The correct answer is option 3: "Hydrogen has no isotopes." Isotopes are atoms of the same element that have a different number of neutrons, resulting in a slightly different atomic weight.

Hydrogen does have isotopes. The most common isotope is protium, which has one proton and no neutrons. However, there are also two other isotopes of hydrogen: deuterium and tritium. Deuterium has one proton and one neutron, while tritium has one proton and two neutrons. These isotopes of hydrogen have slightly different atomic weights and can behave differently in certain chemical reactions.

Option 1: "Hydrogen is an element." This statement is correct. Hydrogen is the first element on the periodic table and is the lightest element. It consists of one proton and one electron.

Option 2: "Hydrogen is the lightest element." This statement is also correct. Hydrogen has an atomic weight of approximately 1 gram per mole, making it the lightest element.

Option 4: "Hydrogen and oxygen form an explosive mixture." This statement is correct. When hydrogen gas combines with oxygen gas in the presence of a spark or flame, it can form water vapor