Arrange with explanation the following elements in the increasing order of their ionisation energies :
Li, Be, B, Na, Mg

Explanation

To arrange the elements Lithium (Li), Beryllium (Be), Boron (B), Sodium (Na), and Magnesium (Mg) in increasing order of their ionization energies, we need to understand the trends in ionization energy across periods and down groups in the periodic table.

### Step-by-Step Solution:

1. **Understanding Ionization Energy**:
Ionization energy is the energy required to remove an electron from an atom in its gaseous state. Generally, ionization energy increases across a period (from left to right) and decreases down a group (from top to bottom).

2. **Identify the Periods and Groups**:
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