Atoms are the basic units of matter. They consist of a nucleus which contains positively charged particles called protons and neutrons, which have a neutral charge. Most of the weight of an atom is located in the nucleus. There is a negatively charged cloud of particles called electrons that surround the nucleus. An element is a "pure substance" consisting of only one kind of atom. For example, oxygen is an element consisting of only oxygen atoms, and gold is a different element consisting of gold atoms. The atoms of different substances differ by atomic weight and number. Compounds consist of two or more chemical elements held together by chemical bonds. For example, water is a compound consisting of two elements--Hydrogen and Oxygen, held together by chemical bonds forming H20. This formula means that in the compound water, there are two hydrogen atoms and one oxygen atom. A mixture is made of two or more different substances that are not chemically combined which can be separated again by mechanical or thermal means. For example, sugar and water is a mixture, as is air, which is a mixture of oxygen, nitrogen, carbon dioxide and water vapor.
An atom is the smallest unit a pure substance may be divided into and still retain all the physical and chemical properties of that pure substance.
An element has a specific atomic particle makeup, with a specific number of protons, neutrons, and electrons. The nuber of protons is called the elements atomic number and the number of protons and neutrons is called the atomic mass. Atoms of an element are all the same, there is no difference between them.
A compound is 2 or more elements that are chemically combined. They cannot be separated by normal physical means. Compounds are formed by the formulation of electron pairings between one element and the other by sharing (covalent bonds) or outright donation of electrons (ionic bonds).
A mixture is a combination of 2 or more elements that can be separated by ordinary physical means. All elements retain their physical and chemical identities in this process.