Why do we need neutrons in the nucleus?

Neutrons are required for the stability of nuclei, with the exception of the single-proton hydrogen nucleus. Neutrons are produced copiously in nuclear fission and fusion. They are a primary contributor to the nucleosynthesis of chemical elements within stars through fission, fusion, and neutron capture processes.

The thread that connects the pearls is like an ionic bond.

Explanation:

Polymers are macromolecules conformed by several monomers (precisely how several pearls connected together form one pearl necklace). These monomers are bonded together through a process known as polymerization (just like stringing the beads together in the necklace).

However, the bonds between the monomers are not ionic, they are covalent. Polymers are usually made from organic compounds, and as such they have C-C, C-H, C-N and C-O bonds, which are covalent.