Elements are the simplest chemical substances and the building blocks of all other materials. There are about 118 elements in the periodic table, which is a list of all known elements arranged according to their properties. The first 90 elements occur naturally, while the remaining 28 have been artificially created in laboratories.
The word “element” comes from the Latin word for “ingredient”. The term was first used by ancient Greek philosophers to refer to the basic substances of which all things are made. In medieval alchemy, the four basic elements were thought to be fire, air, earth, and water. Modern science has shown that these four substances are not actually elements, but rather compounds composed of different proportions of other elements.
All matter is made up of atoms, which are themselves made up of smaller particles called protons and neutrons (in the nucleus) and electrons (in shells around the nucleus). The number of protons in an atom’s nucleus determines its element; for example, all atoms with six protons in their nuclei are carbon atoms. The number of protons in an element’s nucleus also determines how strong its atomic interactions are—the so-called “atomic number”.
The types of atoms present in a sample determine its physical properties such as density and melting point. For example, iron has a higher density than aluminum because its atoms are larger and heavier. Similarly, lead has a higher melting point than zinc because it takes more energy to break apart lead atoms than zinc atoms.
Atoms can join together to form molecules by sharing electrons between them; when this happens, they create bonds that hold the molecules together. Different types of bonds result in different kinds of molecules: ionic bonds result in salts like sodium chloride (table salt), covalent bonds result in molecules like water or methane gas, while metallic bonds result in metals like iron or copper.