Introduction:
In the given reaction, sodium and hydrogen combine to form sodium hydride in the presence of heat. We are required to identify the oxidizing agent in this reaction.

Explanation:
To determine the oxidizing agent, we need to understand the concept of oxidation and reduction. Oxidation is the process in which an atom, ion, or molecule loses electrons, while reduction is the process in which an atom, ion, or molecule gains electrons.

In this reaction, sodium is combined with hydrogen to form sodium hydride. Let's analyze the electron transfer that occurs in this reaction:

1. Sodium (Na): Sodium has an electronic configuration of 1s² 2s² 2p⁶ 3s¹. In the reaction, sodium loses one electron to form Na⁺ ion, which has a stable noble gas configuration of neon (1s² 2s² 2p⁶).

2. Hydrogen (H): Hydrogen has an electronic configuration of 1s¹. In the reaction, hydrogen gains one electron to form H⁻ ion.

3. Sodium Hydride (NaH): Sodium hydride is formed when sodium donates its electron to hydrogen. The resulting compound has a stable noble gas configuration for both sodium and hydrogen.

Oxidizing Agent:
The oxidizing agent is the species that causes oxidation by accepting electrons. In this reaction, hydrogen gains one electron and is therefore being reduced. Since the hydrogen species is gaining electrons, it cannot be the oxidizing agent.

Therefore, the oxidizing agent in this reaction is sodium. Sodium loses one electron and is being oxidized. It is responsible for the electron transfer in the reaction.

Conclusion:
In the reaction where sodium and hydrogen combine to form sodium hydride in the presence of heat, the oxidizing agent is sodium. Sodium loses one electron and undergoes oxidation, while hydrogen gains one electron and undergoes reduction.