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    Everything in our world is made of atoms, which we can visualize as negatively charged electrons orbiting rapidly around a nucleus, which is composed of positively charged protons and neutral neutrons.  The different types of atoms are called the chemical elements, substances that do not break down into simpler substances by chemical means.  There are 90 elements found naturally on Earth, but there are 109 presently on the periodic table of the elements. 

    Protons and neutrons have more mass than electrons, so we find an atomís total mass using only the masses of its nuclear particles.  Hydrogen has a mass of 1.67 x 10 -24 grams, about one atomic mass unit (u).  To find an atom's mass, simply find its mass number on the periodic table. All elements have isotopes, forms of elements with different numbers of neutrons in the nucleus.  The number of protons in a nucleus, its atomic number, shows identity. If the number of protons change, then it becomes a different element. The atomic number also determines the number of electrons in a neutral atom which also determines its chemistry.

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    Two forces hold atoms together.  The electric force, a force vastly stronger than gravity, holds an atom's electrons in the space around its nucleus.  The nuclear force holds an atom's protons and neutrons together in its nucleus.  It is the strongest known force in nature, but is effective only over a short range.

    The chemical properties of an atom depend mainly on the electrons in its outer shell or valence shell.  If this shell is full, the atoms are called inert, or non-reactive.  If it is partially filled an atom can enter a chemical reaction and form chemical bonds.  Chemical bonds bind different elements tightly together in compounds.  Both compounds and elements can exist as molecules, which are the smallest units of combined atoms with their own chemical identity. 

Infoplease periodic table

There are four principal bonds between atoms: ionic, covalent, metallic, and hydrogen bonds.

Metals, elements whose outer shells are less than half full, react with nonmetals, elements whose outer shells are nearly full, through ionic bonds. In ionic bonds,  metal atoms transfer their valence electrons to nonmetal atoms. This process forms salts.

Nonmetals react with nonmetals through covalent bonds, in which two atoms share electrons in the region between them.  Remember the Bohr models we did in class?

Metal atoms stick together with metallic bonds, in which the outermost electrons of the atoms wander freely, shared by all the connected metal atoms.  The van der Waals bond (Van der Waals AKA London Dispersion Force) occurs when the motions of an atom's electrons polarize it, creating positively and negatively charged regions that attract any other neutral atoms in close proximity.  If energy is released when bonds are formed or broken the process is exothermic. If energy is absorbed  in the chemical reaction the process is endothermic.

See chemistry page for more detail on bonding.