Elements on the periodic table are classified by their atomic number and their atomic mass. The atomic number is the number of protons in an atom. This determines the elements properties. The atomic mass is the mass of the protons and neutrons. The electrons are not taken into account.
Elements on the periodic table are orginized into columns and rows. The vertical columns,called groups, contain elements that have similar properties. The horizontal rows are called periods. All the Elements left of the "stair-step line" are metals. The elements next to the line are metalloids, mixes of metal and non-metal properties. All of the elements to the right of the line are non-metals. These are either gasses or brittle solids.
An atom has 7 "shells" outside the nucleus. The first shell can hold 2 electrons, the second shell can hold 6 electrons, and the third shell can hold 18 electrons. The stability of an atom is dependent upon two factors, how close the "shell" is to the nucleus and how full the shell is. All of the noble gases are extremely stable because they fill up the "shells" completely.
An isotope is an atom of the same element but with a different number of neutrons. Take Boron for example, it can have such isotopes as
The arrangment of electrons in an atom determines the properties.
