ATOMIC NUMBER AND MASS NUMBERS

                                                                                                        

                            Atomic Number  

                                                                    

In the modern periodic table, the elements are actually arranged in order of increasing atomic number--that's the number of protons in one atom of a particular element. An undisturbed atom is electrically neutral, so the number of electrons in it is the same as its atomic number.

Atomic weight almost always increases with atomic number, so Mendeleev's sequence of elements was almost exactly the same as the one used today, though there are a couple of weird exceptions. In general, it's correct to think of atoms getting heavier as you go down a column or to the right across a row. 

                                  

                           Mass number

                                     

A mass number is the total number of protons and neutrons found in an atom. 

To find the mass number, you must know the number of neutrons in that particular isotope of the element and then add the atomic number (same as the number of protons) to it. 

When this number is definite it is called a nuclide and the symbol is written with a superscript giving the mass number and a subscript giving the atomic number (number of protons) placed in front of atomic symbol: 2311Na. It can also be written after the elements name, sodium-23.