Explain the relationship among atoms, elements, and compounds.
Atoms are the basic building blocks of matter. They are composed of positively charged particles (protons) and negatively charged particles (electrons). Many also have neutrally charged particles (neutrons). Elements are identified by the number of protons they possess—this is called the atomic number. While elements can have different atomic weights (the sum of their protons and neutrons), all elements have a specific atomic number. This atomic number allows elements to be classified on a periodic table.
Compounds are formed because elements are seldom found in a pure state—they often bond as valence electrons go to other elements in order to provide stability for the atom. Compounds are combinations of one or more elements which join chemically. There are two different kinds of chemical bonding—ionic and covalent. Ionic compounds are formed when atoms pull electrons away from another element. Covalent allow for two atoms to share electrons.
Atoms, elements, and compounds are part of a classification scheme used by chemists. At the top are the categories of matter and energy. These can be converted into each other through nuclear reactions according to Einstein's equation for special relativity: E = mc^2, where E is energy, m is mass, and c is the speed of light. Since c is a large number, this equation implies that a small amount of matter can be converted into a large amount of energy. Energy comes in two basic categories, kinetic and potential.
Under matter are the categories of pure substances and mixtures. Mixtures can be made in any ratio and can be physically separated using such techniques as distillation, condensation, smelting, evaporation, and filtration. Examples of mixtures are alloys, solutions, air, smoke, etc. They can be heterogeneous ("lumpy"), homogeneous ("smooth"), or in-between, such as suspensions or colloids.
There are two categories of pure substances: compounds and elements. A compound contains more than one element in a fixed, or definite ratio. The smallest unit of a compound is a molecule, and the individual atoms in a molecule are chemically bonded. A chemical reaction (or separation) is needed to break the atoms apart.
Elements are pure substances that have only one type of atom in them, so they cannot be separated either chemically or physically. The smallest unit of an element is an atom. Specific elements always have the same number of protons, but can have different numbers of neutrons. These are called isotopes.