The character of a chemical bond depends strongly on the electronegativity difference between the bonded atoms. When two elements differ greatly in electronegativity, the more electronegative atom attracts the shared electrons so strongly that it effectively gains them, while the less electronegative atom loses them, producing oppositely charged ions held together by electrostatic attraction. This is an ionic bond, typical of combinations such as a reactive metal with a reactive non-metal like sodium and chlorine. A pure covalent bond forms when the electronegativity difference is zero, as between identical atoms. A metallic bond forms among metal atoms sharing delocalised electrons, not between very different elements. A coordinate bond involves one atom donating both shared electrons and does not specifically arise from large electronegativity differences. The relationship between electronegativity difference and bond type is a central NCERT idea linking periodicity to bonding. Plausibility check: a large difference such as between sodium and chlorine indeed yields the ionic compound sodium chloride, confirming the answer.