What is the oxidation state of the K atom in KClO4?

+1

Rules to remember when trying to find out the oxidation state of an element:

(1) The total charge of a stable compound is always equal to zero (meaning no charge).

For example, the ##H_2O## molecule exists as a neutrally charged substance.

(2) If the substance is an ion (either there is a positive or negative charge) the total oxidation state of the ion is the charge (i.e. oxidation state of ##NO_3^"-1"## ion is -1).

(3) All from Group 1A has an oxidation state of +1 (e.g. ##Na^"+1"##, ##Li^"+1"##). All Group 2A and 3A have an oxidation state of +2 and +3, respectively. (e.g. ##Ca^"2+"##, ##Mg^"2+"##, ##Al^"3+"##)

(4) Oxygen always have a charge -2 except for peroxide ion (##O_2^"2-"##) which has a charge of -1.

(5) Hydrogen always have a charge of +1 if it is bonded with a non-metal (as in the case of ##HCl##) and always have a -1 charge if it is bonded with a metal (as in ##AlH_3##).

For your question, ##KClO_4##, based on rule 3, the answer is +1.