The Periodic Table
The periodic table was developed by Russian chemist Dmitri Mendeleev (1834-1907). It is designed to group the elements, and therefore atoms, according to their characteristics. The atoms in the periodic table are electrically neutral. That means that they have the same number of protons and electrons, cancelling out the positive and negative charges. The number of protons can be determined by looking at the atomic number. The atomic number is the number of protons. The atomic mass is the average of the atomic masses of all the isotopes for that atom (based on their percentage of availability) This is why the atomic mass shown on a periodic table is rarely a whole number. The mass is representative of the number of protons plus the number of neutrons. So to determine the number of neutrons, you take the nearest whole number and subtract the number of protons.
In the periodic table, every atom is in a particular period (horizontal row) and in a particular group (vertical column). The atomic number of every atom in a period increases by one if you read the atoms from left to right. All the atoms in a group share the same binding characteristics. They have the same number of valence electrons (electrons in their outter shell) See the tab on bonding to learn more.