Sodium is metal while bromine is non metal. The bond formed between them is ionic.
Ionic bond:
Ionic bond is formed by the complete transfer of electron form metal to non metal.
Sodium is belongs to the group one while bromine is present in group seventeen. The number of valance electron of bromine are seven. Bromine require one electron to complete the octet, while sodium lose its one valance electron to gain noble gas stable electronic configuration. Thus sodium transfer its electron to bromine and gain positive charge and bromine gain its electron and form negative ion. The forces present between them are electrostatic forces which are very strong.
Explanation The number of energy levels (n) increases, and so does the distance between the nucleus and the outermost orbital. The increased distance and the increased shielding weaken the nuclear attraction, and so an atom can't attract electrons as strongly.
