Chapter 20, Problem 40A

Solution

(a)

To find: Whether the statement is true or false.

The statement is true.

The given statement is:

The oxidation number of an uncombined element is zero.

The oxidation number of an element is a way that can give the charge of a particular chemical compound. When it is present in the uncombined form or elemental form there is no charge present. So, the oxidation number of an uncombined element such as O, N, H, Xe is zero.

(b)

To find: Whether the statement is true or false.

The statement is true.

The given statement is:

The sum of the oxidation numbers of atoms in a polyatomic ion must equal the charge of the ion.

Atoms combine to form polyatomic ions having a net charge on them. For such species, the sum of the oxidation number of each element is equal to the net charge of that compound. So, the sum of the oxidation number of each element in the polyatomic ions is equal to the net charge.

(c)

To find: Whether the statement is true or false.

The statement is false.

The given statement is:

Every element has a single oxidation number.

Depending on the element there exist certain elements that possess more than one oxidation number. But some elements have only one oxidation number. So, it is not obvious that every element has a single oxidation number.

(d)

To find: Whether the statement is true or false.

The statement is true.

The given statement is:

The oxidation number of oxygen in a compound or polyatomic ion is almost always $-2$ .

In most cases, the oxidation number of oxygen is $-2$ . But there is an exception in which the oxidation number of oxygen becomes $-1$ in the case of peroxides. So, in most cases the oxidation number is $-2$ .