[JNW2]

Why is the acid strength of NaH <LiH<RbH<CsH ?

[Mitch]

The strength of an acid is often a combination of three factors: the polarity of the HM bond, the strength of the HM bond, and the stability of the conjugate base, M^-.

[JNW2]

The strength of an acid is often a combination of three factors: the polarity of the HM bond, the strength of the HM bond, and the stability of the conjugate base, M^-.

Thank you.
The polarity NaH >LiH. The more polarity, the more acidic.
The strength of the bond  LiH> NaH . The more strength ,the less acidic .
These 2 factors lead to NaH > LiH.
But factor 3 ( stability of M-) I don't know how to compare.

[galpinj]

The strength of an acid is often a combination of three factors: the polarity of the HM bond, the strength of the HM bond, and the stability of the conjugate base, M^-.

Thank you.
The polarity NaH >LiH. The more polarity, the more acidic.
The strength of the bond  LiH> NaH . The more strength ,the less acidic .
These 2 factors lead to NaH > LiH.
But factor 3 ( stability of M-) I don't know how to compare.

The larger the radius (the further down the periodic table an element is), the greater the ability of an acid to "spread out" the negative charge. This means that acidity increases as you move down the periodic table. For this reason, along with what you mentioned about polarity and bond strength, NaH would be a better/stronger acid than LiH.

[galpinj]

I've been thinking about your question, and I think I can add some more to the discussion.

In all the examples you provided, hydrogen is acting as the acid (it is accepting the electron) and the alkali elements are acting as bases. So, given that the acid for all the examples is the same (hydrogen), we need to analyze the quality of the base. A base with a very low electronegativity, weaker bond, and larger radius will be stronger. With this in mind, I would have to say that the order should still be CsH > RbH > NaH > LiH.