Use the shell model to explain why a lithium atom Li, is larger than a beryllium atom, Be.

I don't think it is larger. It has a lower atomic number 3 vs 4. It is therefore lighter
see below link and click on elements.

To understand why a lithium atom is larger than a beryllium atom, we need to examine their electron configurations using the shell model or the quantum mechanical model of the atom.

The shell model describes the arrangement of electrons in energy levels called shells or orbitals. Each shell can accommodate a specific number of electrons, and the shells are filled in a specific order from the innermost shell to the outermost shell.

In the case of lithium (Li), it has an atomic number of 3, meaning it has three electrons. These three electrons are distributed as follows: two in the first shell (closest to the nucleus) and one in the second shell. The first shell is fully occupied, while the second shell has only one electron.

On the other hand, beryllium (Be) has an atomic number of 4, meaning it has four electrons. The electron configuration of beryllium is two in the first shell and two in the second shell.

Now, the size or radius of an atom is determined by the average distance of its outermost electrons from the nucleus. In the case of lithium, its outermost electron is in the second shell, which is further from the nucleus compared to beryllium's outermost electron in the second shell.

Since lithium's outermost electron is in a higher energy level and further from the nucleus, it experiences less attraction, resulting in a larger atomic radius compared to beryllium, where the outermost electron is closer to the nucleus.

Therefore, based on the electron configuration and the shell model, we can conclude that a lithium atom is larger than a beryllium atom.