Water is composed of two fundamental elements: Hydrogen and Oxygen. These are the only two elements that make up water molecules. The chemical formula H₂O tells us that each water molecule contains two hydrogen atoms and one oxygen atom.
Hydrogen is the first element in the periodic table with atomic number 1. It's the lightest and most abundant element in the universe. Hydrogen atoms consist of just one proton and one electron. It's a colorless, odorless gas that is highly flammable and readily forms chemical bonds with other elements.
Oxygen is the eighth element in the periodic table with atomic number 8. It has 8 protons in its nucleus and 8 electrons arranged in two shells. Oxygen is essential for life as we know it, supporting both breathing and combustion. It's a highly reactive element that readily forms compounds with other elements.
When hydrogen and oxygen combine to form water, two hydrogen atoms each share an electron with one oxygen atom. This creates covalent bonds, forming a stable water molecule with the chemical formula H₂O. The bonding occurs because oxygen needs two more electrons to complete its outer shell, while each hydrogen needs one electron.
To summarize, water contains exactly two elements from the periodic table: Hydrogen and Oxygen. Each water molecule has two hydrogen atoms and one oxygen atom, giving us the familiar chemical formula H₂O. These are the only two elements present in pure water - no other elements are needed or found in water molecules.