1)The smallest complete unit of elemental oxygen gas is an atom of oxygen. (True)

Since an oxygen gas is made up of two oxygen atoms bonded by two bonds, so oxygen atom is the smallest complete unit of elemental oxygen gas.

2)The usual products of an acid-base reaction are salt and water. (True)

An acid-base reaction is a neutralization reaction that involves the reaction between the acid and the base to form a neutral salt and water.

3)A reaction that leads to a decrease in the enthalpy of the system is always spontaneous. (False)

For a spontaneous reaction, there is always a decrease in the free energy change of the system but there can be either a decrease or an increase in the enthalpy of the system.