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Atomic number and atomic mass number
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- The division of lelectrons from an atom
Atomic number and atomic mass number The chemical properties of an element are determined by the charge of its nucleus, i.e. by the number of protons. This number is called the atomic number and is denoted by the letter Z. Definition: Atomic number (Z) The number of protons in an atom. The mass of an atom depends on how many nucleons its nucleus contains. The number of
Definition: Atomic mass number (A) The number of protons and neutrons in the nucleus of an atom. Standard notation shows the chemical symbol, the atomic mass number and the atomic number of an element as follows: 455
 A
For example, the iron nucleus which has 26 protons and 30 neutrons, is denoted as 
where the total nuclear charge is Z = 26 and the mass number A = 56. The number of neutrons is simply the difference N = A − Z. Important: Don’t confuse the notation we have used above, with the way this information appears on the Periodic Table. On the Periodic Table, the atomic number usually appears in the top lefthand corner of the block or immediately above the element’s symbol. The number below the element’s symbol is its relative atomic mass. This is not exactly the same as the atomic mass number. This will be explained in section 3.5. The example of iron is used again below. 
You will notice in the example of iron that the atomic mass number is more or less the same as its atomic mass. Generally, an atom that contains n protons and neutrons (i.e. Z = n), will have a mass approximately equal to n u. The reason is that a C-12 atom has 6 protons, 6 neutrons and 6 electrons, with the protons and neutrons having about the same mass and the electron mass being negligible in comparison. The division of lelectrons from an atom A large local charge separation usually results when a shared electron pair is donated unilaterally. The three Kekulé formulas shown here illustrate this condition. 
In the formula for ozone the central oxygen atom has three bonds and a full positive charge while the right hand oxygen has a single bond and is negatively charged. The overall charge of the ozone molecule is therefore zero. Similarly, 456
nitromethane has a positive-charged nitrogen and a negative-charged oxygen, the total molecular charge again being zero. Finally, azide anion has two negative-charged nitrogens and one positive-charged nitrogen, the total charge being minus one. In general, for covalently bonded atoms having valence shell electron octets, if the number of covalent bonds to an atom is greater than its normal valence it will carry a positive charge. If the number of covalent bonds to an atom is less than its normal valence it will carry a negative charge. The formal charge on an atom may also be calculated by the following formula: 
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