Chemistry · Periodic Classification Of Properties · NEET
It INCREASES down a group. Going down, the metal atom gets bigger and loses its outer electron more easily (more electropositive). A bigger, more electropositive metal gives a more ionic bond. So for group 2 hydrides the order is BeH2 < CaH2 < BaH2. Do not mix this up with atomic radius — both go up together here, which makes it easy to remember.
Be2+ is a very small ion with a +2 charge, so it has very high 'polarising power' — it pulls and distorts the electron cloud of the H- ion. When the cloud is distorted a lot, electrons get shared, so the bond becomes covalent. Ba2+ is large, so it cannot distort the cloud much; the electrons stay put and the bond stays ionic. This is Fajans' rule in action.
Fajans' rule tells you when an 'ionic-looking' bond is actually more covalent. A bond has MORE covalent character (less ionic) when: (1) the cation is small, (2) the cation has a high charge, and (3) the anion is large. So small high-charge cations like Li+, Be2+, Al3+ make covalent bonds. Large low-charge cations like K+, Ba2+ make ionic bonds.
NaCl is more ionic. The cation (Na+) is the same, but the anion changes. I- is much larger than Cl-, so its big soft electron cloud is easily distorted by Na+. More distortion means more covalent character. So among sodium halides, ionic character order is NaI < NaBr < NaCl < NaF. Bigger anion = more covalent = less ionic.
Because they are the smallest atoms in their groups, their ions (Li+, Be2+) are tiny and have high polarising power. This high power distorts anion clouds and pulls electrons into sharing, so their compounds have strong COVALENT character. This is a common NEET trap — people assume 'metal = ionic', but small metal ions break that rule.
A simple rule: the bigger the electronegativity difference between the two atoms, the more ionic the bond. In hydrides, going down group 1 or 2 the metal becomes less electronegative, so the gap between metal and hydrogen grows, and the bond becomes more ionic. This gives the same answer as Fajans' rule, so you can use whichever is faster in the exam.
Among CaH2, BeH2, BaH2, the order of ionic character is
Assertion (A): Lithium and beryllium, unlike their other respective group members, form compounds with pronounced ionic character. Reason (R): Lithium and magnesium have similar properties due to the diagonal relationship.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
It increases down the group. For group 2 hydrides: BeH2 < MgH2 < CaH2 < SrH2 < BaH2. The metal becomes more electropositive going down, making the hydride more ionic.
For the same cation, ionic character increases as the halide anion gets smaller: NaI < NaBr < NaCl < NaF. Smaller anion is harder to distort, so the bond stays ionic.
A small cation has more covalent character. Small size means high polarising power, which distorts the anion cloud and leads to electron sharing (Fajans' rule).
NEET asks direct ordering questions (like the 2018 hydride question) and Assertion-Reason traps (like 2023 on Li/Be). Knowing the trend and Fajans' rule lets you answer both in seconds without memorising every compound.
Yes. As metallic (electropositive) character increases down a group, the ionic character of that metal's compounds also increases. That is why the next topic, metallic and non-metallic character, connects directly to this one.
