Q.Write the expected formulas for the hydrides of 4th period elements. What is the trend in the formulas? In what way the first two members of the series different from the others?

Step 1. The 4th period begins with the s-block metals potassium (Group 1) and calcium (Group 2), which form simple ionic hydrides matching their fixed valence: $KH$ and $CaH_2$.

Step 2. From scandium onward, the 4th period enters the d-block (transition metals), whose hydrides are generally metallic/interstitial rather than simple ionic compounds -- hydrogen occupies lattice voids in these metals, giving hydrides of variable, non-stoichiometric composition (e.g. $TiH_{1.5-1.8}$, $PdH_{0.6-0.8}$ as seen for related transition metals in Section 4.8), rather than a single clean formula.

Step 3. So the trend in formulas is: simple, predictable, stoichiometric hydrides (KH, CaH₂) for the first two (s-block) elements, giving way to variable, non-stoichiometric interstitial hydrides for the transition metals that follow. …

Split the 4th period into its s-block start (K, Ca) and its d-block continuation (Sc onward), and match each to the hydride TYPE (ionic vs in …

• Trying to write one single stoichiometric formula (like MH or MH2) for every 4th-period transition metal -- most of their hydrides are genuine …