The atomic number refers to the number of protons per atomic nucleus.  Atomic mass (weight on many periodic tables) is the combined mass of the protons and neutrons.  You can subtract the atomic number from the atomic mass and find the number of neutrons.  The atomic mass on most Periodic Tables includes the different isotopes of that element, so the number is an average.  An isotope is an element that has the same protons, but its different isotopes have varying number of neutrons. Hydrogen has three isotopes deuterium with one neutron and tritium with two neutrons. If it has no neutrons this is usually referred to as hydrogen.  When a Periodic Table gives you the atomic mass of all the isotopes, you should round off the atomic mass to the nearest whole number to derive the number of neutrons. The number of protons equals the number of electrons.  

Each of the elements has not only a unique atomic mass and number but also have a crystal structure in which they form on the atomic level. The crystal structure refers to the way in which the atoms come together to form a pattern.  On the recommended Periodic Table each of the natural occurring elements will have a crystal symbol in the element box.  Refer to Symmetry of Matter, Lesson 5 for a further explanation (under Math Skills).

The importance of melting and boiling points helps to define some physical properties of the element.  The melting point is the temperature that a substances changes from a solid to liquid.  The boiling point is when a material changes from a liquid to gas.  These properties help to measure the strength of that element.    Density is sometimes referred to as Specific Gravity on some Periodic Tables.  It is a physical property that measures how a substance is “packed.”