Q4. Show the electron transfer when magnesium and oxygen form magnesium oxide, and give the electrovalence of each ion.
Mg ([Ne] 3s2) → Mg2+ ([Ne]) + 2e−; O ([He] 2s2 2p4) + 2e− → O2− ([Ne]). Mg2+ + O2− → MgO. Electrovalence: magnesium +2, oxygen −2. Forming O2−(g) from O(g) is strongly endothermic, because oxygen’s second electron gain enthalpy is positive; MgO forms only because its very large lattice enthalpy repays this.
Q5. Why are most ionic compounds formed between metals on the left of the periodic table and non-metals on the right?
Metals on the left have low ionisation enthalpies and lose electrons easily; non-metals on the right (especially the halogens and oxygen) have large negative electron gain enthalpies and gain electrons readily. These are the conditions NCERT names for ionic bonding.
Q6. Why can the lattice enthalpy of a crystal not be found from the attraction and repulsion between one pair of ions alone?
In a three-dimensional crystal each ion is attracted by many oppositely charged neighbours and repelled by many like-charged ones at different distances. The total depends on the crystal geometry, which must be included.
Q7. Sodium forms Na+, never Na2+, in its ionic compounds. Suggest why.
Removing a second electron from Na+ means breaking into its stable [Ne] core, which needs a very large second ionisation enthalpy (about nine times the first). The extra energy released by lattice formation with a 2+ ion would not repay it.