- They are the simplest, most fundamental units of matter. Elements cannot be broken down into simpler substances by chemical means. Each element has its own unique properties and atomic structure, which distinguishes it from other elements.

Hydrogen and oxygen

- Hydrogen and oxygen are both elements because they meet the criteria of being the simplest, most fundamental units of matter that cannot be broken down further by chemical means. They have their own unique atomic structures and properties that distinguish them from other elements.

Atomic structure of hydrogen

- Hydrogen has the simplest atomic structure of all elements, with one proton, one electron, and no neutrons. Its atomic number is 1, which means it has one proton in its nucleus.

Atomic structure of oxygen

- Oxygen has a slightly more complex atomic structure than hydrogen, with eight protons, eight electrons, and eight neutrons. Its atomic number is 8, indicating it has eight protons in its nucleus.

Chemical properties of hydrogen

- Hydrogen is a highly reactive element that can form covalent bonds with many other elements. It is the most abundant element in the universe and is found in a variety of compounds, including water, hydrocarbons, and acids.

Chemical properties of oxygen

- Oxygen is also a highly reactive element that can form covalent bonds with many other elements. It is the second most abundant element in the universe and is found in a variety of compounds, including water, carbon dioxide, and metal oxides.

In summary, hydrogen and oxygen are both considered elements because they are composed of unique atoms that cannot be broken down into simpler substances by chemical means. They have distinct atomic structures and chemical properties that set them apart from other elements.