Introduction:
Ionic radius refers to the size of an ion. It is determined by the attraction between the nucleus and the outermost electron shell. The ionic radius of an ion can differ due to various factors, including the number of protons, the number of electrons, and the electron configuration. In the case of Lithium (Li+) and Magnesium (Mg2+), the difference in their ionic radii can be explained by considering their atomic structure and electron configuration.

Atomic Structure:
Lithium has an atomic number of 3, meaning it has three protons in its nucleus. Magnesium, on the other hand, has an atomic number of 12, indicating the presence of 12 protons in its nucleus. The number of protons determines the positive charge of the nucleus, which attracts the electrons in the outermost shell.

Electron Configuration:
Lithium (Li) has an electron configuration of 1s2 2s1, while Magnesium (Mg) has an electron configuration of 1s2 2s2 2p6 3s2. The valence electrons in the outermost shell of an atom are responsible for the atom's chemical properties. In the case of Li+, it loses one electron from its outermost shell to attain a stable electron configuration of helium (1s2). Mg2+, on the other hand, loses two electrons from its outermost shell to achieve a stable electron configuration of neon (1s2 2s2 2p6).

Effect of Electron Configuration on Ionic Radius:
The loss of electrons in the formation of cations reduces the repulsion between electrons in the outermost shell, leading to a contraction in the ionic radius. However, the number of protons in the nucleus remains the same.

Explanation of the Difference:
1. Lithium ion (Li+):
- Lithium loses one electron, resulting in the removal of the 2s1 electron from its electron configuration.
- The loss of this electron reduces the repulsion between electrons in the outermost shell, causing it to contract.
- The ionic radius of Li+ is smaller compared to its neutral atom due to the decreased electron-electron repulsion.

2. Magnesium ion (Mg2+):
- Magnesium loses two electrons, removing the 3s2 electrons from its electron configuration.
- The loss of these two electrons decreases the electron-electron repulsion in the outermost shell, leading to a contraction in the ionic radius.
- However, the increased number of protons in the nucleus compared to lithium results in a stronger attraction towards the remaining electrons.
- The increased nuclear charge offsets the decrease in electron-electron repulsion, resulting in a smaller ionic radius compared to lithium.

Conclusion:
The ionic radius of Lithium (Li+) is greater than that of Magnesium (Mg2+) due to the difference in their electron configurations and the resulting balance between electron-electron repulsion and nuclear attraction. While both ions experience a contraction in their ionic radii due to electron loss, the increased nuclear charge in magnesium causes a greater overall attraction towards the remaining electrons, resulting in a smaller ionic radius.