Los Alamos National Laboratory
Periodic Table of Elements: LANL

How to use the Periodic Table

When you open any file of an element in the periodic table, you will find a small table with some basic information about that element. Here's how you use that table.

1
– Atomic Number
H – Atomic Symbol
1.008 – Atomic Weight
  • Atomic Number
    The number of protons in an atom defines what element it is. For example carbon atoms have six protons, hydrogen atoms have one, and oxygen atoms have eight. The number of protons in an atom is referred to as the atomic number of that element. The number of protons in an atom also determines the chemical behavior of the element.
  • Atomic Symbol
    The atomic symbol is one or two letters chosen to represent an element ("H" for "hydrogen," etc.). These symbols are used internationally. Typically, a symbol is the truncated name of the element or the truncated Latin name of the element. Click here for a list of the elements and their symbols.
  • Standard Atomic Weight
    The standard atomic weight is the average mass of an element in atomic mass units ("amu"). Though individual atoms always have an integer number of atomic mass units, the atomic mass on the periodic table is stated as a decimal number because it is an average of the various isotopes of an element. The average number of neutrons for an element can be found by subtracting the number of protons (atomic number) from the atomic mass.
    Atomic weight for elements 93-118. For naturally-occurring elements, the atomic weight is calculated from averaging the weights of the natural abundances of the isotopes of that element. However, for man-made trans-uranium elements there is no "natural" abundance. The IUPAC convention is to list the atomic weight of the longest-lived isotope in the periodic table. These atomic weights should be considered provisional since a new isotope with a longer half-life could be produced in the future.
  • Electron Configuration
    The electron configuration is the orbital description of the locations of the electrons in an unexcited atom. Using principles of physics, chemists can predict how atoms will react based upon the electron configuration. They can predict properties such as stability, boiling point, and conductivity. Typically, only the outermost electron shells matter in chemistry, so we truncate the inner electron shell notation by replacing the long-hand orbital description with the symbol for a noble gas in brackets. This method of notation vastly simplifies the description for large molecules.
    Example: The electron configuration for Be is 1s22s2, but we write [He]2s2 where [He] is equivalent to all the electron orbital's in the helium atom. The Letters, s, p, d, and f designate the shape of the orbital's and the superscript gives the number of electrons in that orbital.
  • Atomic Radius
    There are a number of different ways to measure atomic radius. Two common ones are covalent radius (measures in pirometers) and Van der Waals radius. The Van der Walls radius is used for this periodic table. For a more in depth discussion of atomic radius, see the CRC Handbook of Chemistry and Physics, "Atomic Radii of the Elements" and Wikipedia.

    Further explanation of the atom may be found here.
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