The graph displays the successive ionisation energies of element X and element Y. Both elements belong to Period 3. Which statement is right?
- AAn atom of element X needs one extra electron for a full outer shell; an atom of element Y needs three extra electrons for a full outer shell.
- BAn atom of element Y has five electrons in the $3p$ subshell.
- CElement X has an oxidation number of $+7$ in most of its compounds.
- DWhen element X combines with element Y, the bonding is ionic.