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I downloaded a periodic table app which gives detailed information for each element. The app shows some elements, however, have two different oxidation states. Hydrogen, for instance, can have either a 1+ or 1- state. What do these two states signify? Why do these elements have more than one oxidation state?
Can someone explain this?
The oxidation state of an element in a compound is simply decided on the basis of electronegativity of other atoms. As the definition of oxidation state says
Oxidation state of an element in a given compound is the charged acquired by its atom on the basis of electronegativity of other atoms in the compound.
The basis of calculating oxidation number is that the more electronegative element acquires the negative charge and the less electronegative one acquires the positive charge. So in molecules in which hydrogen is less electronegetive it acquires a $+1$ charge while in molecules in which it is more electronegetive it will acquire a $+1$ charge ($1$ because it has the valency of $1$ and usually oxidation number does not exceed valency).
Let's understand it with the help of examples:
In the $\ce{HCl}$ molecule, $\ce{H}$ is less electronegetive that's why charge of $+1$,for the same reason,$+1$ in $\ce{H2O}$.
In the compound $\ce{ZnH2}$, hydrogen is more electronegetive so it will have a charge of $-1$, for similar reason $-1$ in $\ce{NaH}$.
Usually metals (that make metal hydrides) are less electronegetive than hydrogen, That's why we also say hydrogen has an oxidation state of $-1$ in metal hydrides.
You see compounds like HCl in which the hydrogen is +1, and then there are compounds like NaH, where the hydrogen is -1.
The oxidation state, often called the oxidation number, is an indicator of the degree of oxidation (loss of electrons) of an atom in a chemical compound. Conceptually, the oxidation state, which may be positive, negative or zero, is the hypothetical charge that an atom would have if all bonds to atoms of different elements were $100 \% $ ionic, with no covalent component. This is never exactly true for real bonds.
So an atom can gain $1,2,3$ or more electrons and also lose $1,2,3$ or more electrons thereby causing it to gain a certain degree of oxidation or reduction. This explains why an element can have multiple oxidation states.
