Substances can classified as elements or compounds An element is the simplest form of matter that has a unique set of properties oxygen and hydrogen are two of the more than 100 known elements A compound is a substance that contains two or more elements chemically combined in a fixed proportion For example carbon oxygen and hydrogen are chemically combined in the compound sucrose sometimes sucrose is called table sugar to distinguish if from other sugar compounds ln every sample of sucrose there are twice as many hydrogen particles as oxygen particles The proportion of hydrogen particles to oxygen particles in sucrose is fixed There is difference between elements and compounds compounds can be broken down into simpler substances by chemical means but elements cannot