In chemistry, a substance is a special class of matter that has definite composition and properties. The simplest substances that exist are elements. Elements are substances that are composed entirely out of one type of atom. Elements are the building blocks that all other substances are made from because atoms cannot be easily divided using chemical means. Every element is a different atom whose arrangement of sub-particles (protons, neutrons, and electrons) gives it its unique properties.
Every entry in the Periodic Table represents a unique element. Examples of elements are: iron, oxygen, carbon, sulfur, chlorine, radon, manganese, etc. Look at the periodic table below and see if you can’t find some more that you recognize!
Atoms consist of three main sub-particles which cannot easily be separated from each other. Protons and neutrons are found in the very tiny, very dense center of the atom called the nucleus. Even though the nucleus takes up a small portion of the atoms overall size, it contains almost all its mass.
The rest of the atom is composed of the third sub particle, electrons. Electrons travel around the nucleus in defined orbitals. Orbitals consist of mostly empty space and one or two electrons that are moving around within them. As more and more electrons are added to the atom these orbitals begin to layer on top of each other in what are called electron shells. Atoms of different elements may associate with each other (make bonds) based on the number of electrons in their outermost shell. When two atoms are associating closely with each other we call that a chemical bond. A complete list of all the elements can be found in the periodic table of elements which is organized in rows that group electron shells, and columns that group the number of electrons in the outermost shell.
When two or more atoms form chemical bonds together, that association is called a molecule. Molecules usually behave differently than the atoms that they are composed of.
Because molecules behave differently than their elemental components, we think of molecules as their own substances, independent of the atoms that compose them. When a substance is made of molecules rather than a single element we call that substance a compound. Compounds can undergo chemical changes (chemical reactions). Reactions can cause compounds to break up to make simpler molecules or to associate further making more complex substances. Substances are usually identified based on the properties that they exhibit.
For instance, iron is a metal that everyone in the modern world is quite familiar with. Likewise, sulfur is a yellow solid with certain physical properties. However, when those two elements interact, they make a new compound together—iron sulfide, commonly known as a mineral pyrite. Pyrite has a distinctly different physical and chemical properties that the elements that made it.
Periodic Table– sciencenotes.org
Iron: HEINRICH PNIOK (https://de.wikipedia.org/wiki/Benutzer:Alchemist-hp)
Sulfur: Daniel Schwen (https://commons.wikimedia.org/wiki/User:Dschwen)
Pyrite (Iron Sulfide): http://images-of-elements.com/sulfur.php