Hydrogen is estimated to make up approximately 75% of the mass of the universe. Hydrogen is classified as an element and is among the 90 elements that occur naturally on Earth. An element is a pure substance that cannot be separated into simpler substances by chemical or physical processes. However, we know from the Periodic Table of Elements that there are more than 90 elements. The remainder of the elements have been made by scientists and are not known to exist naturally on Earth.
Many elements on the periodic table you have probably heard of – iron, gold, oxygen, carbon, tin, copper, etc. Each of these elements will have a unique chemical name and symbol, which can be found on the periodic table.
In contrast, a compound is made up of two or more different elements that are chemically combined. That is, they cannot be separated by physical means. Table salt’s chemical name is Sodium Chloride, or NaCl. This compound is one part sodium and one part chlorine – together they are salt. You cannot physically separate the sodium from the chloride because they have been chemically combined. Another common compound is water, or H2O. Water consists of two parts hydrogen and one part oxygen.
Can you think of some other common compounds used in daily life?