Hydrogen has an oxidation number of +1 in all compounds excepts when it binds with metals and has an oxidation state of -1.​

See explanation

Explanation:

In the periodic table, hydrogen is placed in group 1 for convenience. Hydrogen is not a metal, however, its common oxidation state is +1. It easily forms a univalent positive ion just like group 1 elements.

So, when hydrogen is combined with strongly electropositive metals such as sodium, lithium, etc, hydrogen accepts one electron and exhibits an oxidation state of -1 in those compounds.

Such compounds in which hydrogen is combined with strongly electropositive metals to give a binary compound is called a hydride.

alkaline metals are lithium, sodium, rubidium, potassium, cesium and francium.

alkali metals have electronegativity from 0.7 to 1, lowest in periodic table of elements, which means that alkali metals (i group in periodic table) has positive oxidation number in compounds.

electronegativity is a chemical property that describes the tendency of an atom to attract a shared pair of electrons towards itself.

alkaline metals (far left in main group) have lowest ionizations energy and easy remove valence electrons,

for example, potassium is the alkali metal and has a single valence electron in the outer electron shell.

electron configuration of potassium is: ₁₉k 1s²2s²2p⁶3s²3p⁶4s¹.

they can not exist alone in nature, because they are very reactive.

answer : a model is often used to explain and make things easier to understand when any scientific topic needs to be demonstrated and explained.

as in this case where a scientist wants to use a model to present the results of his detailed scientific investigation. he should do it as it will him to make the audience understand the concept in a much better way. also, it can be changed according to new scientific understandings.