Who was the first to propose the idea of atoms

 Greek philosopher Leucippus of Miletus and hisstudent Democritus of Abdera in the 5th century B.C

Ifound this on google “these two compounds have a different ionic structure. caesium ion is larger in size when compared to the sodium ion. the charge gets more area to spread out evenly. so caesium chloride has a lower melting point than sodium chloride.” i hope this you for what your looking for