1. THE PERIODIC TABLE
In the modern periodic table, you find the elements placed in vertical columns, called groups, and horizontal lines, called periods.

2. THE PERIODS
The elements are placed in horizontal lines, on crescent atomic number. Each line is called period. There are 7 periods. Each period starts with an element with only one electron in a new principal level of energy.

3. THE FIRST PERIOD
It contains only 2 elements, H and He because at the most there are 2 electrons in the first level. H He

4. Li Be B C N O F Ne THE SECOND PERIOD
It contains 8 elements, from Li to Ne, because at the most there are 8 electrons in the second level. Li Be B C N O F Ne

5. Na Mg Al Si P S Cl Ar THE THIRD PERIOD
The third period should contain 18 elements, but the sublevel 3d follows the 4s in the energetic diagram, so in the third period we have only 8 elements corresponding to filling of sublevels 3s and 3p. Na Mg Al Si P S Cl Ar

6. THE FOURTH PERIOD K Br Kr
The fourth period is composed by 18 elements from K to Kr. This period contains 10 elements, from Sc to Zn, corresponding to filling of sublevel 3d. K Ca Sc Ti V Cr Mn Fe Co Ni Cu Zn Ga Ge As Se Br Kr

7. THE FIFTH PERIOD
The fifth period is composed by 18 elements from Rb to Xe. They complete the d sublevel. Rb Sr Y Zr Nb Mo Tc Ru Rh Pd Ag Cd In Sn Sb Te I Xe

8. THE SIXTH PERIOD Lu Tb Cs Pr Nd Eu Gd Dy Ho Er Yb
The sixth period is composed by 18 elements from Cs to Rn. They fill the f sublevel. In this period there are other fourteen elements called “Lantanidi”. They have the same electronic configuration of La. Cs Ba
La* Hf Ta W Re Os Ir Pt Au Hg Tl Pb Bi Po At Rn
*Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

9. THE SEVENTH PERIOD The seventh period is composed by 3 elements from Fr to Ac. They complete the f sublevel. In this period there are other fourteen elements called “Attinidi”. They have the same electronic configuration of Ac. The elements from U to Lr are artificial and their names derive from the ones of their discoverers. Fr Ra
Ac*
*Th Pa U Np Pu Am Cm Bk Cf Es Fm Md No Lr