Lithium (Li) has three protons, three electrons, and typically three neutrons in its most common isotope. It is in Group 1 of the periodic table, which contains the alkali metals.
The element that has properties most similar to lithium is sodium (Na). Sodium is directly below lithium in Group 1 and has one more electron shell, making it similar in chemical behavior. Both lithium and sodium are highly reactive metals, have similar atomic and ionic radii trends, and share similar properties due to their position in the periodic table.
In summary, sodium (Na) is the element with properties most similar to lithium.