* The basic building blocks of matter.

* Composed of protons, neutrons, and electrons.

* The number of protons in the nucleus determines the element.

Elements:

* Pure substances that cannot be broken down into simpler substances by chemical means.

* Each element is represented by a one- or two-letter symbol on the periodic table.

Compounds:

* Substances composed of two or more elements chemically bonded together.

* The properties of a compound are different from the properties of its constituent elements.

* Examples of compounds include water (H2O), carbon dioxide (CO2), and sodium chloride (NaCl).

Molecules:

* A molecule is a group of two or more atoms that are held together by chemical bonds.

* Molecules can be diatomic (two atoms), triatomic (three atoms), or polyatomic (more than three atoms).

* Examples of molecules include hydrogen gas (H2), oxygen gas (O2), and water (H2O).

Here is a table summarizing the key differences between atoms, elements, compounds, and molecules:

| Feature | Atom | Element | Compound | Molecule |

|---|---|---|---|---|

| Composition | Protons, neutrons, electrons | Same number of protons | Two or more elements | Two or more atoms |

| Properties | Determined by number of protons | Represented by one or two-letter symbol | Different from constituent elements | Determined by chemical bonds |

| Examples | Hydrogen, carbon, oxygen | Gold, silver, copper | Water, carbon dioxide, sodium chloride | Hydrogen gas, oxygen gas, water |