Explanation:

The metallic character of an element is determined by its ability to lose electrons and form positive ions. Generally, metallic character increases as we move down a group in the periodic table.

In Group 1 of the periodic table, also known as the alkali metals, the metallic character increases as we move down the group. This is because the outermost electron in these elements is located in an S orbital, which is farther from the nucleus and more shielded by inner electron shells as we move down the group. As a result, the outermost electron is less tightly held by the nucleus, making it easier to lose and form a positive ion.

Comparison of Group 1 elements:
1. Lithium (Li): Lithium is the first element in Group 1. It has 3 protons in its nucleus and its outermost electron is located in the 2s orbital. Due to its small atomic size and relatively higher effective nuclear charge, lithium has a stronger hold on its outermost electron compared to the other elements in Group 1. As a result, it requires more energy to remove the outermost electron and form a positive ion. Therefore, lithium is the least metallic element in Group 1.

2. Sodium (Na): Sodium is the second element in Group 1. It has 11 protons in its nucleus and its outermost electron is also located in the 2s orbital. Sodium is larger in size compared to lithium, and its outermost electron is less tightly held. It requires less energy to remove the outermost electron compared to lithium, making sodium more metallic than lithium.

3. Potassium (K): Potassium is the third element in Group 1. It has 19 protons in its nucleus and its outermost electron is located in the 3s orbital. Potassium is larger in size compared to sodium, and its outermost electron is even less tightly held. It requires even less energy to remove the outermost electron compared to sodium, making potassium more metallic than sodium.

4. Caesium (Cs): Caesium is the fourth and last element in Group 1. It has 55 protons in its nucleus and its outermost electron is also located in the 6s orbital. Caesium is the largest element in Group 1, and its outermost electron is the least tightly held. It requires the least energy to remove the outermost electron compared to all the other elements in Group 1, making caesium the most metallic element.

In conclusion, lithium is the least metallic element in Group 1 because it requires the most energy to remove its outermost electron and form a positive ion.