What is an atom, element, molecule, and compound? What is an example of each?

Atom: An atom is the basic unit of matter, composed of a nucleus containing protons and neutrons, surrounded by electrons. Atoms are the building blocks of all elements and molecules. For example, a single hydrogen atom consists of one proton, one electron, and usually zero neutrons.

Element: An element is a substance made up of only one type of atom. Each element is defined by the number of protons in its nucleus, which is known as its atomic number. Examples of elements include hydrogen (H), oxygen (O), and carbon (C).

Molecule: A molecule is a group of two or more atoms chemically bonded together. These atoms can be of the same element or different elements. Molecules are the smallest units of compounds and can exist independently. For instance, a molecule of water (H2O) consists of two hydrogen atoms bonded to one oxygen atom.

Compound: A compound is a substance composed of two or more different elements that are chemically bonded in a fixed ratio. Compounds have unique properties distinct from the elements that compose them. An example of a compound is carbon dioxide (CO2), which is made up of one carbon atom and two oxygen atoms bonded together.

In summary, atoms are the fundamental particles that make up matter, elements are substances consisting of a single type of atom, molecules are groups of atoms held together by chemical bonds, and compounds are substances made up of different elements chemically combined in fixed ratios.