Oxidation in its original sense means combining with oxygen. In this case it is clear which element is combining with more oxygen (oxidized) and which is losing oxygen (reduced)
The concept was extended to mean oxidation is like losing an electron because oxygen is an electron hog (high electronegativity. So, it's like living with a mooch - he takes your stuff ( This can be remembered by LeO goes GeR) Metals are oxidize because they lose electrons. Nonmetals are reduced because they gain electrons.
The oxidizing agent is the compound that has the element that is reduced. The Reducing agent is the compound that has the element that is oxidized. Metals and Hydrogen are reducing agents and non-metals (like oxygen and fluorine) are oxidizing agents.
If the situation is complicated, the final arbiter is oxidation state (more positive, more oxidized). You should learn the rules for this in your class. If you check it, you will see that As goes up and Sb goes down in ox. no. The hydrogens complicate things as does the one charge, but the oxygens dominate.
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