Explain why the atomic radius of lithium is larger than that of fluorine. - Leaving Cert Chemistry - Question (d) - 2021

The atomic radius of lithium (Li) is larger than that of fluorine (F) due to several key factors relating to their positions in the periodic table and their electronic configurations.

Firstly, lithium is located in Group 1 (the alkali metals) and has the atomic number 3, while fluorine is in Group 17 (the halogens) with an atomic number of 9. The number of electron shells is a significant factor in determining atomic size. Lithium, having two electron shells (1s² 2s¹), has a larger atomic radius than fluorine, which has two electron shells as well but with a greater effective nuclear charge due to its higher atomic number.

Secondly, as we move from left to right across a period in the periodic table, the atomic radius tends to decrease. This decrease occurs because the increasing positive charge of the nucleus attracts the electrons more strongly, pulling them closer, which results in a smaller atomic radius.

Although both lithium and fluorine have their outermost electrons in the second shell, fluorine's increased nuclear charge (9 protons) compared to lithium (3 protons) causes a stronger attraction towards the nucleus, leading to a smaller atomic size for fluorine.

In summary, the larger atomic radius of lithium compared to fluorine is primarily due to the fewer protons in lithium's nucleus, resulting in weaker nuclear attraction and thus a greater atomic radius.