Be (112 pm) $>$ B (85 pm)
Thus, the decreasing order is Mg $>$ Li $>$ Al $>$ Be $>$ B.

Step 4: Final Answer:
The correct order is Mg $>$ Li $>$ Al $>$ Be $>$ B.