Atomic number = Number of protons  


Number of Protons = number of electrons for a  neutral atom


Protons have a charge of  +1     

Electrons have a charge of -1

Neutrons have a neutral charge of 0


Nucleus – Contains Protons and Neutrons.   These do not change during a basic chemical reaction.  It requires a nuclear reaction.


  1. Electrons  - are in the outer orbital of the atom
  2. Mass of an atom    = number of protons + number of neutrons
  3. Number of Neutrons  =   Mass number  -  number of  Protons
  4. Isotopes   -   Atoms with the same atomic number but a different number of neutrons.

 Notation for Isotopes     AB X


A =   Mass number;     B =   Atomic number


Example:    An isotope of Carbon can be expressed as: 136 C


The atomic number for this element is 6;

The mass number is 13

The number of neutrons = 13 - 6 =   7

The atom is an isotope for Carbon, C.  =    136 C


Electron configuration - location of electrons in an atom (SEE General Orbitals' Chart)


  1. Atom - A neutral species.  It contains the same number of electrons and protons.  All elements listed in the periodic table are listed as neutral species.
  2. Ion - An ‘ion’ is an atom that will have an electrical charge of either a positive (+) or negative (-) charge.  It will have an unequal number of electrons & protons.   It has either gained or lost electrons.  (The number of protons of an atom will remain constant, they are in the nucleus).
  3. Valence electrons:  The electrons in the outer (highest) energy level of an atom.  These are the ones that are most effected during chemical reactions.
  4. Predicting the charge of an ion.  The charge acquired by an atom when it forms a compound with other atoms is primarily based on its valence electrons (V.E., number of electrons contained in its outer energy level.  The number of .valence electrons can influence whether the atom is most likely to lose electrons or gain electrons.  Atoms with more than 4 electrons, in its valence level, tend to gain additional electrons, thus acquire a negative charge; atoms with less than 4 electrons, in its valence level, tend to lose those electrons, thus acquire a positive charge...

Rational: The most stable valence level (outer energy level) for an atom is one that contains eight electrons. This configuration has the same number of electrons (isoelectronic) with the most stable elements, noble gases.  Atoms with more than 4 electrons strive to gain additional electrons to obtain a total of eight.  Atoms with less than 4 electrons tend to lose their electrons to obtain an "empty" status, so that its next lower level is stable.   Atoms with exactly 4 electrons tend to share electrons.

Example:    Magnesium has 12 electrons and 12 protons; however, it has two electrons in

                    its valence energy level.  It tends to lose those two electrons so that its next

                    lower level is stable.  Losing those two electrons, it now has 10 electrons with

                    its 12 protons. This is a net +2 charge.  It forms an ion with a positive two

                    charge,   (Mg+2).


                   Oxygen has 8 electrons with its 8 protons; however, it has 6 electrons in its

                   valence energy level.  It tends to gain two electrons to obtain eight electrons to

                   stabilize the valence level.  Gaining two electrons, it now has 10 electrons with

                   it 8 protons.  This is a net -2 charge.  It forms an ion with a negative two

                   charge, (O-2).


                   The Carbon atom (C) atom has four electrons in its valence energy level. It  

                   tends to share electrons when it forms compounds.